Fig. 157—Outline sketch showing the principal rock belts of Peru along the seventy-third meridian. They are: 1, Pleistocene and Recent gravels and sands, the former partly indurated and slightly deformed, with the degree of deformation increasing toward the mountain border (south). 2, Tertiary sandstones, inclined from 15° to 30° toward the north and unconformably overlain by Pleistocene gravels. 3, fossil-bearing Carboniferous limestones with vertical dip. 4, non-fossiliferous slates, shales, and slaty schists (Silurian) with great variation in degree of induration and in type of structure. South of the parallel of 13° is a belt of Carboniferous limestones and sandstones bordering (5), the granite axis of the Cordillera Vilcapampa. For its structural relations to the Cordillera see Figs. 141 and 142. 6, old and greatly disturbed volcanic agglomerates, tuffs and porphyries, and quartzitic schists and granite-gneiss. 7, principally Carboniferous limestones north of the axis of the Central Ranges and Cretaceous limestones south of it. Local granite batholiths in the axis of the Central Ranges. 8, quartzites and slates predominating with thin limestones locally. South of 8 is a belt of shale, sandstone, and limestone with a basement quartzite appearing on the valley floors. 9, a portion of the great volcanic field of the Central Andes and characteristically developed in the Western or Maritime Cordillera, throughout northern Chile, western Bolivia, and Peru. At Cotahuasi (see also Fig. 20) Cretaceous limestones appear beneath the lavas. 10, Tertiary sandstones of the coastal desert with a basement of old volcanics and quartzites appearing on the valley walls. The valley floor is aggraded with Pleistocene and Recent alluvium. 11, granite-gneiss of the Coast Range. 12, late Tertiary or Pleistocene sands and gravels deposited on broad coastal terraces. For rock structure and character see the other figures in this chapter. For a brief designation of index fossils and related forms see Appendix B. For the names of the drainage lines and the locations of the principal towns see Figs. 20 and 204.
SCHISTS AND SILURIAN SLATES [50]
The oldest series of rocks along the seventy-third meridian of Peru extends eastward from the Vilcapampa batholith nearly to the border of the Cordillera, 157 . It consists of (1) a great mass of slates and shales with remarkable uniformity of composition and structure over great areas, and (2) older schists and siliceous members in restricted belts. They are everywhere thoroughly jointed; near the batholith they are also mineralized and altered from their original condition; in a few places they have been intruded with dikes and other form of igneous rock.
The slates and shales underlie known Carboniferous strata on their eastern border and appear to be a physical continuation of the fossiliferous slates of Bolivia; hence they are provisionally referred to the Silurian, though they may possibly be Devonian. Certainly the known Devonian exceeds in extent the known Silurian in the Central Andes but its lithological character is generally quite unlike the character of the slates here referred to the Silurian. The schists are of great but unknown age. They are unconformably overlain by known Carboniferous at Puquiura in the Vilcapampa Valley (Fig. 158), and near Chuquibambilla on the opposite side of the Cordillera Vilcapampa. The deeply weathered fissile mica schists east of Pasaje (see Appendix C for all locations) are also unconformably overlain by conglomerate and sandstone of Carboniferous age. While the schists vary considerably in lithological appearance and also in structure, they are everywhere the lowest rocks in the series and may with confidence be referred to the early Palaeozoic, while some of them may date from the Proteriozoic.
Fig. 158—Geologic sketch map of the lower Urubamba Valley. A single traverse was made along the valley, hence the boundaries are not accurate in detail. They were sketched in along a few lateral traverses and also inferred from the topography. The country rock is schist and the granite intruded in it is an arm of the main granite mass that constitutes the axis of the Cordillera Vilcapampa. The structure and to some degree the extent of the sandstone on the left are represented in Figs. 141 and 142.
