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.