Fig. 140—Glacial sculpture in the heart of the Cordillera Vilcapampa. In places the topography has so high a relief that the glaciers seem almost to overhang the valleys. See Figs. 96 and 179 for photographs.
The topography conspired to increase this contrast. In place of many streams, direct descents, a dispersion of snow in many valleys, as on the east, the western slopes had indirect descents, gentler valley profiles, and that higher degree of concentration of drainage which naturally goes with topographic maturity. For example, there is nothing in the east to compare with the big spurless valley near the pass above Arma. The side walls were so extensively trimmed that the valley was turned into a trough. The floor was smoothed and deepened and all the tributary glaciers were either left high up on the bordering slopes or entered the main valley with very steep profiles; their lateral and terminal moraines now hang in festoons on the steep side walls. Moreover, the range crest is trimmed from the west so that the serrate skyline is a feature rarely seen from eastern viewpoints. This may not hold true for more than a small part of the Cordillera. It was probably emphasized here less by the contrasts already noted than by the geologic structure. The eastward-flowing glaciers descended over dip slopes on highly inclined sandstones, as at Pampaconas. Those flowing westward worked either in a jointed granite or on the outcropping edges of the sandstones, where the quarrying process known as glacial plucking permitted the development of excessively steep slopes.
There are few glacial steps in the eastern valleys. The western valleys have a marvelous display of this striking glacial feature. The accompanying hachure maps show them so well that little description is needed. They are from 50 to 200 feet high. Each one has a lake at its foot into which the divided stream trickles over charming waterfalls. All of them are clearly associated with a change in the volume of the glacier that carved the valley. Wherever a tributary glacier entered, or the side slopes increased notably in area, a step was formed. By retreat some of them became divided, for the process once begun would push the step far up valley after the manner of an extinguishing waterfall.
The retreat of the steps, the abrasion of the rock, and the sapping of the cirques at the valley heads excavated the upper valleys so deeply that they are nearly all, as W. D. Johnson has put it, “down at the heel.” Thus, above Arma, one plunges suddenly from the smooth, grassy glades of the strongly glaciated valley head down over the outer slopes of the lowermost terminal moraine to the steep lower valley. Above the moraine are fine pastures, in the steep valley below are thickets and rocky defiles. There are long quiet reaches in the streams of the glaciated valley heads besides pretty lakes and marshes. Below, the stream is swift, almost torrential. Arma itself is built upon alluvial deposits of glacial origin. A mile farther down the valley is constricted and steep-walled—really a canyon.
Though the glaciers have retreated to the summit region, they are by no means nearing extinction. The clear blue ice of the glacier descending from Mt. Soiroccocha in the Arma Valley seems almost to hang over the precipitous valley border. In curious contrast to its suggestion of cold and storm is the patch of dark green woodland which extends right up to its border. An earthquake might easily cause the glacier to invade the woodland. Some of the glaciers between Choquetira and Arma rest on terminal moraines whose distal faces are from 200 to 300 feet high. The ice descending southeasterly from Panta Mt. is a good illustration. Earlier positions of the ice front are marked by equally large moraines. The one nearest that engaged by the living glacier confines a large lake that discharges through a gap in the moraine and over a waterfall to the marshy floor of the valley.
Retreat has gone so far, however, that there are only a few large glacier systems. Most of the tributaries have withdrawn toward their snowfields. In place of the twenty distinct glaciers now lying between the pass and the terminal moraine below Choquetira, there was in glacial times one great glacier with twenty minor tributaries. The cirques now partly filled with damp snow must then have been overflowing with dry snow above and ice below. Some of the glaciers were over a thousand feet thick; a few were nearly two thousand feet thick, and the cirques that fed them held snow and ice at least a half mile deep. Such a remarkably complete set of glacial features only 700 miles from the equator is striking evidence of the moist climate on the windward eastern part of the great Andean Cordillera, of the universal change in climate in the glacial period, and of the powerful dominating effects of ice erosion in this region of unsurpassed Alpine relief.
THE VILCAPAMPA BATHOLITH AND ITS TOPOGRAPHIC EFFECTS
Fig. 141—Composite geologic section on the northeastern border of the Cordillera Vilcapampa, in the vicinity of Pampaconas, to show the deformative effects of the granite intrusion. There is a limited amount of limestone near the border of the Cordillera. Both limestone and sandstone are Carboniferous. See Appendix B. See also Figs. 142 and 146. The section is about 15 miles long.
