The most complete succession of strata (Tertiary) occurs just below Cantas on the trail to Jaguey (Fig. 171). Upon a floor of granite-gneiss, and alternating beds of quartzite and shale belonging to an older series, are deposited heavy beds of red sandstone with many conglomerate lenses. The sandstone strata are measurably deformed and their upper surfaces moderately dissected. Upon them have been deposited unconformably a thicker series of deposits, conglomerates, sandstones, and finer wind-blown material. The basal conglomerate is very coarse—much like beach material in both structure and composition, and similar to that along and south of the present coast at Camaná. Higher in the section the material is prevailingly sandy and is deposited in regular beds from a few inches to a few feet in thickness. Near the top of the section are a few hundred feet of strata chiefly wind deposited. Unconformably overlying the whole series and in sharp contrast to the fine wind-blown stuff below it, is a third series of coarse deposits about five hundred feet thick. The topmost material, that forming the surface of the desert upland, consists of wind-blown sand now shifted by the wind and gathered into sand dunes or irregular drifts, banks of white earth, “tierra blanca,” and a pebble pavement a few inches thick.

If the main facts of the above section are now summarized they will facilitate an understanding of other sections about to be described, inasmuch as the summary will in a measure anticipate our conclusions concerning the origin of the deposits and their subsequent history. The sediments in the Majes Valley between Cantas and Jaguey consist of three series separated by two unconformities. The lowermost series is evenly bedded and rather uniform in composition and topographic expression, standing forth in huge cliffs several hundred feet high on the eastern side of the valley. This lower series is overlain by a second series, which consists of coarse conglomerate grading into sand and ultimately into very fine fluffy wind-deposited sands and silts. The lower series is much more deformed than the upper, showing that the deforming movements of later geologic times have been much less intense than the earlier, as if there had been a fading out or weakening of the deforming agents. Finally there is a third series several hundred feet thick which forms the top of the section.

Three other sections may now be examined, one immediately below Cantas, one just above, and one opposite Aplao. The section below Cantas is shown in 173 , and indicates a lower series of red sandstones crossed by vertical faults and unconformably overlain by nearly horizontal conglomerates, sandstones, etc., and the whole faulted again with an inclined fault having a throw of nearly 25°. A white to gray sandstone unconformably overlying the red sandstone is shown interpolated between the lowermost and uppermost series, the only example of its kind, however. No important differences in lithographical character may be noted between these and the beds of the preceding section.

A similar succession of strata was observed at Aplao, still farther up the Majes Valley, 174 . A greatly deformed and metamorphosed older series is unconformably overlaid by a great thickness of younger strata. The younger strata may be again divided into two series, a lower series consisting chiefly of red sandstones and an upper consisting of gray to yellow, and only locally red sands of finer texture and more uniform composition. The two are separated by an erosion surface and only the upper series is tilted regionally seaward with faint local deformation; the lower series is both folded and faulted with overthrusts aggregating several thousand feet of vertical and a half mile of horizontal displacement.

The above sections all lie on the eastern side of the Majes Valley. From the upper edge of the valley extensive views were gained of the strata on the opposite side, and two sections, though they were not examined at close range, are at least worth comparing with those already given. From the narrows below Cantas the structure appears as in Figs. 175-176, and shows a deforming movement succeeded by erosion in a lower series. The upper series of sedimentary rock has suffered but slight deformation. A still more highly deformed basal series occurs on the right of the section, presumably the older quartzites. At Huancarqui, opposite Aplao, an extensive view was gained of the western side of the valley, but the lower Tertiary seems not to be represented here, as the upper undeformed series rests unconformably upon a tilted series of quartzites and slates. Farther up the Cantas valley (an hour’s ride above Aplao) the Tertiary rests upon volcanic flows or older quartzites or the granite-gneiss exposed here and there along the valley floor.

In no part of the sedimentaries in the Majes Valley were fossils found, save in the now uplifted and dissected sands that overlie the upraised terraces along the coast immediately south of Camaná and also back of Mollendo. Like similar coastal deposits elsewhere along the Peruvian littoral, the terrace sands are of Pliocene or early Pleistocene age. The age of the deposits back of the Coast Range is clearly greater than that of the coastal deposits, (1) since they involve two unconformities, a mile or more of sediments, and now stand at least a thousand feet above the highest Pliocene (or Pleistocene) in the Camaná Valley, and (2) because the erosion history of the interior sediments may be correlated with the physiographic history of the coastal terraces and the correlation shows that uplift and dissection of the terraces and of the interior deposits went hand in hand, and that the deposits on the terraces may similarly be correlated with alluvial deposits in the valley.