The Silurian beds are composed of shale, sandstone, shaly sandstone, limestone, and slate with some slaty schist, among which the shales are predominent and the limestones least important. Near their contact with the granite the slate series is composed of alternating beds of sandstone and shale arranged in beds from one to three feet thick. At Santa Ana they become more fissile and slaty in character and in several places are quarried and used for roofing. At Rosalina they consist of almost uniform beds of shale so soft and so minutely and thoroughly jointed as to weather easily. Under prolonged erosion they have, therefore, given rise to a well-rounded and soft-featured landscape. Farther down the Urubamba Valley they again take on the character of alternating beds of sandstone and shale from a few feet to fifteen and more feet thick. In places the metamorphism of the series has been carried further—the shales have become slates and the sandstones have been altered to extremely resistant quartzites. The result is again clearly shown in the topography of the valley wall which becomes bold, inclosing the river in narrow “pongos” or canyons filled with huge bowlders and dangerous rapids. The hills become mountains, ledges appear, and even the heavy forest cover fails to smooth out the natural ruggedness of the landscape.
It is only upon their eastern border that the Silurian series includes calcareous beds, and all of these lie within a few thousand yards of the contact with the Carboniferous limestones and shales. At first they are thin paper-like layers; nearer the top they are a few inches wide and finally attain a thickness of ten or twelve feet. The available limestone outcrops were rigorously examined for fossils but none were found, although they are lavishly distributed throughout the younger Carboniferous beds just above them. It is also remarkable that though the Silurian age of these beds is reasonably inferred they are not separated from the Carboniferous by an unconformity, at least we could find none in this locality. The later beds disconformably overlie the earlier beds, although the sharp differences in lithology and fossils make it easy to locate the line of separation. The limestone beds of the Silurian series are extremely compact and unfossiliferous. At least in this region those of Carboniferous age are friable and the fossils varied and abundant. The Silurian beds are everywhere strongly inclined and throughout the eastern half or third of their outcrop in the Urubamba Valley they are nearly vertical.
In view of the enormous thickness of the repeated layers of shale and sandstone this series is of great interest. Added importance attaches to their occurrence in a long belt from the eastern edge of the Bolivian highlands northward through Peru and possibly farther. From the fact that their disturbance has been on broad lines over wide areas with extreme metamorphism, they are to be separated from the older mica-schists and the crumpled chlorite schists of Puquiura and Pasaje. Further reasons for this distinction lie in their lithologic difference and, to a more important degree, in the strong unconformity between the Carboniferous and the schists in contrast to the disconformable relations shown between the Carboniferous and Silurian fifty miles away at Pongo de Mainique. The mashing and crumpling that the schists have experienced at Puquiura is so intense, that were they a part of the Silurian series the latter should exhibit at least a slight unconformity in relation to the Carboniferous limestones deposited upon them.
If our interpretation of the relation of the schists to the slates and shales be correct, we should have a mountain-making period introduced in pre-Silurian time, affecting the accumulated sediments and bringing about their metamorphism and crumpling on a large scale. From the mountains and uplands thus created on the schists, sediments were washed into adjacent waters and accumulated as even-bedded and extensive sheets of sands and muds (the present slates, shales, quartzites, etc.). Nowhere do the sediments of the slate series show a conglomeratic phase; they are remarkably well-sorted and consist of material disposed with great regularity. Though they are coarsest at the bottom the lower beds do not show cross-bedding, ripple marking, or other signs of shallow-water conditions. Toward the upper part of the series these features, especially the ripple-marking, make their appearance. During the deposition of the last third of the series, and again just before the deposition of the limestone, the beds took on a predominantly arenaceous character associated with ripple marks and cross-bedding characteristic of shallow-water deposits.
In the persistence of arenaceous sediments throughout the series and the distribution of the ripple marks through the upper third of the beds, we have a clear indication that the degree of shallowness was sufficient to bring the bottom on which the sediments accumulated into the zone of current action and possibly wave action. It is also worth considering whether the currents involved were not of similar origin to those now a part of the great counter-clockwise movements in the southern seas. If so, their action would be peculiarly effective in the wide distribution of the sediment derived from a land mass on the eastern edge of a continental coast, since they would spread out the material to a greater and greater degree as they flowed into more southerly latitudes. Among geologic agents a broad ocean current of relatively uniform flow would produce the most uniform effects throughout a geologic period, in which many thousand feet of clastic sediments were being accumulated. A powerful ocean current would also work on flats (in contrast to the gradient required by near-shore processes), and at the same time be of such deep and steady flow as to result in neither ripple marks nor cross-bedding.