The main axis of the Cordillera Vilcapampa consists of granite in the form of a batholith between crystalline schists on the one hand (southwest), and Carboniferous limestones and sandstones and Silurian shales and slates on the other (northeast). It is not a domal uplift in the region in which it was observed in 1911, but an axial intrusion, in places restricted to a narrow belt not more than a score of miles across. As we should expect from the variable nature of the invaded material, the granite belt is not uniform in width nor in the character of its marginal features. In places the intrusion has produced strikingly little alteration of the country rock; in other localities the granite has been injected into the original material in so intimate a manner as almost completely to alter it, and to give rise to a very broad zone of highly metamorphosed rock. Furthermore, branches were developed so that here and there tributary belts of granite extend from the main mass to a distance of many miles. Outlying batholiths occur whose common petrographic character and similar manner of occurrence leave little doubt that they are related abyssally to a common plutonic mass.
The Vilcapampa batholith has two highly contrasted borders, whether we consider the degree of metamorphism of the country rock, the definition of the border, or the resulting topographic forms. On the northeastern ridge at Colpani the contact is so sharp that the outstretched arms in some places embrace typical granite on the one hand and almost unaltered shales and slates on the other. Inclusions or xenoliths of shale are common, however, ten and fifteen miles distant, though they are prominent features in a belt only a few miles wide. The lack of more intense contact effects is a little remarkable in view of the altered character of the inclusions, all of which are crystalline in contrast to the fissile shales from which they are chiefly derived. Inclusions within a few inches of the border fall into a separate class, since they show in general but trifling alteration and preserve their original cleavage plains. It appears that the depth of the intrusion must have been relatively slight or the intrusion sudden, or both shallow and sudden, conditions which produce a narrow zone of metamorphosed material and a sharp contact.
Fig. 142—The deformative effects of the Vilcapampa intrusion on the northeastern border of the Cordillera. The deformed strata are heavy-bedded sandstones and shales and the igneous rocks are chiefly granites with bordering porphyries. Looking northwest near Puquiura. For conditions near Pampaconas, looking in the opposite direction, see Fig. 141. For conditions on the other side of the Cordillera, see Fig. 146.
The relation between shale and granite at Colpani is shown in 143 . Projections of granite extend several feet into the shale and slate and generally end in blunt barbs or knobs. In a few places there is an intimate mixture of irregular slivers and blocks of crystallized sediments in a granitic groundmass, with sharp lines of demarcation between igneous and included material. The contact is vertical for at least several miles. It is probable that other localities on the contact exhibit much greater modification and invasion of the weak shales and slates, but at Colpani the phenomena are both simple and restricted in development.
Fig. 143—Relation of granite intrusion to schist on the northeastern border of the Vilcapampa batholith near the bridge of Colpani, lower end of the granite Canyon of Torontoy. The sections are from 15 to 25 feet high and represent conditions at different levels along the well-defined contact.
The highly mineralized character of the bordering sedimentary strata, and the presence of numbers of complementary dikes, nearly identical in character to those in the parent granite now exposed by erosion over a broad belt roughly parallel to the contact, supplies a basis for the inference that the granite may underlie the former at a slight depth, or may have had far greater metamorphic effects upon its sedimentary roof than the intruded granite has had upon its sedimentary rim.
The physiographic features of the contact belt are of special interest. No available physiographic interpretation of the topography of a batholith includes a discussion of those topographic and drainage features that are related to the lithologic character of the intruded rock, the manner of its intrusion, or the depth of erosion since intrusion. Yet each one of these factors has a distinct topographic effect. We shall, therefore, turn aside for a moment from the detailed discussion of the Vilcapampa region to an examination of several physiographic principles and then return to the main theme for applications.
It is recognized that igneous intrusions are of many varieties and that even batholithic invasions may take place in rather widely different ways. Highly heated magmas deeply buried beneath the earth’s surface produce maximum contact effects, those nearer the surface may force the strata apart without extreme lithologic alterations of the displaced beds, while through the stoping process a sedimentary cover may be largely absorbed and the magmas may even break forth at the surface as in ordinary vulcanism. If the sedimentary beds have great vertical variation in resistance, in attitude, and in composition, there may be afforded an opportunity for the display of quite different effects at different levels along a given contact, so that a great variety of physical conditions will be passed by the descending levels of erosion. At one place erosion may have exposed only the summit of the batholith, at another the associated dikes and sheets and ramifying branches may be exposed as in the zone of fracture, at a third point the original zone of flowage may be reached with characteristic marginal schistosity, while at still greater depths there may be uncovered a highly metamorphosed rim of resistant sedimentary rock.
