If all the structural and topographic conditions were known in a great variety of gathering basins we should undoubtedly find in them, and not in special forms of ice erosion, an explanation of the various forms assumed by cirques. The limitations inherent in a high-altitude field and a limited snow cover prevented me from solving the problem, but it offered sufficient evidence at least to indicate the probable lines of approach to a solution. For example it is noteworthy that in all the cases examined the schrundline was better developed the further glacial erosion had advanced. So constantly did this generalization check up, that if at a distance a short valley was observed to end in a cirque, I knew at once and long before I came to the valley head that a shoulder below the schrundline did not exist. At the time this observation was made its significance was a mystery, but it represents a condition so constant that it forms one of the striking features of the glacial forms in the headwater region.
The meaning of this feature is represented in 199 , in which three successive stages in cirque development are shown. In A, as displayed in small valleys or mountainside alcoves which were but temporarily occupied by snow and ice, or as in all higher valleys during the earlier stages of the advancing hemicycle of glaciation, snow collects, a short glacier forms, and a bergschrund develops. As a result of the concentrated frost action at the base of the bergschrund a rapid deepening and steepening takes place at a. As long as the depth of snow (or snow and névé) is slight the bergschrund may remain open. But its existence at this particular point is endangered as the cirque grows, since the increasing steepness of the slope results in more rapid snow movement. Greater depth of snow goes hand in hand with increasing steepness and thus favors the formation of névé and even ice at the bottom of the moving mass and a constantly accelerated rate of motion. At the same time the bergschrund should appear higher up for an independent reason, namely, that it tends to form between a mass of slight movement and one of greater movement, which change of function, as already pointed out, would appear to be controlled by change from snow to névé or ice on the part of the bottom material.
The first stages in the upward migration of the bergschrund will not effect a marked change from the original profile, since the converging slopes, the great thickness of névé and ice at this point, and the steep gradient all favor powerful erosion. When, however, stage C is reached, and the bergschrund has retreated to c″, a broader terrace results below the schrundline, the gradient is decreased, the ice and névé (since they represent a constant discharge) are spread over a greater area, hence are thinner, and we have the cirque taking on a compound character with a lower, less steep and an upper, precipitous section.
It is clear that a closely jointed and fragile rock might be quarried by moving ice at c′-c″ and the cirque wall extended unbroken to x; it is equally clear that a homogeneous, unjointed granite would offer no opportunities for glacial plucking and would powerfully resist the much slower process of abrasion. Thus Gilbert[64] observed the schrundline in the granites of the Sierra Nevada, which are “in large part structureless” and my own observations show the schrundline well developed in the open-jointed granites of the Cordillera Vilcapampa and wholly absent in the volcanoes of the Maritime Cordillera, where ashes and cinders, the late products of volcanic action, form the easily eroded walls of the steep cones. Somewhere between these extremes—lack of a variety of observations prevents our saying where—the resistance and the internal structure of the rock will just permit a cirque wall to extend from x to c′ ″ of 199 .
A common feature of cirques that finds an explanation in the proposed hypothesis is the notch that commonly occurs at some point where a convergence of slopes above the main cirque wall concentrates snow discharge. It is proposed to call this type the notched cirque. It is highly significant that these notches are commonly marked by even steeper descents at the point of discharge into the main cirque than the remaining portion of the cirque wall, even when the discharge was from a very small basin and in the form of snow or at the most névé. The excess of discharge at a point on the basin rim ought to produce the form we find there under the conditions of snow motion outlined in earlier paragraphs. It is also noteworthy that it is at such a point of concentrated discharge that crevasses no sooner open than they are closed by the advancing snow masses. To my mind the whole action is eminently representative of the action taking place elsewhere along the cirque wall on a smaller scale.
What seems a good test of the explanation of cirques here proposed was made in those localities in the Maritime Cordillera, where large snowbanks but not glaciers affect the form of the catchment basins. A typical case is shown in 201 . As in many other cases we have here a great lava plateau broken frequently by volcanic cones of variable composition. Some are of lava, others consist of ashes, still others of tuff and lava and ashes. At lower elevations on the east, as at 16,000 feet between Antabamba and Huancarama, evidences of long and powerful glaciers are both numerous and convincing. But as we rise still higher the glaciated topography is buried progressively deeper under the varying products of volcanic action, until finally at the summit of the lava fields all evidences of glaciation disappear in the greater part of the country between Huancarama and the main divide. Nevertheless, the summit forms are in many cases as significantly altered as if they had been molded by ice. Precipitous cirque walls surround a snow-filled amphitheater, and the process of deepening goes forward under one’s eyes. No moraines block the basin outlets, no U-shaped valleys lead forward from them. We have here to do with post-glacial action pure and simple, the volcanoes having been formed since the close of the Pleistocene.
Likewise in the pass on the main divide, the perpetual snow has begun the recessing of the very recent volcanoes bordering the pass. The products of snow action, muds and sands up to very coarse gravel, glaciated in texture with an intermingling of blocks up to six inches in diameter in the steeper places, are collected into considerable masses at the snowline, where they form broad sheets of waste so boggy as to be impassable except by carefully selected routes. No ice action whatever is visible below the snowline and the snow itself, though wet and compact, is not underlain by ice. Yet the process of hollowing goes forward visibly and in time will produce serrate forms. In neither case is there the faintest sign of a bergschrund; the gradients seem so well adjusted to the thickness and rate of movement of the snow from point to point that the marginal crack found in many snowfields is absent.
The absence of bergschrunds is also noteworthy in many localities where formerly glaciation took place. This is notoriously the case in the summit zone of the Cordillera Vilcapampa, where the accumulating snows of the steep cirque walls tumble down hundreds of feet to gather into prodigious snowbanks or to form névé fields or glaciers. From the converging walls the snowfalls keep up an intermittent bombardment of the lower central snow masses. It is safe to say that if by magic a bergschrund could be opened on the instant, it would be closed almost immediately by the impetus supplied by the falling snow masses. The explanation appears to be that the thicker snow and névé concentrated at the bottom of the cirque results in a corresponding concentration of action and effect; and cirque development goes on without reference to a bergschrund. The chief attraction of the bergschrund hypothesis lies in the concentration of action at the foot of the cirque wall. But in the thickening of the snow far beyond the minimum thickness required for motion at the base of the cirque wall and its change of function with transformation into névé, we need invoke no other agent. If a bergschrund forms, its action may take place at the foot of the cirque wall or high up on the wall, and yet sapping at the foot of the wall continue.