The increasing volume of shallow-water sediments of uniform character near the end of the Silurian, indicates great crustal stability at a level which brought about neither a marked gain nor loss of material to the region. At any rate we have here no Devonian sediments, a characteristic shared by almost all the great sedimentary formations of Peru. At the beginning of the Carboniferous the water deepened, and great heavy-bedded limestones appear with only thin shale partings through a vertical distance of several hundreds of feet. The enormous volume of Silurian sediments indicates the deep and prolonged erosion of the land masses then existing, a conclusion further supported (1) by the extensive development of the Silurian throughout Bolivia as well as Peru, (2) by the entire absence of coarse material whether at the top or bottom of the section, and (3) by the very limited extent of older rock now exposed even after repeated and irregular uplift and deep dissection. Indeed, from the latter very striking fact, it may be reasonably argued that in a general way the relief of the country was reduced to sea level at the close of the Silurian. Over the perfected grades of that time there would then be afforded an opportunity for the effective transportation of waste to the extreme limits of the land.
Further evidence of the great reduction of surface during the Silurian and Devonian is supplied by the extensive development of the Carboniferous strata. Their outcrops are now scattered across the higher portions of the Andean Cordillera and are prevailingly calcareous in their upper portions. Upon the eastern border of the Silurian they indicate marine conditions from the opening of the period, but at Pasaje in the Apurimac Valley they are marked by heavy beds of basal conglomerate and sandstone, and an abundance of ripple marking and other features associated with shallow-water and possibly near-shore conditions.
CARBONIFEROUS
Carboniferous strata are distributed along the seventy-third meridian and rival in extent the volcanic material that forms the western border of the Andes. They range in character from basal conglomerates, sandstones, and shales of limited development, to enormous beds of extremely resistant blue limestone, in general well supplied with fossils. On the eastern border of the Andes they are abruptly terminated by a great fault, the continuation northward of the marginal fault recognized in eastern Bolivia by Minchin[51] and farther north by the writer.[52] Coarse red sandstones with conglomeratic phase abut sharply and with moderate inclination against almost vertical sandstones and limestones of Carboniferous age. The break between the vertical limestones and the gently inclined sandstones is marked by a prominent scarp nearly four thousand feet high (Fig. 159), and the limestone itself forms a high ridge through which the Urubamba has cut a narrow gateway, the celebrated Pongo de Mainique.
Fig. 159—Topographic and structural section at the northeastern border of the Peruvian Andes. The slates are probably Silurian, the fossiliferous limestones are known Carboniferous, and the sandstones are Tertiary grading up to Pleistocene.
At Pasaje, on the western side of the Apurimac, the Carboniferous again appears resting upon the old schists described on p. 236. It is steeply upturned, in places vertical, is highly conglomeratic, and in a belt a half-mile wide it forms true badlands topography. It is succeeded by evenly bedded sandstones of fine and coarse composition in alternate beds, then follow shales and sandstones and finally the enormous beds of limestone that characterize the series. The structure is on the whole relatively simple in this region, the character and attitude of the beds indicating their accumulation in a nearly horizontal position. Since the basal conglomerate contains only pebbles and stones derived from the subjacent schists and does not contain granites like those in the Cordillera Vilcapampa batholith on the east it is concluded that the batholithic invasion was accompanied by the compression and tilting of the Carboniferous beds and that the batholith itself is post-Carboniferous. From the ridge summits above Huascatay and in the deep valleys thereabouts the Carboniferous strata may be seen to extend far toward the west, and also to have great extent north and south. Because of their dissected, bare, and, therefore, well-exposed condition they present exceptional opportunities for the study of Carboniferous geology in central Peru.
Fig. 160—The deformative effects of the granite intrusion of the Cordillera Vilcapampa are here shown as transmitted through ancient schists to the overlying conglomerates, sandstones, and limestones of Carboniferous age, in the Apurimac Valley at Pasaje.
Carboniferous strata again appear at Puquiura, Vilcapampa, and Pampaconas. They are sharply upturned against the Vilcapampa batholith and associated volcanic material, chiefly basalt, porphyry, and various tuffs and related breccias. The Carboniferous beds are here more arenaceous, consisting chiefly of alternating beds of sandstone and shale. The lowermost beds, as at Pongo de Mainique, are dominantly marine, fossiliferous limestone beds having a thickness estimated to be over two miles.
