PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT 241 



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. 



CABBONIFEBOTJS 



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 develop- 

 ment, to enormous beds of extremely resistant blue limestone, in 

 general well supplied with fossils. On the eastern border of the 



FRONT RANGE 



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. 



Andes they are abruptly terminated by a great fault, the continua- 

 tion northward of the marginal fault recognized in eastern 

 Bolivia by Minchin 2 and farther north by the writer. 3 Coarse 

 red sandstones with conglomeratic phase abut sharply and with 

 moderate inclination against almost vertical sandstones and lime- 

 stones of Carboniferous age. The break between the vertical lime- 

 stones and the gently inclined sandstones is marked by a promi- 

 nent 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. 



At Pasaje, on the western side of the Apurimac, the Carbonifer- 

 ous 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. 



* Eastern Bolivia and the Gran Chaco, Proc. Royal Geogr. Soc, Vol. 3, 1881, pp. 

 401-420. 



• The Physiography of the Central Andes, Am. Journ. Sci., Vol. 28, 1909, p. 395. 