From which we conclude (1) that where frost action occurs at the bottom of a bergschrund opening to the foot of the cirque wall it aids in the retreat of the wall; (2) that a sapping action takes place at this point whether or not a bergschrund exists and that bergschrund action is not a necessary part of cirque formation; (3) that when a more or less persistent bergschrund opens on the cirque wall above its foot it tends to develop a schrundline with a marked terrace below it; (4) that schrundlines are best developed in the mature stages of topographic development in the glacial cycle; (5) that the varying rates of snow, névé, and ice motion at a valley head are the persistent features to which we must look for topographic variations; (6) that the hypothesis here proposed is applicable to all cases whether they involve the presence of snow or névé or ice or any combination of these, and whether bergschrunds are present or not; and (7) at the same time affords a reasonable explanation for such variations in forms as the compound cirque with its schrundline and terrace, the unbroken cirque wall, the notched cirque, and the recessed, snow-covered mountain slopes unaffected by ice.
ASYMMETRICAL CREST LINES AND ABNORMAL VALLEY PROFILES IN THE CENTRAL ANDES
To prove that under similar conditions glacial erosion may be greater than subaërial denudation quantitative terms must be sought. Only these will carry conviction to the minds of many opponents of the theory that ice is a vigorous agent of erosion. Gilbert first showed in the Sierra Nevada that headwater glaciers eroded more rapidly than nonglacial agents under comparable topographic and structural conditions.[65] Oddly enough none of the supporters of opposing theories have replied to his arguments; instead they have sought evidence from other regions to show that ice cannot erode rock to an important degree. In this chapter evidence from the Central Andes, obtained in 1907 and 1911, will be given to show the correctness of Gilbert’s proposition.
The data will be more easily understood if Gilbert’s argument is first outlined. On the lower slopes of the glaciated Sierra Nevada asymmetry of form resulted from the presence of ice on one side of each ridge and its absence on the other (Fig. 200). The glaciers of these lower ridges were the feeblest in the entire region and were formed on slopes of small extent; they were also short-lived, since they could have existed only when glacial conditions had reached a maximum. Let the broken line in the upper part of the figure represent the preglacial surface and the solid line beneath it the present surface. It will not matter what value we give the space between the two lines on the left to express nonglacial erosion, since had there been no glaciers it would be the same on both sides of the ridge. The feeble glacier occupying the right-hand slope was able in a very brief period to erode a depression far deeper than the normal agents of denudation were able to erode in a much longer period, i.e., during all of interglacial and postglacial time. Gilbert concludes: “The visible ice-made hollows, therefore, represent the local excess of glacial over nonglacial conditions.”
Fig. 200—Diagrammatic cross-section of a ridge glaciated on one side only; with hypothetical profile (broken line) of preglacial surface. |
Fig. 201—Postglacial volcano recessed on shady southern side by the process of nivation. Absolute elevation 18,000 feet (5,490 m.), latitude 14° S., Maritime Cordillera, Peru. |
In the Central Andes are many volcanic peaks and ridges formed since the last glacial epoch and upon them a remarkable asymmetry has been developed. Looking southward one may see a smoothly curved, snow-free, northward-facing slope rising to a crest line which appears as regular as the slope leading to it. Looking northward one may see by contrast (Fig. 194) sharp ridges, whose lower crests are serrate, separated by deeply recessed, snow-filled mountain hollows. Below this highly dissected zone the slopes are smooth. The smooth slope represents the work of water; the irregular slopes are the work of snow and ice. The relation of the north and south slopes is diagrammatically shown in Fig. 201.
To demonstrate the erosive effects of snow and ice it must be shown: (1) that the initial slopes of the volcanoes are of postglacial age; (2) that the asymmetry is not structural; (3) that the snow-free slopes have not had special protection, as through a more abundant plant cover, more favorable soil texture, or otherwise.
Proof of the postglacial origin of the volcanoes studied in this connection is afforded: (1) by the relation of the flows and the ash and cinder beds about the bases of the cones to the glacial topography; (2) by the complete absence of glacial phenomena below the present snowline. Ascending a marginal valley (Fig. 202), one comes to its head, where two tributaries, with hanging relations to the main valley, come down from a maze of lesser valleys and irregular slopes. Glacial features of a familiar sort are everywhere in evidence until we come to the valley heads. Cirques, reversed grades, lakes, and striæ are on every hand. But at altitudes above 17,200 feet, recent volcanic deposits have over large areas entirely obscured the older glacial topography. The glacier which occupied the valley of 202 was more than one-quarter of a mile wide, the visible portion of its valley is now over six miles long, but the extreme head of its left-hand tributary is so concealed by volcanic material that the original length of the glacier cannot be determined. It was at least ten miles long. From this point southward to the border of the Maritime Cordillera no evidence of past glaciation was observed, save at Solimana and Coropuna, where slight changes in the positions of the glaciers have resulted in the development of terminal moraines a little below the present limits of the ice.
From the wide distribution of glacial features along the northeastern border of the Maritime Cordillera and the general absence of such features in the higher country farther south, it is concluded that the last stages of volcanic activity were completed in postglacial time. It is equally certain, however, that the earlier and greater part of the volcanic material was ejected before glaciation set in, as shown by the great depth of the canyons (over 5,000 feet) cut into the lava flows, as contrasted with the relatively slight filling of coarse material which was accumulated on their floors in the glacial period and is now in process of dissection. Physiographic studies throughout the Central Andes demonstrate both the general distribution of this fill and its glacial origin.
So recent are some of the smaller peaks set upon the lava plateau that forms the greater part of the Maritime Cordillera, that the snows massed on their shadier slopes have not yet effected any important topographic changes. The symmetrical peaks of this class are in a few cases so very recent that they are entirely uneroded. Lava flows and beds of tuff appear to have originated but yesterday, and shallow lava-dammed lakes retain their original shore relations. In a few places an older topography, glacially modified, may still be seen showing through a veneer of recent ash and cinder deposits, clear evidence that the loftier parts of the lava plateau were glaciated before the last volcanic eruption.