From Huascatay westward and southward the Carboniferous is in part displaced by secondary batholiths of granite, in part cut off or crowded aside by igneous intrusions of later date, and in still larger part buried under great masses of Tertiary volcanic material. Nevertheless, it remains the dominating rock type over the whole stretch of country from Huascatay to Huancarama. In the northwestern part of the Abancay sheet its effect on the landscape may be observed in the knife-like ridge extending from west to east just above Huambo. Above Chuquibambilla it again outcrops, resting upon a thick resistant quartzite of unknown age, 162 . It is strongly developed about Huadquirca and Antabamba and, still associated with a quartzite floor, it finally disappears under the lavas of the great volcanic field on the western border of the Andes. Figs. 141 and 142 show its relation to the invading granite batholiths and 162 shows further structural features as developed about Antabamba where the great volcanic field of the Maritime Cordillera begins.
Fig. 161—Types of deformation north of Lambrama near Sotospampa. A dark basaltic rock has invaded both granite-gneiss and slate. Sills and dikes occur in great numbers. The topographic depression in the profile is the Lambrama Valley. See the Lambrama Quadrangle.
Both the enormous thickness of the Carboniferous limestone series and the absence of clastic members over great areas in the upper portion of the series prove the widespread extent of the Carboniferous seas and their former occurrence in large interlimestone tracts from which they have since been eroded. At Puquiura they extend far over the schist, in fact almost completely conceal it; at Pasaje they formerly covered the mica-schists extensively, their erosion in both cases being conditioned by the pronounced uplift and marginal deformation which accompanied the development of the Vilcapampa batholith.
Fig. 162—Sketch sections at Antabamba to show (a) deformed limestones on the upper edge of the geologic map, 163 A; and (b) the structural relations of limestone and quartzite. See also Fig. 163.
The degree of deformation of the Carboniferous sediments varies between simple uplift through moderate folding and complex disturbances resulting in nearly vertical attitudes. The simplest structures are represented at Pasaje, where the uplift of the intruded schists, marginal to the Vilcapampa batholith, has produced an enormous monoclinal fold exposing the entire section from basal conglomerates and sandstones to the thickest limestone. Above Chuquibambilla the limestones have been uplifted and very gently folded by the invasion of granite associated with the main batholith and several satellitic batholiths of limited extent. A higher degree of complexity is shown at Pampaconas (Fig. 141), where the main monoclinal fold is traversed almost at right angles by secondary folds of great amplitude. The limestones are there carried to the limit of the winter snows almost at the summit of the Cordillera. The crest of each secondary anticline rises to form a group of conspicuous peaks and tabular ridges. Higher in the section, as at Puquiura, the sandstones are thrown into a series of huge anticlines and synclines, apparently by the marginal compression brought about at the time of the intrusion of the granite core of the range. At Pongo de Mainique the whole of the visible Carboniferous is practically vertical, and is cut off by a great fault marking the abrupt eastern border of the Cordillera.
Fig. 163—Geologic sketch section to show the relation of the volcanic flows of Fig. 164 to the sandstones and quartzites beneath.
It is noteworthy that the farther east the Carboniferous extends the more dominantly marine it becomes, though marine beds of great thickness constitute a large part of the series in whatever location. From Huascatay westward the limestones become more and more argillaceous, and finally give way altogether to an enormous thickness of shales, sandstones, and thin conglomerates. These were observed to extend with strong inclination westward out of the region studied and into and under the volcanoes crowning the western border of the Cordillera. Along the line of traverse opportunity was not afforded for further study of this aspect of the series, since our route led generally along the strike rather than along the dip of the beds. It is interesting to note, however, that these observations as to the increasing amounts of clastic material in a westward direction were afterwards confirmed by Señor José Bravo, the Director of the Bureau of Mines at Lima, who had found Carboniferous land plants in shales at Pacasmayo, the only fossils of their kind found in Peru. Formerly it had been supposed that non-marine Carboniferous was not represented in Peru. From the varied nature of the flora, the great thickness of the shales in which the specimens were collected, and the fact that the dominantly marine Carboniferous elsewhere in Peru is of great extent, it is concluded that the land upon which the plants grew had a considerable area and probably extended far west of the present coast line. Since its emergence it has passed through several orogenic movements. These have resulted in the uplift of the marine portion of the Carboniferous, while the terrestrial deposits seem to have all but disappeared in the down-sunken blocks of the ocean floor, west of the great fault developed along the margin of the Cordillera. The following figures are graphic representations of this hypothesis.
