294 PATAGONIAN EXPEDITIONS: GEOGRAPHY. 



low bluffs extending along the heads of both of these bays are largely- 

 composed of sedimentary rocks covered over with only a thin layer of 

 glacial detritus, proving conclusively that the former connection that doubt- 

 less existed between these two bodies of water has been broken, not by a 

 damming up by glacial materials, but by an elevation sufficient to bring 

 the sedimentary rocks at the bottom above the water level. 



From the observations and conditions already referred to, and many 

 other facts bearing directly upon these questions, I believe that the longi- 

 tudinal valleys separating the main range from the two lateral ranges of 

 the Andes, and also the great transverse valleys crossing Patagonia from 

 east to west, had their origin previous to the last submergence, which took 

 place over this region in late Pliocene times and continued only for a rela- 

 tively short period. This submergence was greater over the western than 

 over the eastern Andes, thus rendering the western channels much deeper 

 than the eastern. Toward the close of the Pliocene there began over this 

 region a process of elevation, which, though general over the entire region, 

 was greatest along a line approximating that of the axis of the eastern 

 lateral range of the Andes, and was also greater over northern than over 

 southern Patagonia. As the elevation proceeded, the general surface of the 

 land would be brought above water level, while the longitudinal and trans- 

 verse valleys would remain submerged and appear respectively as channels 

 from the Pacific and as straits connecting the two oceans. This condition 

 may be termed the first stage in the process of elevation and is now seen 

 in the Magellan Strait. 



After a time a second stage would be reached, in which the, transverse 

 valleys would no longer appear as straits, but as land valleys, while the 

 channel of the eastern longitudinal valley would be broken up into a 

 series of fiords, extending inland from the continuous channels of the 

 deeper western longitudinal valley. This second stage is now seen in 

 the region lying between the Strait of Magellan on the south and Lake 

 Argentino on the north. 



A third stage appears when the amount of elevation accomplished is 

 sufficient to sever the connection existing between the east and west 

 longitudinal valleys and reduce the fiords entering the eastern valley to a 

 series of lakes discharging their waters by rivers into the channels of the 

 western valley, still submerged beneath the sea. This third stage is seen 

 in the region north of Lake Argentino, while a fourth stage, in which the 