The asymmetry of the peaks and ridges in the Maritime Cordillera cannot be ascribed to the manner of eruption, since the contrast in declivity and form is persistently between northern and southern slopes. Strong and persistent winds from a given direction undoubtedly influence the form of volcanoes to at least a perceptible degree. In the case in hand the ejectamenta are ashes, cinders, and the like, which are blown into the air and have at least a small component of motion down the wind during both their ascent and descent. The prevailing winds of the high plateaus are, however, easterly and the strongest winds are from the west and blow daily, generally in the late afternoon. Both wind directions are at right angles to the line of asymmetry, and we must, therefore, rule out the winds as a factor in effecting the slope contrasts which these mountains display.
It remains to be seen what influence a covering of vegetation on the northern slopes might have in protecting them from erosion. The northern slopes in this latitude (14° S.) receive a much greater quantity of heat than the southern slopes. Above 18,000 feet (5,490 m.) snow occurs on the shady southern slopes, but is at least a thousand feet higher on the northern slopes. It is therefore absent from the northern side of all but the highest peaks. Thus vegetation on the northern slopes is not limited by snow. Bunch grass—the characteristic ichu of the mountain shepherds—scattered spears of smaller grasses, large ground mosses called yareta, and lichens extend to the snowline. This vegetation, however, is so scattered and thin above 17,500 feet (5,330 m.) that it exercises no retarding influence on the run-off. Far more important is the porous nature of the volcanic material, which allows the rainfall to be absorbed rapidly and to appear in springs on the lower slopes, where sheets of lava direct it to the surface.
The asymmetry of the north and south slopes is not, then, the result of preglacial erosion, of structural conditions, or of special protection of the northern slopes from erosion. It must be concluded, therefore, that it is due to the only remaining factor—snow distribution. The southern slopes are snow-clad, the northern are snow-free—in harmony with the line of asymmetry. The distribution of the snow is due to the contrasts between shade and sun temperatures, which find their best expression in high altitudes and on single peaks of small extent. Frankland’s observations with a black-bulb thermometer in vacuo show an increase in shade and sun temperatures contrasts of over 40° between sea level and an elevation of 10,000 feet. Violle’s experiments show an increase of 26 per cent in the intensity of solar radiation between 200 feet and 16,000 feet elevation. Many other observations up to 16,000 feet show a rapid increase in the difference between sun and shade temperatures with increasing elevation. In the region herein described where the snowline is between 18,000 and 19,000 feet (5,490 to 5,790 m.) these contrasts are still further heightened, especially since the semi-arid climate and the consequent long duration of sunshine and low relative humidity afford the fullest play to the contrasting forces. The coefficient of absorption of radiant energy by water vapor is 1,900 times that of air, hence the lower the humidity the more the radiant energy expended upon the exposed surface and the greater the sun and shade contrasts. The effect of these temperature contrasts is seen in a canting of the snowline on individual volcanoes amounting to 1,500 feet in extreme instances. The average may be placed at 1,000 feet.
The minimum conditions of snow motion and the bearing of the conclusions upon the formation of cirques have been described in the chapters immediately preceding. It is concluded that snow moves upon 20° slopes if the snow is at least forty feet deep, and that through its motion under more favorable conditions of greater depth and gradient and the indirect effects of border melting there is developed a hollow occupied by the snow. Actual ice is not considered to be a necessary condition of either movement or erosion. We may at once accept the conclusion that the invariable association of the cirques and steepened profiles with snowfields proves that snow is the predominant modifying agent.
An argument for glacial erosion based on profiles and steep cirque walls in a volcanic region has peculiar appropriateness in view of the well-known symmetrical form of the typical volcano. Instead of varied forms in a region of complex structure long eroded before the appearance of the ice, we have here simple forms which immediately after their development were occupied by snow. Ever since their completion these cones have been eroded by snow on one side and by water on the other. If snow cannot move and if it protects the surface it covers, then this surface should be uneroded. All such surfaces should stand higher than the slopes on the opposite aspect eroded by water. But these assumptions are contrary to fact. The slopes underneath the snow are deeply recessed; so deeply eroded indeed, that they are bordered by steep cliffs or cirque walls. The products of erosion also are to some extent displayed about the border of the snow cover. In strong contrast the snow-free slopes are so slightly modified that little of their original symmetry is lost—only a few low hills and shallow valleys have been formed.
The measure of the excess of snow erosion over water erosion is therefore the difference between a northern or water-formed and a southern or snow-formed profile, 200 . This difference is also shown in 201 and from it and the restored initial profiles we conclude that the rate of water erosion is to that of nivation as 1:3. This ratio has been derived from numerous observations on cones so recently formed that the interfluves without question are still intact.
Fig. 202—Graphic representation of amount of glacial erosion during the glacial period. In the background are mature slopes surmounted by recessed asymmetrical peaks. The river entrenched itself below the mature slopes before it began to aggrade, and, when aggradation set in, had cut its valley floor to a′-b′-c. By aggradation the valley floor was raised to a-b while ice occupied the valley head. By degradation the river has again barely lowered its channel to a′-b′, the ice has disappeared, and the depression of the profile represents the amount of glacial erosion.
a′-b′-c = preglacial profile.
a-b-d-c = present profile.
b′-d-c-b = total ice erosion in the glacial period.
a-b = surface of an alluvial valley fill due to excessive erosion at valley head.
b-b′ = terminal moraine.
d-c = cirque wall.
e, e′ e″ = asymmetrical summits.
Thus far only those volcanoes have been considered which have been modified by nivation. There are, however, many volcanoes which have been eroded by ice as well as by snow and water. It will be seen at once that where a great area of snow is tributary to a single valley, the snow becomes compacted into névé and ice, and that it then erodes at a much faster rate. Also a new force—plucking—is called into action when ice is present, and this greatly accelerates the rate of erosion. While it lies outside the limits of my subject to determine quantitatively the ratio between water and ice action, it is worth pointing out that by this method a ratio much in excess of 1:3 is determined, which even in this rough form is of considerable interest in view of the arguments based on the protecting influence of both ice and snow. I have, indeed, avoided the question of ice erosion up to this point and limited myself to those volcanoes which have been modified by nivation only, since the result is more striking in view of the all but general absence of data relating to this form of erosion.
