1. If movements in the earth’s crust are predominantly downward, sedimentation may be carried on continuously, and a clear geologic record may be made.
2. Even if crustal movements are alternately downward and upward, satisfactory conclusions may be drawn from both (a) the nature of the buried surfaces of erosion, and (b) the alternating character of the sediments.
3. If, however, the deformative processes effect steady or intermittent uplifts, there may be no sediments, at least within the limits of the positive crustal units, and a geologic record must be derived not from sedimentary deposits but from topographic forms. We speak of the lost intervals represented by stratigraphic breaks or unconformities and commonly emphasize our ignorance concerning them. The longest, and, from the human standpoint, the most important, break in the sedimentary record is that of the present wherever degradation is the predominant physiographic process. Unlike the others the lost interval of the present is not lost, if we may so put it, but is in our possession, and may be definitely described as a concrete thing. It is the physiography of today.
Even where long-buried surfaces of erosion are exposed to view, as in northern Wisconsin, where the Pre-Cambrian paleo-plain projects from beneath the Paleozoic sediments, or, as in New Jersey and southeastern Pennsylvania, where the surface developed on the crystalline rocks became by depression the floor of the Triassic and by more recent uplift and erosion has been exposed to view,—even in such cases the exposures are of small extent and give us at best but meager records. In short, many of the breaks in the geologic record are of such long duration as to make imperative the use of physiographic principles and methods. The great Appalachian System of eastern North America has been a land area practically since the end of the Paleozoic. In the Central Andes the “lost interval,” from the standpoint of the sedimentary, record, dates from the close of the Cretaceous, except in a few local intermont basins partially filled with Tertiary or Pleistocene deposits. Physiographic interpretations, therefore, serve the double purpose of supplying a part of the geologic record while at the same time forming a basis for the scientific study of the surface distribution of living forms.
The geologic dates of origin of the principal topographic forms of the Central Andes may be determined with a fair degree of accuracy. Geologic studies in Peru and Bolivia have emphasized the wide distribution of the Cretaceous formations. They consist principally of thick limestones above and sandstones and conglomerates below, and thus represent extensive marine submergence of the earth’s crust in the Cretaceous where now there are very lofty mountains. The Cretaceous deposits are everywhere strongly deformed or uplifted to a great height, and all have been deeply eroded. They were involved, together with other and much older sediments, in the erosion cycle which resulted in the development of the widely extended series of mature slopes already described. From low scattered island elevations projecting above sea level, as in the Cretaceous period, the Andes were transformed by compression and uplift to a rugged mountain belt subjected to deep and powerful erosion. The products of erosion were in part swept into the adjacent seas, in part accumulated on the floors of intermont basins, as in the great interior basins of Titicaca and Poopó.
Since the early Tertiary strata are themselves deformed from once simple and approximately horizontal structures and subjected to moderate tilting and faulting, it follows that mountain-making movements again affected the region during later Tertiary. They did not, however, produce extreme effects. They did stimulate erosion and bring about a reorganization of all the slopes with respect to the new levels.
This agrees closely with a second line of evidence which rests upon an independent basis. The alluvial fill which lies upon all the canyon and valley floors is of glacial origin, as shown by its interlocking relations with morainal deposits at the valley heads. It is now in process of dissection and since its deposition in the Pleistocene had been eroded on the average about 200 feet. Clearly, to form a 3,000-foot canyon in hard rock requires much more time than to deposit and again partially to excavate an alluvial fill several hundred feet deep. Moreover, the glacial material is coarse throughout, and was built up rapidly and dissected rapidly. In most cases, furthermore, coarse material at the bottom of the glacial series rests directly upon the rock of a narrow and ungraded valley floor. From these and allied facts it is concluded that there is no long time interval represented by the transitions from degrading to aggrading processes and back again. The early Pleistocene, therefore, seems quite too short a period in which to produce the bold forms and effect the deep erosion which marks the period between the close of the mature cycle and the beginnings of deposition in the Pleistocene.
The alternative conclusion is that the greater part of the canyon cutting was effected in the late Tertiary, and that it continued into the early Pleistocene until further erosion was halted by changed climatic conditions and the augmented delivery of land waste to all the streams. The final development of the well-graded high-level slopes is, therefore, closely confined to a small portion of the Tertiary. The closest estimate which the facts support appears to be Miocene or early Pliocene. It is clear, however, that only the culmination of the period can be definitely assigned. Erosion was in full progress at the close of the Cretaceous and by middle Tertiary had effected vast changes in the landscape. The Tertiary strata are marked by coarse basal deposit and by thin and very fine top deposits. Though their deformed condition indicates a period of crustal disturbance, the Tertiary beds give no indication of wholesale transformations. They indicate chiefly tilting and moderate and normal faulting. The previously developed effects of erosion were, therefore, not radically modified. The surface was thus in large measure prepared by erosion in the early Tertiary for its final condition of maturity reached during the early Pliocene.
It seems appropriate, in concluding this chapter, to summarize in its main outlines the physiography of southern Peru, partly to condense the extended discussion of the preceding paragraphs, and partly to supply a background for the three chapters that follow. The outstanding features are broad plateau areas separated by well-defined “Cordilleras.” The plateau divisions are not everywhere of the same origin. Those southwest of Cuzco ([Fig. 130]), and in the Anta Basin ([Fig. 124]), northwest of Cuzco, are due to prolonged erosion and may be defined as peneplane surfaces uplifted to a great height. They are now bordered on the one hand by deep valleys and troughs and basins of erosion and deformation; and, on the other hand, by residual elevations that owe their present topography to glacial erosion superimposed upon the normal erosion of the peneplane cycle. The residuals form true mountain chains like the Cordillera Vilcanota and Cordillera Vilcapampa; the depressions due to erosion or deformation or both are either basins like those of Anta and Cuzco or valleys of the canyon type like the Urubamba canyon; the plateaus are broad rolling surfaces, the punas of the Peruvian Andes.
There are two other types of plateaus. The one represents a mature stage in the erosion cycle instead of an ultimate stage; the other is volcanic in origin. The former is best developed about Antabamba (Figs. 122 and 123), where again deep canyons and residual ranges form the borders of the plateau remnants. The latter is well developed above Cotahuasi and in its simplest form is represented in [133] . Its surface is the top of a vast accumulation of lavas in places over a mile thick. While rough in detail it is astonishingly smooth in a broad view ([Fig. 29]). Above it rise two types of elevations: first, isolated volcanic cones of great extent surrounded by huge lava flows of considerable relief; and second, discontinuous lines of peaks where volcanic cones of less extent are crowded closely together. The former type is displayed on the Coropuna Quadrangle, the latter on the Cotahuasi and La Cumbre Quadrangles.