We shall now see what further ground there is for the determination of the age of these sediments. Just below Chuquibamba, where they first appear, the sediments rest upon a floor of volcanic and older rock belonging to the great field now known from evidence in many localities to have been formed in the early Tertiary, and here known to be post-Cretaceous from the relations between Cretaceous limestones and volcanics in the Cotahuasi Valley (see p. 247). Although volcanic flows were noted interbedded with the desert deposits, these are few in number, insignificant in volume, and belong to the top of the volcanic series. The same may be said of the volcanic flows that locally overlie the desert deposits. We have then definite proof that the sandstones, conglomerates, and related formations of the Majes Valley and bordering uplands are older than the Pliocene or early Pleistocene and younger than the Cretaceous and the older Tertiary lavas. Hence it can scarcely be doubted that they represent a considerable part of the Tertiary period, especially in view of the long periods of accumulation which the thick sediments represent, and the additional long periods represented by the two well-marked unconformities between the three principal groups of strata.

If we now trace the physical history of the region we have first of all a deep depression between the granite range along the coast and the western flank of the Andes. Here and there, as in the Vitor, the Majes, and other valleys, there were gaps through the Coast Range. Nowhere did the relief of the coastal chain exceed 5,000 feet. The depression had been partly filled in early geologic (probably early Paleozoic) time by sediments later deformed and metamorphosed so that they are now quartzites and shales. The greater resistance of the granite of the Coast Range resulted in superior relief, while the older deformed sedimentaries were deeply eroded, with the result that by the beginning of the Tertiary the basin quality of the depression was again emphasized. All these facts are expressed graphically in 171 . On the western flanks of the granite range no corresponding sedimentary deposits are found in this latitude. The sea thus appears to have stood farther west of the Coast Range in Paleozoic times than at present.

For the later history it is necessary to assemble the various Tertiary sections described on the preceding pages. First of all we recognize three quite distinct types of accumulations, for which we shall have to postulate three sets of conditions and possibly three separate agents. The first or lowermost consists of even-bedded deposits of red and gray sandstones, the former color predominating. The material is in general well-sorted save locally, where lenses and even thin beds of conglomerate have been developed. There is, however, about the whole series a uniformity and an orderliness in striking contrast to the coarse, cross-bedded, and irregular material above the unconformity. On their northeastern or inner margin the sandstones are notably coarser and thicker, a natural result of proximity to the mountains, the source of the material. The general absence of wind-blown deposits is marked; these occur entirely along the eastern and northern portions of the deposits and are recognized (1) by their peculiar cross-bedding, and (2) by the fact that the cross-bedding is directed northeastward in a direction contrary to the regional dip of the series, a condition attributable to the strong sea breezes that prevail every afternoon in this latitude.

The main body of the material is such as might be deposited on the wide flood plains of piedmont streams during a period of prolonged erosion on surrounding highlands that served as the feeding grounds of the streams. The alternations in the character of the deposits, alternations which, in a general view, give a banded appearance to the rock, are produced by successions of beds of fine and coarse material, though all of it is sandstone. Such successions are probably to be correlated with seasonal changes in the volume and load of the depositing streams.

Along the inner edge of the Desert of Tarapacá, roughly between the towns of Tarapacá and Quillagua, Chile, the piedmont gravels, sands, silts, and muds extend for over a hundred miles, flanking the western Andes and forming a transition belt between these mountains and the interior basins of the coast desert. The silts and muds constitute the outer fringe of the piedmont and are interrupted here and there where sands are blown upon them from the higher portions of the piedmont, or from the desert mountains and plains on the seaward side. Practically no rain falls upon the greater part of the desert and the only water it receives is that borne to it by the piedmont streams in the early summer, from the rains and melted snows of the high plateau and mountains to the eastward. These temporary streams spread upon the outer edge of the piedmont a wide sheet of mud and silt which then dries and becomes cracked, the curled and warped plates retaining their character until the next wet season or until covered with wind-blown sand. The wind-driven sand fills the cracks in the muds and is even drifted under the edges of the upcurled plates, filling the spaces completely. Over this combined fluvial and æolian deposit is spread the next layer of mud, which frequently is less extensive than the earlier deposits, thus giving abundant opportunity for the observation of the exact manner of burial of the older sand-covered stratum.