Fig. 203—A composite sketch to represent general conditions in the Peruvian Andes. In order to have the actual facts represented the profiles of this figure were taken from the accompanying topographic sheets. The main depression on the right and the corresponding depression of the tributary profiles bear out most strikingly the conclusions concerning the erosive power of the ice. At A and B the spurs have been cut off to exhibit the profiles of tributary valleys. At 2 and 3 were tributary glaciers of such size that they entered the main valley at grade. Lesser tributaries had floors elevated above those they joined and now have a hanging character, as just above 2. D is a matterhorn; C is deeply recessed by cirques; E represents a peak just below the limit of glaciation. At F are the undissected post-mature slopes of an earlier cycle of erosion. G lies on the steep lower slopes formed during the canyon cycle of erosion. The down-cutting of the stream in the canyon cycle was generally checked by glaciation and was superseded by aggradation.
If we now turn to the valley profiles of the glaciated portions of the Peruvian Andes, we shall see the excess of ice over water erosion expressed in a manner equally convincing. To a thoughtful person it is one of the most remarkable features of any glaciated region that the flattest profiles, the marshiest valley flats, and the most strongly meandering stretches of the streams should occur near the heads of the valleys. The mountain shepherds recognize this condition and drive their flocks up from the warmer valley into the mountain recesses, confident that both distance and elevation will be offset by the extensive pastures of the finest ichu grass. Indeed, to be near the grazing grounds of sheep and llamas which are their principal means of subsistence, the Indians have built their huts at the extraordinarily lofty elevations of 16,000 to 17,000 feet.
An examination of a large number of these valleys and the plotting of their gradients discloses the striking fact that the heads of the valleys were deeply sunk into the mountains. It is thus possible by restoring the preglacial profiles to measure with considerable certainty the excess of ice over water erosion.
The results are graphically expressed in 202 . It will be seen that until glacial conditions intervened the stream was flowing on a rock floor. During the whole of glacial time it was aggrading its rock floor below b′ and forming a deep valley fill. A return to warmer and drier conditions led to the dissection of the fill and this is now in progress. The stream has not yet reached its preglacial profile, but it has almost reached it. We may, therefore, say that the preglacial valley profile below b′ fixes the position of the present profile just as surely as if the stream had been magically halted in its work at the beginning of the period of glaciation. There, b′-d-c-b represents the amount of ice erosion. To be sure the line b-c is inference, but it is reasonable inference and, whatever position is assigned to it, it cannot be coincident with b′-d, nor can it be anywhere near it. The break in the valley profile at b′ is always marked by a terminal moraine, regardless of the character of the rock. This is not an accidental but a causal association. It proves the power of the ice to erode. In glacial times it eroded the quantity b-c-d-b′. This is not an excess of ice over water erosion, but an absolute measure of ice erosion, since a′-b′ has remained intact. The only possible error arises from the position assigned b-c, and even if we lower it to b-c′ (for which we have no warrant but extreme conservatism) we shall still have left b′-c′-d-b as a striking value for rock erosion (plucking and abrasion) by a valley glacier.
A larger diagram, 203 , represents in fuller detail the topographic history of the Andes of southern Peru and the relative importance of glaciation. The broad spurs with grass-covered tops that end in steep scarps are in wonderful contrast to the serrate profiles and truncated spurs that lie within the zone of past glaciation. In the one case we have minute irregularities on a canyon wall of great dimensions; in the other, more even walls that define a glacial trough with a flat floor. Before glaciation on a larger scale had set in the right-hand section of the diagram had a greater relief. It was a residual portion of the mountain and therefore had greater height also. Glaciers formed upon it in the Ice Age and glaciation intensified the contrast between it and the left-hand section; not so much by intensifying the relief as by diversifying the topographic forms.
Fig. 204—Topographic map of the Andes between Abancay and the Pacific Coast at Camaná. Compiled from the seven accompanying topographic sheets (see Contents, p. xi). Scale 1:1,000,000. Contour interval 1,000 feet. Longitude west of Greenwich. The Central Ranges of the Maritime Cordillera are not confined to the area covered by these names. In the one case the term includes all the ranges between Lambrama and Huichihua; in the other case, the peaks and ranges from 14° 30′ S. to Mt. Coropuna.
APPENDIX A
SURVEY METHODS EMPLOYED IN THE CONSTRUCTION OF THE SEVEN ACCOMPANYING TOPOGRAPHIC SHEETS
By Kai Hendriksen, Topographer
The main part of the topographical outfit consisted of (1) a 4-inch theodolite, Buff and Buff, the upper part detachable, (2) an 18 x 24 inch plane-table with Johnson tripod and micro-meteralidade. These instruments were courteously loaned the expedition by the U. S. Coast and Geodetic Survey and the U. S. Geological Survey respectively.
The method of survey planned was a combination of graphic triangulation and traverse with the micro-meteralidade. All directions were plotted on the plane-table which was oriented by backsight; distances were determined by the micro-meteralidade or triangulation, or both combined; and elevations were obtained by vertical angles. Finally, astronomical observations, usually to the sun, were taken at intervals of about 60 miles for latitude and azimuth to check the triangulation. No observations were made for differences in longitude because this would probably not have given any reliable result, considering the time and instruments at our disposal. Because the survey was to follow very closely the seventy-third meridian west of Greenwich, directions and distances, checked by latitude and azimuth observations, undoubtedly afforded far better means of determining the longitude than time observations. In other words, the time observations made in connection with azimuth observations were not used for computing longitudinal differences. Absolute longitude was taken from existing observations of principal places.
Principal topographical points were located by from two to four intersections from the triangulation and plane-table stations; and elevations were determined by vertical angle measurements. Whenever practicable, the contours were sketched in the field; the details of the topography otherwise depend upon a great number of photographs taken by Professor Bowman from critical stations or other points which it was possible to locate on the maps.
Cross-Section Map from Abancay to Camaná at the Pacific Ocean
Seven sheets. Scale, 1:125,000; contour interval, 200 feet. Datum is mean sea level. Astronomical control: 5 latitude and 5 azimuth observations as indicated on the accompanying topographic sheets.
On September 10th, returning from a reconnaissance survey of the Pampaconas River, I joined Professor Bowman’s party, Dr. Erving acting as my assistant. We crossed the Cordillera Vilcapampa and the Canyon of the Apurimac and after a week’s rest at Abancay started the topographic work near Hacienda San Gabriel south of Abancay. Working up the deep valley of Lambrama, observations for latitude and azimuth were made midway between Hacienda Matara and Caypi.