Fig. 164—Geologic sketch map and section, Antabamba region. The Antabamba River has cut through almost the entire series of bedded strata.
Fig. 165—The upper diagram (A) represents the hypothetical distribution of land and sea during the Carboniferous Period, as inferred from the present distribution and character of Carboniferous limestones and slates. The lower diagram (B) represents the present relief. The dotted line at the left of the two diagrams connects identical points. The fragmentation of the former continental border is believed to have left only a small portion of a former coastal chain and to have been contemporaneous with the development of ocean abysses near the present shore.
The wide distribution of the Carboniferous sediments and especially the limestones, together with the uniformity of the fossil faunas, makes it certain that the sea extended entirely across the region now occupied by the Andes. However, from the relation of the Carboniferous to the basal schists, and the most conservative extension of the known Carboniferous, it may be inferred that the Carboniferous sea did not completely cover the entire area but was broken here and there by island masses in the form of an elongated archipelago. The presence of land plants in the Carboniferous of Pisco warrants the conclusion that a second island mass, possibly an island chain parallel to the first, extended along and west of the present shore.
CRETACEOUS
The Cretaceous formations are of very limited extent in the belt of country under consideration, in spite of their generally wide distribution in Peru. They are exposed distinctly only on the western border of the Cordillera and in special relations. In the gorge of Cotahuasi, over seven thousand feet deep, about two thousand feet of Cretaceous limestones are exposed. The series includes only a very resistant blue limestone and terminates abruptly along a well-marked and highly irregular erosion surface covered by almost a mile of volcanic material, chiefly lava flows. The character of the bottom of the section is likewise unknown, since it lies apparently far below the present level of erosion.
Fig. 166—Geologic sketch map and cross-section in the Cotahuasi Canyon at Cotahuasi. With a slight gap this figure continues Fig. 167 to the left. The section represents a spur of the main plateau about 1,500 feet high in the center of the map.
The Cretaceous limestones of the Cotahuasi Canyon are everywhere greatly and irregularly disturbed. Typical conditions are represented in the maps and sections, Figs. 166 and 167. They are penetrated and tilted by igneous masses, apparently the feeders of the great lava sheets that form the western summit of the Cordillera. From the restricted development of the limestones along a western border zone it might be inferred that they represent a very limited marine invasion. It is certainly clear that great deformative movements were in progress from at least late Palæozoic time since all the Palæozoic deposits are broken abruptly down in this direction, and, except for such isolated occurrences as the land Carboniferous at Pacasmayo, are not found anywhere in the coastal region today. The Cretaceous is not only limited within a relatively narrow shore zone, but also, like the Palæozoic, it is broken down toward the west, not reappearing from beneath the Tertiary cover of the desert region or upon the granite-gneisses that form the foundation for all the known sedimentary strata of the immediate coast.
Fig. 167—Geologic sketch map and cross-section in the Cotahuasi Canyon at Taurisma, above Cotahuasi. The relations of limestone and lava flows in the center of the map and on a spur top near the canyon floor. Thousands of feet of lava extend upward from the flows that cap the limestone.
From these considerations I think we have a strong suggestion of the geologic date assignable to the development of the great fault that is the most strongly marked structural and physiographic feature of the west coast of South America. Since the development of this fault is so intimately related to the origin of the Pacific Ocean basin its study is of special importance. The points of chief interest may be summarized as follows:
(1) The character of the land Carboniferous implies a much greater extent of the land than is now visible.
(2) The progressive coarsening of the Carboniferous deposits westward and their land derivation, together with the great thickness of the series, point to an elevated land mass in process of erosion west of the series as a whole, that is west of the present coast.