Uplift and erosion of the earliest of the Tertiary deposits of the Majes Valley is indicated in two ways: (1) by the deformed character of the beds, and (2) by the ensuing coarse deposits which were derived from the invigorated streams. Without strong deformations it would not be possible to assign the increased erosion so confidently to uplift; with the coarse deposits that succeed the unconformity we have evidence of accumulation under conditions of renewed uplift in the mountains and of full streams competent to remove the increasing load.

It is well known that the front of a sand dune generally consists of sand deposited on a slope inclined at the angle of repose, say between 30° and 35°, and rolled into place up the long back slope of the dune by the wind. It has not, however, been generally recognized that the angle of repose may be exceeded (a) when there exists a strong back eddy or (b) when the wind blows violently and for a short time in the opposite direction. In either case sand is carried up the short steep slope of the dune front and accumulated at an angle not infrequently running up to 43° and 48° and locally, and under the most favorable circumstances, in excess of 50°. The conditions under which these steep angles are attained are undoubtedly not universal, but they can be found in some parts of almost any desert in the world. They appear not to be present where the sand grains are of uniform size throughout, since that leads to rolling. They are found rather where there is a certain limited variation in size that promotes packing. Packing and the development of steep slopes are also facilitated in parts of the coastal desert of Peru by a cloud canopy that hangs over the desert in the early morning, that in the most favorable places moistens even the dune surfaces and that has least penetration on the steep semi-protected dune fronts. Sand later blown up the dune front or rolled down from the dune crest is encouraged to remain near the cornice on an abnormally steep slope by the attraction which the slightly moister sand has for the dry grains blown against it. Since dunes travel and since their front layers, formed on steep slopes, are cut off to the level of the surface in the rear of the dune, it follows that the steepest dips in exposed sections are almost always less than those in existing dunes. Exceptions to the rule will be noted in filled hollows not re-excavated until deeply covered by wind-blown material. These, re-exposed at the end of a long period of wind accumulation, may exhibit even the maximum dips of the dune cornices. Such will be conspicuously the case in sections in aggraded desert deposits. On the border of the Majes Valley, from 400 to 500 feet of wind-accumulated deposits may be observed, representing a long period of successive dune burials.

It is to the ability of the wind to transport material against, as well as with, gravity, that we owe the third distinct quality of dune material, the succession of flowing lines, in contrast to the succession of now flat-lying now steeply inclined beds characteristic of cross-bedded material deposited by water. One dune travels across the face of the country only to be succeeded by another.[54] Even if wind aggradation is in progress, the plain-like surface in the rear of a dune may be excavated to the level of steeply inclined beds upon whose truncated outcrop other inclined beds are laid, 178 . The contrast to these conditions in the case of aggradation by water is so clearly and easily inferred that space will not be taken to point them out. It is also true as a corollary to the above that the greater part of a body of wind-drifted material will consist of cross-bedded layers, and not a series of evenly divided and alternating flat-lying and cross-bedded layers which result from deposition in active and variable currents of water.

The caution must of course be observed that wind action and water action may alternate in a desert region, as already described in Tarapacá in northern Chile, so that the whole of a deposit may exhibit an alternation of cross-bedded and flat-lying layers; but the former only are due to wind action, the latter to water action.

Finally it may be noted that the sudden, frequent, and diversified dips in the cross-bedding are peculiarly characteristic of wind action. Although one sees in a given cross-section dips apparently directed only toward the left or the right, excavation will supply a third dimension from which the true dips may be either observed or calculated. These show an almost infinite variety of directions of dip, even in restricted areas, a condition due to the following causes:

(1) the curved fronts of sand dunes, which produce dips concentric with respect to a point and ranging through 180° of arc; (2) the irregular character of sand dunes in many places, a condition due in turn to (a) the changeful character of the strong wind (often not the prevailing wind) to which the formation of the dunes is due, and (b) the influence of the local topography upon wind directions within short distances or upon winds of different directions in which a slight change in wind direction is followed by a large change in the local currents; (3) the fact that all combinations are possible between the erosion levels of the wind in successive generations of dunes blown across a given area, hence any condition at a given level in a dune may be combined with any other condition of a succeeding dune; (4) variations in the sizes of successive dunes will lead to further contrasts not only in the scale of the features but also in the direction and amount of the dips.