The mere enumeration of these variable structural features is sufficient to show how variable we should expect the associated land forms to be. Were the forms of small extent, or had they but slight distinction upon comparison with other erosional effects, they would be of little concern. They are, on the contrary, very extensively developed; they affect large numbers of lofty mountain ranges besides still larger areas of old land masses subjected to extensive and deep erosion, thus laying bare many batholiths long concealed by a thick sedimentary roof.
The differences between intruded and country rock dependent upon these diversified conditions of occurrence are increased or diminished according to the history of the region after batholithic invasion takes place. Regional metamorphism may subsequently induce new structures or minimize the effects of the old. Joint systems may be developed, the planes widely spaced in one group of rocks giving rise to monolithic masses very resistant to the agents of weathering, while those of an adjacent group may be so closely spaced as greatly to hasten the rate of denudation. There may be developed so great a degree of schistosity in one rock as to give rise (with vigorous erosion) to a serrate topography; on the other hand the forms developed on the rocks of a batholith may be massive and coarse-textured.
To these diversifying conditions may be added many others involving a large part of the field of dynamic geology. It will perhaps suffice to mention two others: the stage of erosion and the special features related to climate. If a given intrusion has been accompanied by an important amount of uplift or marginal compression, vigorous erosion may follow, whereupon a chance will be offered for the development of the greatest contrast in the degree of boldness of topographic forms developed upon rocks of unequal resistance. Ultimately these contrasts will diminish in intensity, as in the case of all regional differences of relief, with progress toward the end of the normal cycle of erosion. If peneplanation ensue, only feeble topographic differences may mark the line of contact which was once a prominent topographic feature. With reference to the effects of climate it may be said simply that a granite core of batholithic origin may extend above the snowline or above timber line or into the timbered belt, whereas the invaded rock may occur largely below these levels with obvious differences in both the rate and the kind of erosion affecting the intruded mass.
Fig. 144—Cliffed canyon wall in the Urubamba Valley between Huadquiña and Torontoy. There is a descent of nearly 2,000 feet shown in the photograph and it is developed almost wholly along successive joint planes. |
Fig. 145—Another aspect of the canyon wall of 144 . The almost sheer descents are in contrast with the cliff and platform type of topography characteristic of the Grand Canyon of Colorado. |
If we apply the foregoing considerations to the Cordillera Vilcapampa, we shall find some striking illustrations of the principles involved. The invasion of the granite was accompanied by moderate absorption of the displaced rock, and more especially by the marginal pushing aside of the sedimentary rim. The immediate effect must have been to give both intruded rock and country rock greater height and marked ruggedness. There followed a period of regional compression and torsion, and the development of widespread joint systems with strikingly regular features. In the Silurian shales and slates these joints are closely spaced; in the granites they are in many places twenty to thirty feet apart. The shales, therefore, offer many more points of attack and have weathered down into a smooth-contoured topography boldly overlooked along the contact by walls and peaks of granite. In some cases a canyon wall a mile high is developed entirely on two or three joint planes inclined at an angle no greater than 15°. The effect in the granite is to give a marked boldness of relief, nowhere more strikingly exhibited than at Huadquiña, below Colpani, where the foot-hill slopes developed on shales and slates suddenly become moderate. The river flows from a steep and all but uninhabited canyon into a broad valley whose slopes are dotted with the terraced chacras, or farms, of the mountain Indians.
The Torontoy granite is also homogeneous while the shales and slates together with their more arenaceous associates occur in alternating belts, a diversity which increases the points of attack and the complexity of the forms. Tending toward the same result is the greater hardness of the granite. The tendency of the granite to develop bold forms is accelerated in lofty valleys disposed about snow-clad peaks, where glaciers of great size once existed, and where small glaciers still linger. The plucking action of ice has an excellent chance for expression, since the granite may be quarried cleanly without the production of a large amount of spoil which would load the ice and diminish the intensity of its plucking action.