(3) The restricted development of the Cretaceous seas upon the western border of the Carboniferous, and the still more restricted development of the Tertiary deposits between the mountains and the present coast, point to increasing definition of the submarine scarp through the Mesozoic and the Tertiary.
(4) The Tertiary deposits are all clearly derived from the present mountains and have been washed seaward down slopes with geographic relations approximately like those of the present.
(5) From the great width, deep dissection, and subsequent burial of the Tertiary terraces of the coast, it is clear that the greater part of the adjustment of the crust to which the bordering ocean basin is due was accomplished at least by mid-Tertiary time.
Fig. 168—Composite structure section representing the succession of rocks in the Urubamba Valley from Urubamba to Torontoy.
Aside from the fossiliferous limestones of known Cretaceous age there have been referred to the Cretaceous certain red sandstones and shales marked, especially in the central portions of the Cordillera, by the presence of large amounts of salt and gypsum. These beds were at first considered Permian, but Steinmann has since found at Potosí related and similar formations with Cretaceous fossils. In this connection it is also necessary to add that the great red sandstone series forming the eastern border of the Andes in Bolivia is of uncertain age and has likewise been referred to the Cretaceous, though the matter of its age has not yet been definitely determined. In 1913 I found it appearing in northwestern Argentina in the Calchaquí Valley in a relation to the main Andean mass, similar to that displayed farther north. It contains fossils and its age was, therefore, readily determinable there.[53]
In the Peruvian field the red beds of questionable age were not examined in sufficient detail to make possible a definite age determination. They occur in a great and only moderately disturbed series in the Anta basin north of Cuzco, but are there not fossiliferous. The northeastern side of the hill back of Puqura (of the Anta basin: to be distinguished from Puquiura in the Vilcabamba Valley) is composed largely of rocks of this class. In a few places their calcareous members have been weathered out in such a manner as to show karst topography. Where they occur on the well-drained brow of a bluff the caves are used in place of houses by Indian farmers. The large and strikingly beautiful Lake Huaipo, ten miles north of Anta, and several smaller, neighboring lakes, appear to have originated in solution depressions formed in these beds.
Fig. 169—The line of unconformity between the igneous basement rocks (agglomerates at this point) and the quartzites and sandstones of the Urubamba Valley, between the town of Urubamba and Ollantaytambo.
Fig. 170—The inclined lower and horizontal upper sandstone on the southeastern wall of the Majes Valley at Hacienda Cantas. The section is a half-mile high.
The structural relation of the red sandstone series to the older rocks is well displayed about half-way between Urubamba and Ollantaytambo in the deep Urubamba Valley. The basal rocks are slaty schist and granite succeeded by agglomerates and basalt porphyries upon whose eroded surfaces (Fig. 169) are gray to yellow cross-bedded sandstones. Within a few hundred feet of the unconformity gypsum deposits begin to appear and increase in number to such an extent that the resulting soil is in places rendered worthless. Copper-stained bands are also common near the bottom of the series, but these are confined to the lower beds. Higher up in the section, for example, just above the gorge between Urubamba and Ollantaytambo, even-bedded sandstones occur whose most prominent characteristic is the regular succession of coarse and fine sandstone beds. Such alternations of character in sedimentary rocks are commonly marked by alternating shales and sandstones, but in this locality shales are practically absent. Toward the top of the section gypsum deposits again appear first as beds and later, as in the case of the hill-slope on the southern shore of Lake Huaipo, as veins and irregular masses of gypsum. The top of the deformed Cretaceous (?) is eroded and again covered unconformably by practically flat-lying Tertiary deposits.
TERTIARY
The Tertiary deposits of the region under discussion are limited to three regions: (1) the extreme eastern border of the main Cordillera, (2) intermontane basins, the largest and most important of which are (a) the Cuzco basin and (b) the Titicaca-Poopó basin on the Peruvian-Bolivian frontier, and (3) in the west-coast desert and in places upon the huge terraces that form a striking feature of the topography of the coast of Peru.