Finally, we may note that a section of dune deposits has a distinctive feature not exhibited by water deposits. If the foreset beds of a cross-bedded water deposit be exposed in a plane parallel to the strike of the beds, the beds will appear to be horizontal. They could not then be distinguished from the truly horizontal beds above and below them. But the conditions of wind deposition we have just noted, and chiefly the facts expressed by 178 , make it impossible to select a position in which both tangency and irregular dips are not well developed in a wind deposit. I believe that we have in the foregoing facts and inferences a means for the definite separation of these two classes of deposits. Difficulties will arise only when there is a quick succession of wind and water action in time, or where the wind produces powerful and persistent effects without the actual formation of dunes.

The latest known deposits in the coastal region are found surmounting the terrace tops along the coast between Camaná and Quilca, where they form deposits several hundred feet thick in places. The age of these deposits is determined by fossil evidence, and is of extraordinary interest in the determination of the age of the great terraces upon which they lie. They consist of alternating beds of coarse and fine material, the coarser increasing in thickness and frequency toward the bottom of the section. It is also near the bottom of the section that fossils are now found; the higher members are locally saline and throughout there is a marked inclination of the beds toward the present shore. The deposits appear not to have been derived from the underlying granite-gneiss. They are distributed most abundantly near the mouths of the larger streams, as near the Vitor at Quilca, and the Majes at Camaná. Elsewhere the terrace summit is swept clean of waste, except where local clay deposits lie in the ravines, as back of Mollendo and where “tierras blancas” have been accumulated by the wind.

These coastal deposits were laid down upon a dissected terrace up to five miles in width. The degree of dissection is variable, and depends upon the relation of the through-flowing streams to the Coast Range. The Vitor and the Majes have cut down through the Coast Range, and locally removed the terrace; smaller streams rising on the flanks of the Coast Range either die out near the foot of the range or cross it in deep and narrow valleys. The present drainage on the seaward slopes of the Coast Range is entirely ineffective in reaching the sea, as was seen in 1911, the wettest season known on the coast in years and one of the wettest probably ever observed on this coast by man.

The broad regional uplift of the Peruvian Andes in late Tertiary and in Pleistocene times carried their summits above the level of perpetual snow. It is still an open question whether or not uplift was sufficiently great in the early Pleistocene to be influenced by the first glaciations of that period. As yet, there are evidences of only two glacial invasions, and both are considered late events on account of the freshness of their deposits and the related topographic forms. The coarse deposits—nearly 500 feet thick—that form the top of the desert section described above clearly indicate a wetter climate than prevailed during the deposition of the several hundred feet of wind-blown deposits beneath them. But if our interpretation be correct these deposits are of late Tertiary age, and their character and position are taken to indicate climatic changes in the Tertiary. They may have been the mild precursors of the greater climatic changes of glacial times. Certain it is that they are quite unlike the mass of the Tertiary deposits. On the other hand they are separated from the deposits of known glacial age by a time interval of great length—an epoch in which was cut a benched canyon nearly a mile deep and three miles wide. They must, therefore, have been formed when the Andes were thousands of feet lower and unable to nourish glaciers. It was only after the succeeding uplifts had raised the mountain crests well above the frost line that the records of oscillating climates were left in erratic deposits, troughed valleys, cliffed cirques and pinnacled divides.

The Pleistocene deposits fall into three well-defined groups: (1) glacial accumulations at the valley heads, (2) alluvial deposits in the valleys, and (3) lacustrine deposits formed on the floors of temporary lakes in inclosed basins. Among these the most variable in form and composition are the true glacier-laid deposits at the valley heads. The most extensive are the fluvial deposits accumulated as valley fill throughout the entire Andean realm. Though important enough in some respects the lacustrine deposits are of small extent and of rather local significance. Practically none of them fall within the field of the present expedition; hence we shall describe only the first two classes.