As a whole the Central Andes passed through a cycle of erosion in late Tertiary time which was interrupted by uplift after the general surface had been reduced to a condition of topographic maturity. Upon the granites mature slopes are not developed except under special conditions (1) of elevation as in the small batholith above Chuquibambilla, and (2) where the granite is itself bordered by resistant schists which have upheld the surface over a broad transitional belt. Elsewhere the granite is marked by exceedingly rugged forms: deep steep-walled canyons, precipitous cirques, matterhorns, and bold and extended escarpments of erosion. In the shale belt the trails run from valley to valley in every direction without special difficulties, but in the granite they follow the rivers closely or cross the axis of the range by carefully selected routes which generally reach the limit of perpetual snow. Added interest attaches to these bold topographic forms because of the ruins now found along the canyon walls, as at Torontoy, or high up on the summit of a precipitous spur, as at Machu Picchu near the bridge of San Miguel.
The Vilcapampa batholith is bordered on the southwest by a series of ancient schists with which the granite sustains quite different relations. No sharp dividing line is visible, the granite extending along the planes of foliation for such long distances as in places to appear almost interbedded with the schists. The relation is all the more striking in view of the trifling intrusions effected in the case of the seemingly much weaker shales on the opposite contact. Nor is the metamorphism of the invaded rock limited to simple intrusion. For several miles beyond the zone of intenser effects the schists have been enriched with quartz to such an extent that their original darker color has been changed to light gray or dull white. At a distance they may even appear as homogeneous and light-colored as the granite. At distant points the schists assume a darker hue and take on the characters of a rather typical mica schist.
It is probable that the Vilcapampa intrusion is one of a family of batholiths which further study may show to extend over a much larger territory. The trail west of Abancay was followed quite closely and accidentally crosses two small batholiths of peculiar interest. Their limits were not closely followed out, but were accurately determined at a number of points and the remaining portion of the contact inferred from the topography. In the case of the larger area there may indeed be a connection westward with a larger mass which probably constitutes the ranges distant some five to ten miles from the line of traverse.
Fig. 146—Deformative effects on limestone strata of the granite intrusion on the southwestern border of the Vilcapampa batholith above Chuquibambilla. Fig. 147 is on the same border of the batholith several miles farther northwest. The granite mass on the right is a small outlier of the main batholith looking south. The limestone is Cretaceous. See Appendix C for locations.
These smaller intrusions are remarkable in that they appear to have been attended by little alteration of either invading or invaded rock, though the granites were observed to become distinctly more acid in the contact zone. Space was made for them by displacing the sedimentary cover and by a marked shortening of the sedimentary rim through such structures as overthrust faults and folds. The contact is observable in a highly metamorphosed belt about twenty feet wide, and for several hundred feet more the granite has absorbed the limestone in small amounts with the production of new minerals and the development of a distinctly lighter color. The deformative effects of the batholithic invasion are shown in their gross details in Figs. 141, 142, and 146; the finer details of structure are represented in 147 , which is drawn from a measured outcrop above Chuquibambilla.
It will be seen that we have here more than a mere crinkling, such as the mica schists of the Cordillera Vilcapampa display. The diversified sedimentary series is folded and faulted on a large scale with broad structural undulations visible for miles along the abrupt valley walls. Here and there, however, the strata become weaker generally through the thinning of the beds and the more rapid alternation of hard and soft layers, and for short distances they have absorbed notable amounts of the stresses induced by the igneous intrusions. In such places not only the structure but the composition of the rock shows the effects of the intrusion. Certain shales in the section are carbonaceous and in all observed cases the organic matter has been transformed to anthracite, a condition generally associated with a certain amount of minute mashing and a cementation of both limestone and sandstone.
Fig. 147—Overthrust folds in detail on the southwestern border of the Vilcapampa batholith near Chuquibambilla. The section is fifteen feet high. Elevation, 13,100 feet (4,000 m.). For comparison with the structural effects of the Vilcapampa intrusion on the northeast see Fig. 142.
The granite becomes notably darker on approach to the northeastern contact near Colpani; the proportion of ferro-magnesian minerals in some cases is so large as to give a distinctly black color in sharp contrast to the nearly white granite typical of the central portion of the mass. Large masses of shale foundered in the invading magma, and upon fusion gave rise to huge black masses impregnated with quartz and in places smeared or injected with granite magma. Everywhere the granite is marked by numbers of black masses which appear at first sight to be aggregations of dark minerals normal to the granite and due to differentiation processes at the time of crystallization. It is, however, noteworthy that these increase rapidly in number on approach to the contact, until in the last half-mile they appear to grade into the shale inclusions. It may, therefore, be doubted that they are aggregations. From their universal distribution, their uniform character, and their marked increase in numbers on approach to lateral contacts, it may reasonably be inferred that they represent foundered masses of country rock. Those distant from present contacts are in almost all cases from a few inches to a foot in diameter, while on approach to lateral contacts they are in places ten to twenty feet in width, as if the smaller areas represented the last remnants of large inclusions engulfed in the magma near the upper or roof contact. They are so thoroughly injected with silica and also with typical granite magma as to make their reference to the country rock less secure on petrographical than on purely distributional grounds.