On October 4th we made our camp in newly fallen snow surrounded by beautiful glacial scenery. The next day on the high plateau, we passed sharp-crested glaciated peaks; a heavy thunder and hail storm broke out while I occupied the station at the pass, the storm continuing all the afternoon—a frequent occurrence. The camp was made 6 miles farther on, and the next morning I returned to finish the latter station. I succeeded in sketching the detailed topography just south of the pass, but shortly after noon, a furious storm arose similar to the one the day before, and made further topographic work impossible; to get connection farther on I patiently kept my eye to the eye-piece for more than an hour after the storm had started, and was fortunate to catch the station ahead in a single glimpse. I had a similar experience some days later at station 16,079, Antabamba Quadrangle, on the rim of the high-level puna, the storm preventing all topographic work and barely allowing a single moment in which to catch a dim sight of the signals ahead while I kept my eye steadily at the telescope to be ready for a favorable break in the heavy clouds and hail.
At Antabamba we got a new set of Indian carriers, who had orders to accompany us to Cotahuasi, the next sub-prefectura. Raimondi’s map indicates the distance between the two cities to be 35 miles, but although nothing definite was stated, we found out in Antabamba that the distance was considerably longer, and moreover that the entire route lay at a high altitude.
From the second day out of Antabamba until Huaynacotas was in sight in the Cotahuasi Canyon, a distance of 50 miles, the route lay at an altitude of from 16,000 to 17,630 feet, taking in 5 successive camps at an altitude from 15,500 to 17,000 feet; 12 successive stations had the following altitudes:
| 16,379 | feet | |
| 16,852 | " | |
| 17,104 | " | |
| 17,559 | " | |
| 17,675 | " | —highest station occupied. |
| 17,608 | " | |
| 17,633 | " | |
| 16,305 | " | |
| 17,630 | " | |
| 17,128 | " | |
| 16,794 | " | |
| 16,260 | " |
The occupation of these high stations necessitated a great deal of climbing, doubly hard in this rarefied air, and often on volcanoes with a surface consisting of bowlders and ash and in the face of violent hailstorms that made extremely difficult the task of connecting up observations at successive stations.
At Cotahuasi a new pack-train was organized, and on October 25th I ventured to return alone to the high altitudes in order to continue the topography at the station at 17,633 feet on the summit of the Maritime Cordillera. Dr. Erving was obliged to leave on October 18th and Professor Bowman left a week later in order to carry out his plans for a physiographic study of the coast between Camaná and Mollendo. Philippi Angulo, a native of Taurisma, a town above Cotahuasi, acted as majordomo on this journey. Knowing the trail and the camp sites, I was able to pick out the stations ahead myself, and made good progress, returning to Cotahuasi on October 29th, three or four days earlier than planned. From Cotahuasi to the coast I had the assistance of Mr. Watkins. The most trying part of the last section of high altitude country was the great Pampa Colorada, crowned by the snow-capped peaks of Solimana and Coropuna, reaching heights of 20,730 and 21,703 feet respectively. The passing of this pampa took seven days and we arrived at Chuquibamba on November 9th. Two circumstances made the work on this stretch peculiarly difficult—the scarcity of camping places and the high temperature in the middle of the day, which heated the rarefied air to a degree that made long-distance shots very strenuous work for the eyes. Although our base signals were stone piles higher than a man, I was often forced to keep my eye to the telescope for hours to catch a glimpse of the signals; lack of time did not allow me to stop the telescope work in the hottest part of the day.
The top of Coropuna was intersected from the four stations: 16,344, 15,545, 16,168, and 16,664 feet elevation, the intersections giving a very small triangular error. The elevation of Mount Coropuna’s high peak as computed from these 4 stations is:
| 21,696 feet | ||
| 21,746 " | ||
| 21,714 " | ||
| 21,657 " | ||
| Mean elevation 21,703 feet above sea level. | ||
The elevation of Coropuna as derived from these four stations has thus a mean error of 18 feet (method of least squares) while the elevation of each of the four stations as carried up from mean sea level through 25 stations—vertical angles being observed in both directions—has an estimated mean error of 30 feet. The result of this is a mean error of 35 feet in Coropuna’s elevation above mean sea level.
The latitude is 15° 31′ 00″ S.; the longitude is 72° 42′ 40″ W. of Greenwich, the checking of these two determinations giving a result unexpectedly close.
On November 11th azimuth and latitude observations were taken at Chuquibamba and two days later we arrived at Aplao in the bottom of the splendid Majes Valley. In the northern part of this valley I was prevented from doing any plane-table work in the afternoons of four successive days. A strong gale set in each noon raising a regular sandstorm, that made seeing almost impossible, and blowing with such a velocity that it was impossible to set up the plane-table.
From Hacienda Cantas to Camaná we had to pass the western desert for a distance of 45 miles. We were told that on the entire distance there was only one camping place. This was at Jaguey de Majes, where there was a brook with just enough water for the animals but no fodder. Thus we faced the necessity of carrying water for ten men and fodder for 14 animals in excess of the usual cargo; and we were unable to foretell how many days the topography over the hot desert would require.
Although plane-table work in the desert was impossible at all except in the earliest and latest hours of the day, we made regular progress. We camped three nights at Jaguey and arrived on the fourth day at Las Lomas.
The next morning, on November 23rd, at an elevation of 2178 feet near the crest of the Coast Range, we were repaid for two months of laborious work by a glorious view of the Pacific Ocean and of the city of Camaná with her olive gardens in the midst of the desert sand.
The next day I observed latitude and azimuth at Camaná and in the night my companion and assistant Mr. Watkins and I returned across the desert to the railroad at Vitor.
Conclusions
The planned methods were followed very closely. In two cases only the plane-table had to be oriented by the magnetic needle, the backsights not being obtainable because of the impossibility of locating the last station, passing Indians having removed the signals.
In one case only the distance between two stations had to be determined by graphic triangulation exclusively, the base signals having been destroyed. Otherwise graphic triangulation was used as a check on distances.
Vertical angles were always measured in both directions with the exception of the above-mentioned cases.
Observations for azimuth were always taken to the sun before and after noon. The direction used in the azimuth observation was also taken with the prismatic compass. The mean of the magnetic declination thus found is: East 8° 30′ plus.