It is noteworthy that the moraines decrease in size up valley since each valley had been largely cleaned out by ice action before the retreat of the glacier began. Each lowermost terminal moraine is fronted by a great mass of unsorted coarse bowldery material forming a fill in places several hundred feet thick, as below Choquetira and in the Vilcapampa Valley between Vilcabamba and Puquiura. This bowldery fill is quite distinct from the long, gently inclined, and stratified valley train below it, or the marked ridge-like moraine above it. It is in places a good half mile in length. Its origin is believed to be due to an overriding action beyond the last terminal moraine at a time when the ice was well charged with débris, an overriding not marked by morainal accumulations, chiefly because the ice did not maintain an extreme position for a long period.

The depth of the alluvial valley fill due to tributary fan accumulation depends upon both the amount of the material and the form of the valley. Below Urubamba in the Urubamba Valley a fine series is displayed, as shown in 180 . The fans head in valleys extending up to snow-covered summits upon whose flanks living glaciers are at work today. Their heads are now crowned by terminal moraines and both moraines and alluvial fans are in process of dissection. The height and extent of the moraines and the alluvial fans are in rough proportion and in turn reflect the height, elevation, and extent of the valley heads which served as fields of nourishment for the Pleistocene glaciers. Where the fans were deposited in narrow valleys the effect was to increase the thickness of the deposits at the expense of their area, to dam the drainage lines or displace them, and to so load the streams that they have not yet cleared their beds after thousands of years of work under torrential conditions.

The fill is composed of both fine and coarse material laid down by water in steep valley floors to a depth of many feet. It breaks the steep slope of each valley, forming terraces with pronounced frontal scarps facing the river. On the raw bluffs at the scarps made by the encroaching stream good exposures are afforded. At Chinche in the Urubamba Valley above Santa Ana, the material is both sand and clay with an important amount of gravel laid down with steep valleyward inclination and under torrential conditions; so that within a given bed there may be an apparent absence of lamination. Almost identical conditions are exhibited frequently along the railway to Cuzco in the Vilcanota Valley. The material is mixed sand and gravel, here and there running to a bowldery or stony mass where accessions have been received from some source nearby. It is modified along its margin not only in topographic form but also in composition by small tributary alluvial fans, though these in general constitute but a small part of the total mass. At Cotahuasi, 29 , there is a remarkable fill at least four hundred feet deep in many places where the river has exposed fine sections. The depth of the fill is, however, not determined by the height of the erosion bluffs cut into it, since the bed of the river is made of the same material. The rock floor of the valley is probably at least an additional hundred feet below the present level of the river.

Similar conditions are well displayed at Huadquiña, where a fine series of terraces at the lower end of the Torontoy Canyon break the descent of the environing slopes; also in the Urubamba Valley below Rosalina, and again at the edge of the mountains at the Pongo de Mainique. It is exhibited most impressively in the Majes Valley, where the bordering slopes appear to be buried knee-deep in waste, and where from any reasonable downward extension of rock walls of the valley there would appear to be at least a half mile of it. It is doubtful and indeed improbable that the entire fill of the Majes Valley is glacial, for during the Pliocene or early Pleistocene there was a submergence which gave opportunity for the partial filling of the valley with non-glacial alluvium, upon which the glacial deposits were laid as upon a flat and extensive floor that gives an exaggerated impression of their depth. However, the head of the Majes Valley contains at least six hundred feet and probably as much as eight hundred feet of alluvium now in process of dissection, whose coarse texture and position indicates an origin under glacial conditions. The fact argues for the great thickness of the alluvial material of the lower valley, even granting a floor of Pliocene or early Pleistocene sediments. The best sections are to be found just below Chuquibamba and again about halfway between that city and Aplao, whereas the best display of the still even-floored parts of the valley are between Aplao and Cantas, where the braided river still deposits coarse gravels upon its wide flood plain.

CHAPTER XVI

GLACIAL FEATURES

It has been found[55] that the changes run in cycles of from thirty to thirty-five years in length and that the northern and southern hemispheres appear to be in opposite phase. For example, since 1885 the snowline in the southern hemisphere has been decreasing in elevation in nine out of twelve cases by the average amount of nine hundred feet. With but a single exception, the snowline in the northern hemisphere has been rising since 1890 with an average increase of five hundred feet in sixteen cases. To be sure, we must recognize that the observations upon which these conclusions rest have unequal value, due both to personal factors and to differences in instrumental methods, but that in spite of these tendencies toward inequality they should agree in establishing a general rise of the snowline in the northern hemisphere and an opposite effect in the southern is of the highest significance.