A parallel line of evidence relates to the distribution of complementary dikes throughout the granite. In the main mass of the batholith the dikes are rather evenly distributed as to kind with a slight preponderance of the dark-colored group. Near the contact, however, aplitic dikes cease altogether and great numbers of melanocratic dikes appear. It may be inferred that we have in this pronounced condition suggestions of strong influence upon the final processes of invasion and cooling of the granite magma, on the part of the country rock detached and absorbed by the invading mass. It might be supposed that the indicated change in the character of the complementary dikes could be ascribed to possible differentiation of the granite magma whereby a darker facies would be developed toward the Colpani contact. It has, however, been pointed out already that the darkening of the granite in this direction is intimately related to a marked increase in the number of inclusions, leaving little doubt that the thorough digestion of the smaller masses of detached shales is responsible for the marked increase in the number and variety of the ferro-magnesian and special contact minerals.
Upon the southwestern border of the batholith the number of aplitic dikes greatly increases. They form prominent features, not only of the granite, but also of the schists, adding greatly to the strong contrast between the schist of the border zone and that outside the zone of metamorphism. In places in the border schists, these are so numerous that one may count up to twenty in a single view, and they range in size from a few inches to ten or fifteen feet. The greater fissility of the schists as contrasted with the shales on the opposite or eastern margin of the batholith caused them to be relatively much more passive in relation to the granite magma. They were not so much torn off and incorporated in the magma, as they were thoroughly injected and metamorphosed. Added to this is the fact that they are petrographically more closely allied to the granite than are the shales upon the northeastern contact.
CHAPTER XIV
THE COASTAL TERRACES
ALONG the entire coast of Peru are upraised and dissected terraces of marine origin. They extend from sea level to 1,500 feet above it, and are best displayed north of Mollendo and in the desert south of Payta. The following discussion relates to that portion of the coast between Mollendo and Camaná.
At the time of the development of the coastal terraces the land was in a state of temporary equilibrium, for the terraces were cut to a mature stage as indicated by the following facts: (1) the terraces have great width—from one to five and more miles; (2) their inner border is straight, or, where curves exist, they are broad and regular; (3) the terrace tops are planed off smoothly so that they now have an even gradient and an almost total absence of rock stacks or unreduced spurs; (4) the mature slopes of the Coast Range, strikingly uniform in gradient and stage of development (Fig. 148), are perfectly organized with respect to the inner edge of the terrace. They descend gradually to the terrace margin, showing that they were graded with respect to sea level when the sea stood at the inner edge of the highest terrace.
From the composition and even distribution of the thick-bedded Tertiary deposits of the desert east of the Coast Range, it is concluded that the precipitation of Tertiary time was greater than that of today (see p. 261). Therefore, if the present major streams reach the sea, it may also be concluded that those of an earlier period reached the sea, provided the topography indicates the perfect adjustment of streams to structure. Lacustrine sediments are absent throughout the Tertiary section. Such through-flowing streams, discharging on a stable coast, would also have mature valleys as a consequence of long uninterrupted erosion at a fixed level. The Majes river must have cut through the Coast Range at Camaná then as now. Likewise the Vitor at Quilca must have cut straight across the Coast Range. An examination of the surface leading down from the Coast Range to the upper edge of these valleys fully confirms this deduction. Flowing and well-graded slopes descend to the brink of the inner valley in each case, where they give way to the gorge walls that continue the descent to the valley floor.
Confirmatory evidence is found in the wide Majes Valley at Cantas and Aplao. (See the Aplao Quadrangle for details.) Though the observer is first impressed with the depth of the valley, its width is more impressive still. It is also clear that two periods of erosion are represented on its walls. Above Aplao the valley walls swing off to the west in a great embayment quite inexplicable on structural grounds; in fact the floor of the embayment is developed across the structure, which is here more disordered than usual. The same is true below Cantas, as seen from the trail, which drops over two scarps to get to the valley floor. The upper, widely opened valley is correlated with the latter part of the period in which were formed the mature terraces of the coast and the mature slopes bordering the larger valleys where they cross the Coast Range.