Observations for latitude were taken to the sun by the method of circum-meridian altitudes, except at the town of Vilcabamba where star observations were taken.
As a matter of course, observations to the sun are not so exact as star observations, especially in low latitudes where one can expect to observe the near zenith. However, working in high altitudes for long periods, moving camp every day and often arriving at camp 2 to 4 hours after sunset, I found it essential to have undisturbed rest at night. It was beyond my capacity to spend an hour or two of the night in finding the meridian and in making the observation. Furthermore, the astronomic observations were to check the topography mainly, the latter being the most exact method with the outfit at hand.
The following table contains the comparisons between the latitude stations as located on the map and by observation:
| Map | Observation | |
| Camaná Quadrangle S | 16° 37′ 34″ | 16° 37′ 34″[66] |
| Coropuna, station 9,691S | 15° 48′ 30″ | (15° 51′ 44″) |
| Cotahuasi, " 12,588S | 15° 11′ 40″ | 15° 12′ 30″ |
| La Cumbre, " 16,852S | 14° 28′ 10″ | 14° 29′ 46″ |
| Lambrama, " 8,341S | 13° 43′ 18″ | 13° 43′ 14″ |
The other observations, with the exception of the one on the Coropuna Quadrangle, check probably as well as can be expected with the small and light outfit which we used, and under the exceptionally hard conditions of work. The observation on the Coropuna Quadrangle just south of Chuquibamba is, however, too much out. An explanation for this is that the meridian zenith distance was 1° 23′ 12″ only (in this case the exact formula was used in computing). Of course, an error or an accumulation of errors might have been made in the distances taken by the micrometer-alidade, but the first cause of error mentioned is the more probable, and this is indicated also by the fact that the location on the top of Mount Coropuna checks closely with the one determined in an entirely independent way by the railroad engineers.
For the cross-section map from Abancay to Camaná, the following statistics are desirable:
Micrometer traverse and graphic triangulation, with contours, field scale 1:90,000.
| Total time required, days | 40.5 |
| Average distance per days in miles | 7.5 |
| Average number of plane-table stations occupied per day | 1.5 |
| Average area per day in square miles | 38. |
| Located points per square mile | 0.25 |
| Approximate elevations in excess of above, per square mile | 0.25 |
| Highest station occupied, feet above sea level | 17,675. |
| Highest point located, feet above sea level | 21,703. |
APPENDIX B
Fossil Determinations
A few fossil collections were gathered in order that age determinations might be made. With the following identifications I have included a few fossils (I and II) collected by W. R. Rumbold and put into my hands in 1907. The Silurian is from a Bolivian locality south of La Paz but in the great belt of shales, slates, and schists which forms one of the oldest sedimentary series in the Eastern Andes of Peru as well as Bolivia. While no fossils were found in this series in Peru the rocks are provisionally referred to the Silurian. Fossil-bearing Carboniferous overlies them but no other indication of their age was obtained save their general position in the belt of schists already mentioned. I am indebted to Professor Charles Schuchert of Yale University for the following determinations.
I. Silurian
San Roque Mine, southwest slope of Santa Vela Cruz, Canton Ichocu, Province Inquisivi, Bolivia.
Sent by William R. Rumbold in 1907.
- Climacograptus?
- Pholidops trombetana Clarke?
- Chonetes striatellus (Dalman).
- Atrypa marginalis (Dalman)?
- Cœlospira n. sp.
- Ctenodonta, 2 or more species.
- Hyolithes.
- Klœdenia.
- Calymene?
- Dalmanites, a large species with a terminal tail spine.
- Acidaspis.
These fossils indicate unmistakably Silurian and probably Middle Silurian. As all are from blue-black shales, brachiopods are the rarer fossils, while bivalves and trilobites are the common forms. The faunal aspect does not suggest relationship with that of Brazil as described by J. M. Clarke and not at all with that of North America. I believe this is the first time that Silurian fossils have been discovered in the high Andes.
II. Lower Devonian
Near north end of Lake Titicaca.
- Leptocœlia flabellites (Conrad), very common.
- Atrypa reticularis (Linnæus)?
This is a part of the well-known and widely distributed Lower Devonian fauna of the southern hemisphere.
III. Upper Carboniferous
All of the Upper Carboniferous lots of fossils represent the well-known South American fauna first noted by d’Orbigny in 1842, and later added to by Orville Derby. The time represented is the equivalent of the Pennsylvanian of North America.
Huascatay between Pasaje and Huancarama.
- Crinoidal limestone.
- Trepostomata Bryozoa.
- Polypora. Common.
- Streptorhynchus hallianus Derby. Common.
- Chonetes glaber Geinitz. Rare.
- Productus humboldti d’Orb. Rare.
- " cora d’Orb. Rare.
- " chandlessii Derby.
- " sp. undet. Common.
- " sp. undet. "
- Spirifer condor d’Orb. Common.
- Hustedia mormoni (Marcou). Rare.
- Seminula argentea (Shepard). "
Pampaconas, Pampaconas valley near Vilcabamba.
- Lophophyllum?
- Rhombopora, etc.
- Productus.
- Camarophoria. Common.
- Spirifer condor d’Orb.
- Hustedia mormoni (Marcou).
- Euomphalus. Large form.
Pongo de Mainique. Extreme eastern edge of Peruvian Cordillera.
- Lophophyllum.
- Productus chandlessii Derby.
- " cora d’Orb.
- Orthotetes correanus (Derby).
- Spirifer condor d’Orb.
River bowlders and stones of Urubamba river, just beyond eastern edge of Cordillera at mouth of Ticumpinea river. (Detached and transported by stream action from the Upper Carboniferous at Pongo de Mainique.)
- Mostly Trepostomata Bryozoa.
- Many Productus spines.
- Productus cora d’Orb.
- Camarophoria. Same as at Pampaconas.
- Productus sp. undet.
Cotahuasi A.
- Lophophyllum.
- Productus peruvianus d’Orb.
- " sp. undet.
- Camarophoria.
- Pugnax near utah (Marcou).
- Seminula argentea (Shepard)?
Cotahuasi B.
- Productus cora d’Orb.
- " near semireticulatus (Martin).
IV. Comanchian or Lower Cretaceous
Near Chuquibambilla.