It has already been pointed out that the eastern border of the Cordillera is marked by a fault of great but undetermined throw, whose topographic importance may be estimated from the fact that even after prolonged erosion it stands nearly four thousand feet high. Cross-bedded and ripple-marked features and small lenses of conglomerate are common. The beds now dip at an angle approximately 20° to 50° northward at the base of the scarp, but have decreasing dip as they extend farther north and east. It is noteworthy that the deposits become distinctly conglomeratic as flatter dips are attained, and that there seems to have been a steady accumulation of detrital material from the mountains for a long period, since the deposits pass in unbroken succession from the highly indurated and massive beds of the mountain base to loose conglomerates that now weather down much like an ordinary gravel bank. In a few places just below the mouth of the Ticumpinea, logs about six inches in diameter were observed embeded in the deposits, but these belong distinctly to the upper horizons.
The border deposits, though they vary in dip from nearly flat to 50°, are everywhere somewhat inclined and now lie up to several hundred feet above the level of the Urubamba River. Their upper surface is moderately dissected, the degree of dissection being most pronounced where the dips are steepest and the height greatest. In fact, the attitude of the deposits and their progressive change in character point toward, if they do not actually prove, the steady and progressive character of the beds first deposited and their erosion and redeposition in beds now higher in the series.
Upon the eroded upper surfaces of the inclined border deposits, gravel beds have been laid which, from evidence discussed in a later paragraph, are without doubt referable to the Pleistocene. These in turn are now dissected. They do not extend to the highest summits of the deformed beds but are confined, so far as observations have gone, to elevations about one hundred feet above the river. From the evidence that the overlying horizontal beds are Pleistocene, the thick, inclined beds are referred to Tertiary age, though they are nowhere fossiliferous.
Observations along the Urubamba River were extended as far northward as the mouth of the Timpia, one of the larger tributaries. Upon returning from this point by land a wide view of the country was gained from the four-thousand-foot ridge of vertical Carboniferous limestone, in which it appeared that low and irregular strike ridges continue the features of the Tertiary displayed along the mountain front far northward as well as eastward, to a point where the higher ridges and low mountains of older rock again appear—the last outliers of the Andean system in Peru. Unfortunately time enough was not available for an extension of the trip to these localities whose geologic characters still remain entirely unknown. From the topographic aspects of the country, it is, however, reasonably certain that the whole intervening depression between these outlying ranges and the border of the main Cordillera, is filled with inclined and now dissected and partly covered Tertiary strata. The elevation of the upper surface does not, however, remain the same; it appears to decrease steadily and the youngest Tertiary strata disappear from view below the sediments of either the Pleistocene or the present river gravels. In the more central parts of the depression occupied by the Urubamba Valley, only knobs or ridges project here and there above the general level.
The Coastal Tertiary
The Tertiary deposits of the Peruvian desert region southwest of the Andes have many special features related to coastal deformation, changes of climate, and great Andean uplifts. They lie between the west coast of Peru at Camaná and the high, lava-covered country that forms the western border of the Andes and in places are over a mile thick. They are non-fossiliferous, cross-bedded, ripple-marked, and have abundant lenses of conglomerate of all sizes. The beds rest upon an irregular floor developed upon a varied mass of rocks. In some places the basement consists of old strata, strongly deformed and eroded. In other places it consists of a granite allied in character and probably in origin with the old granite-gneiss of the Coast Range toward the west. Elsewhere the rock is lava, evidently the earliest in the great series of volcanic flows that form this portion of the Andes.
The deposits on the western border of the Andes are excellently exposed in the Majes Valley, one of the most famous in Peru, though its fame rests rather upon the excellence and abundance of its vineyards and wines than its splendid geologic sections. Its head lies near the base of the snow-capped peaks of Coropuna; its mouth is at Camaná on the Pacific, a hundred miles north of Mollendo. It is both narrow and deep; one may ride across its floor anywhere in a half hour. In places it is a narrow canyon. Above Cantas it is sunk nearly a mile below the level of the desert upland through which it flows. Along its borders are exposed basal granites, old sedimentaries, and lavas; inter-bedded with it are other lavas that lie near the base of the great volcanic series; through it still project the old granites of the Coast Range; and upon it have been accumulated additional volcanic rocks, wind-blown deposits, and, finally, coarse wash formed during the glacial period. From both the variety of the formations, the small amount of marginal dissection, and the excellent exposures made possible by the deep erosion and desert climate, the Majes Valley is one of the most profitable places in Peru for physiographic and geologic study.