After its mature development the well-graded marine terrace was upraised and dissected. The deepest and broadest incisions in it were made where the largest streams crossed it. Shallower and narrower valleys were formed where the smaller streams that headed in the Coast Range flowed across it. Their depth and breadth was in general proportional to the height of that part of the Coast Range in which their headwaters lay and to the size of their catchment basins.
When the dissection of the terrace had progressed to the point where about one-third of it had been destroyed, there came depression and the deposition of Pliocene or early Pleistocene sands, gravels, and local clay beds. Everywhere the valleys were partly or wholly filled and over broad stretches, as in the vicinity of stream mouths and upon lower portions of the terrace, extensive deposits were laid down. The largest deposits lie several hours’ ride south of Camaná, where locally they attain a thickness of several hundred feet. Their upper surface was well graded and they show a prolonged period of deposition in which the former coastal terrace was all but concealed.
Fig. 148—The Coast Range between Mollendo and Arequipa at the end of June, 1911. There is practically no grass and only a few dry shrubs. The fine network over the hill slopes is composed of interlacing cattle tracks. The cattle roam over these hills after the rains which come at long intervals. (See page 141 for description of the rains and the transformations they effect. For example, in October, 1911, these hills were covered with grass.)
Fig. 149—The great marine terrace at Mollendo. See Fig. 150 for profile.
The uplift of the coast terrace and its subsequent dissection bring the physical history down to the present. The uplift was not uniform; three notches in the terrace show more faintly upon the granite-gneiss where the buried rock terrace has been swept clean again, more strongly upon the softer superimposed sands. They lie below the 700-foot contour and are insignificant in appearance beside the slopes of the Coast Range or the ragged bluff of the present coast.
The effect of the last uplift of the coast was to impel the Majes River again to cut down its lower course nearly to sea level. The Pliocene terrace deposits are here entirely removed over an area several leagues wide. In their place an extensive delta and alluvial fan have been formed. At first the river undoubtedly cut down to base level at its mouth and deposited the cut material on the sea floor, now shoal, for a considerable distance from shore. We should still find the river in that position had other agents not intervened. But in the Pleistocene a great quantity of waste was swept into the Majes Valley, whereupon aggradation began; and in the middle and lower valley it has continued down to the present.
Fig. 150—Profile of the coastal terraces at Mollendo. At 1, in a tributary gorge, fossiliferous clay occurs at 800 feet elevation above the sea. At 2 is a characteristic change of profile marking a drop from a higher to a lower terrace. On the extreme left is the highest terrace, just under 1,500 feet (460 m.).
Figs. 151-154—These four diagrams represent the physical history and the corresponding physiographic development of the coastal region of Peru between Camaná and Mollendo. The sedimentary beds in the background of the first diagram are hypothetical and are supposed to correspond to the quartzites of the Majes Valley at Aplao.
The effect has been not only the general aggradation of the valley floor, but also the development of a combined delta and superimposed alluvial fan at the valley mouth. The seaward extension of the delta has been hastened by the gradation of the shore between the bounding headlands, thus giving rise to marine marshes in which every particle of contributed waste is firmly held. The plain of Camaná, therefore, includes parts of each of the following: a delta, a superposed alluvial fan, a salt-water marsh, a fresh-water marsh, a series of beaches, small amounts of piedmont fringe at the foot of Pliocene deposits once trimmed by the river and by waves, and extensive tracts of indefinite fill. (See the Camaná Quadrangle for details.)
With the coastal conditions now before us it will be possible to attempt a correlation between the erosion features and the deposits of the coast and those of the interior. An understanding of the comparisons will be facilitated by the use of diagrams, Figs. 151-154, and by a series of concise summary statements. From the relations of the figure it appears that:
1. The Tertiary deposits bordering the Majes Valley east of the Coast Range were in process of deposition when the sea planed the coastal terrace (Fig. 151).
2. A broad mature marine terrace without stacks or sharply alternating spurs and reëntrants (though the rock is a very resistant granite) is correlated with the mature grades of the Coast Range, with which they are integrated and with the mature profiles of the main Cordillera.
3. Such a high degree of topographic organization requires the dissection in the late stages of the erosion cycle of at least the inner or eastern border of the piedmont deposits of the desert, largely accumulated during the early stages of the cycle.