- Pecten near quadricostatus Sowerby.
- Undet. bivalves and gastropods.
- The echinid Laganum? colombianum d’Orb. A clypeasterid.
This Lower Cretaceous locality is evidently of the same horizon as that of Colombia illustrated by d’Orbigny in 1842 and described on pages 63-105.
APPENDIX C
KEY TO PLACE NAMES
A, C, E, H, J, L, M, O, P, Q, R, S, T, U, V, Y
Abancay, town, lat. 12° 35′, Figs. 20, 204.
Abra Tocate, pass, between Yavero and Urubamba valleys, leaving latter at Rosalina, (Fig. 8).
See also Fig. 55.
Anta, town, lat. 13° 30′, Fig. 20.
Antabamba, town, lat. 14° 20′, Figs. 20, 204.
Aplao, town, lat. 16°, Figs. 20, 204.
Apurimac, river, Fig. 20.
Arequipa, town, lat. 16° 30′, Fig. 66.
Arica, town, northern Chile, lat. 18° 30′.
Arma, river, tributary of Apurimac, lat. 13° 25′, (Fig. 20);
tributary of Ocoña, lat. 15° 30′, (Fig. 20).
Arma, village, lat. 13° 15′, Fig. 20.
See also Fig. 140.
Auquibamba, hacienda, lat. 13° 40′, Fig. 204.
Callao, town, lat. 12°, Fig. 66.
Camaná, town, lat. 16° 40′, Figs. 20, 66, 204.
Camisea, river, tributary of Urubamba entering from right, lat. 11° 15′.
Camp 13, lat. 14° 30′.
Cantas, hacienda, lat. 16° 15′, Fig. 204.
Caraveli, town, lat. 16°, Fig. 66.
Catacaos, town, lat. 5° 30′, Fig. 66.
Caylloma, town and mines, lat. 15° 30′, Fig. 66.
Caypi, village, lat. 13° 45′.
Central Ranges, lat. 14°, Fig. 20.
See also Fig. 157.
Cerro Azul, town, lat. 13°, Fig. 66.
Chachani, mt., overlooking Arequipa, lat. 16° 30′, (Fig. 66).
Chaupimayu, river, tributary of Urubamba entering at Sahuayaco, q.v.
Chili, river, tributary of Vitor River, lat. 16° 30′, (Fig. 66).
Chinche, hacienda, Urubamba Valley above Santa Ana, lat. 13°, (Fig. 20).
Chira, river, lat. 5°, Fig. 66.
Choclococha, lake, lat. 13° 30′, Figs. 66, 68.
Choqquequirau, ruins, canyon of Apurimac above junction of Pachachaca River, lat. 13° 25′, (Fig. 20).
Choquetira, village, lat. 13° 20′, Fig. 20.
See also Fig. 136.
Chosica, village, lat. 12°, Fig. 66.
Chuquibamba, town, lat. 15° 50′, Figs. 20, 204.
Chuquibambilla, village, lat. 14°, Figs. 20, 204.
Chuquito, pass, Cordillera Vilcapampa between Arma and Vilcabamba valleys, lat. 13° 10′, (Fig. 20).
See also Fig. 139.
Coast Range, Figs. 66, 204.
Cochabamba, city, Bolivia, lat. 17° 20′, long. 66° 20′.
Colorada, pampa, lat. 15° 30′, Fig. 204.
Colpani, village, lower end of Canyon of Torontoy (Urubamba River), lat. 13° 10′. See Fig. 158.
Copacavana, village, Bolivia, lat. 16° 10′, long. 69° 10′.
Coribeni, river, lat. 12° 40′, Fig. 8.
Coropuna, mt., lat. 15° 30′, Figs. 20, 204.
Corralpata, village, Apurimac Valley near Incahuasi.
Cosos, village, lat. 16°, Fig. 204.
Cotabambas, town, Apurimac Valley, lat. 13° 45′, (Fig. 20).
Cotahuasi, town, lat. 15° 10′, Figs. 20, 204.
Cuzco, city, lat. 13° 30′, Fig. 20.
Echarati, hacienda, on the Urubamba River between Santa Ana and Rosalina, lat. 12° 40′.
See inset map, Fig. 8, and also Fig. 54.
Huadquiña, hacienda, Urubamba River above junction with Vilcabamba, lat. 13° 10′, (Fig. 20).
See also Fig. 158.
Huadquirca, village, lat. 14° 15′, Figs. 20, 204.
Huaipo, lake, north of Anta, lat. 13° 25′, (Fig. 20).
Huambo, village, left bank Pachachaca River between Huancarama and Pasaje, lat. 13° 35′, (Fig. 20).
Huancarama, town, lat. 13° 40′, Fig. 20.
Huancarqui, village, lat. 16° 5′, Fig. 204.
Huascatay, village, left bank of Apurimac above Pasaje, lat. 13° 30′, (Fig. 20).
Huaynacotas, village, lat. 15° 10′, Fig. 204.
Huichihua, village, lat. 14° 10′, Fig. 204.
(Tablazo de) Ica, plateau, lat. 14°-15° 30′, Fig. 66.
Ica, town, lat. 14°, Figs. 66, 67.
Incahuasi, village, lat. 13° 20′, Fig. 20.
Iquique, town, northern Chile, lat. 20° 15′.
(Pampa de) Islay, south of Vitor River, (Fig. 66).
Jaguey, village, Pampa de Sihuas, q.v.
La Joya, pampa, station on Mollendo-Puno R.R., 16° 40′, (Fig. 66).
Lambrama, village, lat. 12° 50′, Fig. 20.
Lima, city, lat. 12°, Fig. 66.
Machu Picchu, ruins, gorge of Torontoy, q.v., lat. 13° 10′.
Majes, river, Fig. 204.
Manugali, river, tributary of Urubamba entering from left above Puviriari River, lat. 12° 20′, (Fig. 8).
Maritime Cordillera, Fig. 204.
Matara, village, lat. 14° 20′, Fig. 204.
(El) Misti, mt., lat. 16° 30′, Fig. 66.
Mollendo, town, lat. 17°, Fig. 66.
Moquegua, town, lat. 17°, Fig. 66.
Morococha, mines, lat. 11° 45′, Fig. 66.
Mulanquiato, settlement, lat. 12° 10′, Fig. 8.