From Central Lagunas, Chile, I went northeastward via Pica and the Huasco Basin to Llica, Bolivia, crossing the Sillilica Pass in May, 1907, at 15,750 feet (4,800 m.). Perpetual snow lay at an estimated height of 2,000-2,500 feet above the pass or 18,000 feet (5,490 m.) above the sea. Two weeks later the Huasco Basin, 14,050 feet (4,280 m.), was covered a half-foot deep with snow and a continuous snow mantle extended down to 13,000 feet. Light snows are reported from 12,000 feet, but they remain a few hours only and are restricted to the height of exceptionally severe winter seasons (June and early July). Three or four distant snow-capped peaks were observed and estimates made of the elevation of the snowline between the Cordillera Sillilica and Llica on the eastern border of the Maritime Cordillera. All observations agreed in giving an elevation much in excess of 17,000 feet. In general the values run from 18,000 to 19,000 feet (5,490 to 5,790 m.). Though the bases of these figures are estimates, it should be noted that a large part of the trail lies between 14,000 and 16,000 feet, passing mountains snow-free at least 2,000 to 3,000 feet higher, and that for general comparisons they have a distinct value.

In taking observations on the snowline along the seventy-third meridian I was fortunate enough to have a topographer the heights of whose stations enabled me to correct the readings of my aneroid barometer whenever these were taken off the line of traverse. Furthermore, the greater height of the passes—15,000 to 17,600 feet—brought me more frequently above the snowline than had been the case in Bolivia and Chile. More detailed observations were made, therefore, not only upon the elevation of the snowline from range to range, but also upon the degree of canting of the snowline on a given range. Studies were also made on the effect of the outline of the valleys upon the extent of the glaciers, the influence on the position of the snowline of mass elevation, precipitation, and cloudiness.

It is well known that with increase of elevation and therefore of the rarity of the air there is less absorption of the sun’s radiant energy, and a corresponding increase in the degree of insolation. It follows, therefore, that at high altitudes the contrasts between sun and shade temperatures will increase. Frankland[56] has shown that the increase may run as high as 500 per cent between 100 to 10,000 feet above the sea. I have noted a fall of temperature of 15° F. in six minutes, due to the obscuring of the sun by cloud at an elevation of 16,000 feet above Huichihua in the Central Ranges of Peru. Since the sun shines approximately half the time in the snow-covered portions of the mountains and since the tropical Andes are of necessity snow-covered only at lofty elevations, this contrast between shade and sun temperatures is by far the most powerful factor influencing differences in elevation of the snowline in Peru.

To the drifting of the fallen snow is commonly ascribed a large portion of this contrast. I have yet to see any evidence of its action near the snowline, though I have often observed it, especially under a high wind in the early morning hours at considerable elevations above the snowline, as at the summits of lofty peaks. It appears that the lower ranges bearing but a limited amount of snow are not subject to drifting because of the wetness of the snow, and the fact that it is compacted by occasional rains and hail storms. Only the drier snow at higher elevations and under stronger winds can be effectively dislodged.

The effect of unequal distribution of precipitation on the windward and leeward slopes of a mountain range is in general to depress the snowline on the windward slopes where the greater amount falls, but this may be offset in high altitudes by temperature contrasts as in the westward trending Cordillera Vilcapampa, where north and south slopes are in opposition. If the Cordillera Vilcapampa ran north and south we should have the windward and leeward slopes equally exposed to the sun and the snowline would lie at a lower elevation on the eastern side. Among all the ranges the slopes have decreasing precipitation to the leeward, that is, westerly. The second and third passes, between Arma and Choquetira, are snow-free (though their elevations equal those of the first pass) because they are to leeward of the border range, hence receive less precipitation. The depressive effect of increased precipitation on the snowline is represented by A-B, 184 ; in an individual range the effect of heavier precipitation may be offset by temperature contrasts between shady and sunny slopes, as shown by the line a-b in the same figure.

The degree of canting of the snowline on opposite slopes of the Cordillera Vilcapampa varies between 5° and 12°, the higher value being represented four hours southwest of Arma on the Choquetira trail, looking northeast. A general view of the Cordillera looking east at this point (Fig. 186), shows the appearance of the snowline as one looks along the flanks of the range. In detail the snowline is further complicated by topography and varying insolation, each spur having a snow-clad and snow-free aspect as shown in the last figure. The degree of difference on these minor slopes may even exceed the difference between opposite aspects of the range in which they occur.