4. Since the graded slopes of the Coast Range on the one side descend to a former shore whose elevation is now but 1,500 feet above sea level, and since only ten to twenty miles inland on the other side of the range, the same kind of slope extends beneath Tertiary deposits 4,000 feet above sea level, it appears that aggradation of the outer (or western) part of the Tertiary deposits on the eastern border of the Coast Range continued down to the end of the cycle of erosion, though
5. There must have been an outlet to the sea, since, as we have already seen, the water supply of the Tertiary was greater than that of today and the present streams reach the sea. Moreover, the mature upper slopes and the steep lower slopes of the large valleys make a pronounced topographic unconformity, showing two cycles of valley development.
6. Upon uplift of the coast and dissection of the marine terraces at the foot of the Coast Range, the streams cut deep trenches on the floors of their former valleys (Fig. 152) and removed (a) large portions of the coast terrace, and (b) large portions of the Tertiary deposits east of the Coast Range.
7. Depression of the coastal terrace and its partial burial meant the drowning of the lower Majes Valley and its partial filling with marine and later with terrestrial deposits. It also brought about the partial filling by stream aggradation of the middle portion of the valley, causing the valley fill to abut sharply against the steep valley walls. (See 155 .)
8. Uplift and dissection of both the terrace and its overlying sediments would be accompanied by dissection of the former valley fill, provided that the waste supply was not increased and that the uplift was regional and approximately equal throughout—not a bowing up of the coast on the one hand, or an excessive bowing up of the mountains on the other. But the waste supply has not remained constant, and the uplift has been greater in the Cordillera than on the coast. Let us proceed to the proof of these two conclusions, since upon them depends the interpretation of the later physical history of the coastal valleys.
Fig. 155—Steep walls in the Majes Valley below Cantas and the abrupt termination against them of a deep alluvial fill. |
Fig. 156—Canyon of the Majes River through the Coast Range north of Camaná. The rock is a granite-gneiss capped by rather flat-lying sedimentaries. |
It is known that the Pleistocene was a time of augmented waste delivery. At the head of the broadly opened Majes Valley there was deposited a huge mass of extremely coarse waste several hundred feet deep and several miles long. Forward from it, interstratified with its outer margin, and continuing the same alluvial grade, is a still greater mass of finer material which descends to lower levels. The fine material is deposited on the floor of a valley cut into Tertiary strata, hence it is younger than the Tertiary. It is now, and has been for some time past, in process of dissection, hence it was not formed under present conditions of climate and relief. It is confidently assigned to the Pleistocene, since this is definitely known to have been a time of greater precipitation and waste removal on the mountains, and deposition on the plains and the floors of mountain valleys. Such a conclusion appears, even on general grounds, to be but a shade less reliable than if we were able to find in the upper Majes Valley, as in so many other Andean valleys, similar alluvial deposits interlocked with glacial moraines and valley trains.
In regard to the second consideration—the upbowing of the Cordillera—it may be noted that the valley and slope profiles of the main Cordillera shown on p. 191, when extended toward the margin of the mountain belt, lie nearly a mile above the level of the sea on the west and the Amazon plains on the east. The evidence of regional bowing thus afforded is checked by the depths of the mountain valleys and the stream profiles in them. The streams are now sunk from one to three thousand feet below their former level. Even in the case of three thousand feet of erosion the stream profiles are still ungraded, the streams themselves are almost torrential, and from one thousand to three thousand feet of vertical cutting must still be accomplished before the profiles will be as gentle and regular as those of the preceding cycle of erosion, in which were formed the mature slopes now lying high above the valley floors.
Further evidence of bowing is afforded by the attitude of the Tertiary strata themselves, more highly inclined in the case of the older Tertiary, less highly inclined in the case of the younger Tertiary. It is noteworthy that the gradient of the present valley floor is distinctly less than that of the least highly inclined strata. This is true even where aggradation is now just able to continue, as near the nodal point of the valley, above Aplao, where cutting ceases and aggradation begins. (See the Aplao Quadrangle for change of function on the part of the stream a half mile above Cosos). Such a progressive steepening of gradients in the direction of the oldest deposits, shows very clearly a corresponding progression in the growth of the Andes at intervals throughout the Tertiary.