Occobamba, river, uniting with Yanatili, q.v.
Ocoña, river, lat. 15°-16° 30′, Figs. 20, 66.
Ollantaytambo, village. Urubamba River below Urubamba town, lat. 13° 15′, (Fig. 20), and see inset map, Fig. 8.
Pabellon, hacienda, Urubamba River above Rosalina, (Fig. 20).
See also Fig. 55.
Pacasmayo, town, lat. 7° 30′, Fig. 66.
Pachatusca (Pachatusun), mt., overlooking Cuzco to northeast, lat. 13° 30′.
Pachitea, river, tributary of Ucayali entering from left, lat. 8° 50′.
Paita, town, lat. 5°, Fig. 66.
Pampacolea, village, south of Coropuna, q.v.
Pampaconas, river, known in lower course as Cosireni, tributary of Urubamba River, (Fig. 8).
Source in Cordillera Vilcapampa west of Vilcabamba.
Pampas, river, tributary of Apurimac entering from left, lat. 13° 20′.
Panta, mt., Cordillera Vilcapampa, northwest of Arma, lat. 13° 15′, (Fig. 20).
See also Fig. 136.
Panticalla, pass, Urubamba Valley above Torontoy, lat. 13° 10′.
Pasaje, hacienda and ferry, lat. 13° 30′, Fig. 20.
Paucartambo (Yavero), river, q.v.
Paucartambo, town, head of Paucartambo (Yavero) River, lat. 13° 20′, long. 71° 40′. Inset map, Fig. 8.
Pichu-Pichu, mt., overlooking Arequipa, lat. 16°, (Fig. 66).
Pilcopata, river, tributary of Upper Madre de Dios east of Paucartambo, lat. 13°.
Piñi-piñi, river, tributary of Upper Madre de Dios east of Paucartambo, lat. 13°.
Pisco, town, lat. 14°, Fig. 66.
Piura, river, lat. 5°-6°, Fig. 66.
Piura, town, lat. 5° 30′, Fig. 66.
Pomareni, river, lat. 12°, Fig. 8.
Pongo de Mainique, rapids, lat. 12°, Fig. 8.
Pucamoco, hacienda, Urubamba River, between Santa Ana and Rosalina, (Fig. 20).
Puquiura, village, lat. 13° 5′, Fig. 20.
See also Fig. 158. Distinguish Puqura in Anta basin near Cuzco.
Puqura, village, Anta basin, east of Anta, lat. 13° 30′, (Fig. 20).
Quilca, town, lat. 16° 40′, Fig. 66.
Quillagua, village, northern Chile, lat. 21° 30′, long. 69° 35′.
Rosalina, settlement, lat. 12° 35′, Fig. 8.
See also Fig. 20.
Sahuayaco, hacienda, Urubamba Valley above Rosalina, (Fig. 20).
See also Fig. 55.
Salamanca, town, lat. 15° 30′, Fig. 20.
Salaverry, town, lat. 8°, Fig. 66.
Salcantay, mt., lat. 13° 20′, Fig. 20.
San Miguel, bridge, canyon of Torontoy near Machu Picchu, lat. 13° 10′.
Santa Ana, hacienda, lat. 12° 50′, Fig. 20.
Santa Ana, river, name applied to the Urubamba in the region about hacienda Santa Ana.
Santa Lucia, mines, lat. 16°, Fig. 66.
Santo Anato, hacienda, La Sama’s hut, 12° 35′, Fig. 8.
Sihuas, Pampa de, lat. 16° 30′, Fig. 204.
Sillilica, Cordillera, east of Iquique, northern Chile.
Sintulini, rapids of Urubamba River above junction of Pomareni, lat. 12° 10′, (Fig. 8).
Sirialo, river, lat. 12° 40′, Fig. 8.
Soiroccocha, mt., Cordillera Vilcapampa north of Arma, lat. 13° 15′, (Fig. 20).
Solimana, mt., lat. 15° 20′, Fig. 204.
Soray, mt., Cordillera Vilcapampa, southeast of Mt. Salcantay, lat. 13° 20′, (Fig. 20).
Sotospampa, village, near Lambrama, lat. 13° 50′, (Fig. 204).
Sullana, town, Chira River, lat. 5°, (Fig. 66).
Taurisma, village, lat. 15° 10′, Fig. 204.
Ticumpinea, river, tributary of Urubamba entering from right below Pongo de Mainique, lat. 11° 50′, (Fig. 8).
Timpia, river, tributary of Urubamba entering from right, lat. 11° 45′.
Tono, river, tributary of Upper Madre de Dios, east of Paucartambo, lat. 13°.
Torontoy, canyon of the Urubamba between the villages of Torontoy and Colpani, lat. 13° 10′-13° 15′.
Torontoy, village at the head of the canyon of the same name, lat. 13° 15′.
See inset map, Fig. 8.
Tumbez, town, lat. 4° 30′, Fig. 66.
Tunari, Cerro de, mt., northwest of Cochabamba, q.v.
Urubamba, river, Fig. 20.
Urubamba, town, lat. 13° 20′, Fig. 20.
Vilcabamba, river, tributary of Urubamba River entering from left above Santa Ana, lat. 13°, Fig. 8.
See also Fig. 158.
Vilcabamba, village, lat. 13° 5′, Fig. 20.
See also Fig. 158.
Vilcanota, Cordillera, southern Peru.
Vilcanota, river, name applied to Urubamba above lat. of Cuzco, 13° 30′, (Fig. 20).
Vilcapampa, Cordillera, lat. 13° 20′, Fig. 20.
Vilque, town, southern Peru, lat. 15° 50′, long. 70° 30′.
Vitor, pampa, lat. 16° 30′, Fig. 66.
Vitor, river, Fig. 66.
Yanahuara, pass, between Urubamba and Yanatili valleys, lat. 13° 10′.
Yanatili, river, tributary of Urubamba entering from right above Rosalina, (Fig. 20).
See also Fig. 65.
Yavero (Paucartambo), river, tributary of Urubamba entering from right, lat. 12° 10′, Fig. 8.
Yavero, settlement, at junction of Yavero and Urubamba rivers, lat. 12° 10′, Fig. 8.
Yunguyo, town, southern Peru, lat. 16° 20′, long. 69° 10′.
Yuyato, river, lat. 12° 5′, Fig. 8.