Thus we have aggradation in the Tertiary at the foot of the growing Andes; aggradation in the Pliocene or early Pleistocene on the floor of a deep valley cut in earlier deposits; aggradation in the glacial epoch; and aggradation now in progress. Basin deposits within the borders of the Peruvian Andes are relatively rare. The profound erosion implied by the development, first of a mature topography across this great Cordillera, and second of many deep canyons, calls for deposition on an equally great scale on the mountain borders. The deposits of the western border are a mile thick, but they are confined to a narrow zone between the Coast Range and the Cordillera. Whatever material is swept beyond the immediate coast is deposited in deep ocean water, for the bottom falls off rapidly. The deposits of the eastern border of the Andes are carried far out over the Amazon lowland. Those of earlier geologic periods were largely confined to the mountain border, where they are now upturned to form the front range of the Andes. The Tertiary deposits of the eastern border are less restricted, though they appear to have gathered chiefly in a belt from fifty to one hundred miles wide.
The deposits of the western border were laid down by short streams rising on a divide only 100 to 200 miles from the Pacific. Furthermore, they drain the dry leeward slopes of the Andes. The deposits of the wet eastern border were made by far larger streams that carry the waste of nearly the whole Cordillera. Their shoaling effect upon the Amazon depression must have been a large factor in its steady growth from an inland sea to a river lowland.
CHAPTER XV
PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT
GENERAL FEATURES
In the preceding chapter we employed geologic facts in the determination of the age of the principal topographic forms. These facts require further discussion in connection with their closest physiographic allies if we wish to show how the topography of today originated. There are many topographic details that have a fundamental relation to structure; indeed, without a somewhat detailed knowledge of geology only the broader and more general features of the landscape can be interpreted. In this chapter we shall therefore refer not to the scenic features as in a purely topographic description, but to the rock structure and the fossils. A complete and technical geologic discussion is not desirable, first, because it should be based upon much more detailed geologic field work, and second because after all our main purpose is not to discuss the geologic features per se, but the physiographic background which the geologic facts afford. I make this preliminary observation partly to indicate the point of view and partly to emphasize the necessity, in a broad, geographic study, for the reconstruction of the landscapes of the past.
The two dominating ranges of the Peruvian Andes, called the Maritime Cordillera and the Cordillera Vilcapampa, are composed of igneous rock—the one volcanic lava, the other intrusive granite. The chief rock belts of the Andes of southern Peru are shown in 157 . The Maritime Cordillera is bordered on the west by Tertiary strata that rest unconformably upon Palaeozoic quartzites. It is bordered on the east by Cretaceous limestones that grade downward into sandstones, shales, and basal conglomerates. At some places the Cretaceous deposits rest upon old schists, at others upon Carboniferous limestones and related strata, upon small granite intrusives and upon old and greatly altered volcanic rock.
The Cordillera Vilcapampa has an axis of granitic rock which was thrust upward through schists that now border it on the west and slates that now border it on the east. The slate series forms a broad belt which terminates near the eastern border of the Andes, where the mountains break down abruptly to the river plains of the Amazon Basin. The immediate border on the east is formed of vertical Carboniferous limestones. The narrow foothill belt is composed of Tertiary sandstones that grade into loose sands and conglomerates. The inclined Tertiary strata were leveled by erosion and in part overlain by coarse and now dissected river gravels, probably of Pleistocene age. Well east of the main border are low ranges that have never been described. They could not be reached by the present expedition on account of lack of time. On the extreme western border of that portion of the Peruvian Andes herein described, there is a second distinct border chain, the Coast Range. It is composed of granite and once had considerable relief, but erosion has reduced its former bold forms to gentle slopes and graded profiles.
The continued and extreme growth of the Andes in later geologic periods has greatly favored structural and physiographic studies. Successive uplifts have raised earlier deposits once buried on the mountain flanks and erosion has opened canyons on whose walls and floors are the clearly exposed records of the past. In addition there have been igneous intrusions of great extent that have thrust aside and upturned the invaded strata exposing still further the internal structures of the mountains. From sections thus revealed it is possible to outline the chief events in the history of the Peruvian Andes, though the outline is still necessarily broad and general because based on rapid reconnaissance. However, it shows clearly that the landscape of the present represents but a temporary stage in the evolution of a great mountain belt. At the dawn of geologic history there were chains of mountains where the Andes now stand. They were swept away and even their roots deeply submerged under invading seas. Repeated uplifts of the earth’s crust reformed the ancient chains or created new ones out of the rock waste derived from them. Each new set of forms, therefore, exhibits some features transmitted from the past. Indeed, the landscape of today is like the human race—inheriting much of its character from past generations. For this reason the philosophical study of topographic forms requires at least a broad knowledge of related geologic structures.