ORIGIN OF SHINGLE 133 



extended inland up to the present foothills of the Cordil- 

 leras. During a part of this time the climate seems to have 

 been even colder than at present, and according to Hatcher, 

 in the central mountain area, ice sheets and glaciers formed 

 extensively, which, as in their flow they came to the sea, 

 brought great quantities of debris. When the ice broke 

 away and floated as icebergs out to sea, the debris (stones 

 and gravel) dropped to the bottom as the ice melted, strew- 

 ing the whole sea floor with several feet of this material. 



In the latter part of the Pliocene and during the Pleis- 

 tocene the final elevation (still going on) of the land took 

 place. Then the action of the tides and waves destroyed 

 much of the former marine deposits, and in particular, 

 worked over the stones and gravel dropped by the ice, 

 making them into a layer of rounded pebbles, which forms 

 the shingle mentioned on page 57, and covers the whole 

 of the Patagonian pampa as far as the foothills of the 

 mountains. This formation of shingle is still going on, 

 as is readily seen anywhere along the beach. Almost every- 

 where the beach is made of rounded pebbles to the depth 

 of ten to fifteen feet, and the material is not from the 

 adjacent bluffs, but consists of all sorts of trap rocks, which 

 occur in abundance far in the interior. No other cause 

 except the ice previously mentioned will explain their 

 presence here on the beach. The rising of the land going 

 on today is clearly indicated at numerous points by the 

 series of beaches one behind the other extending far inland. 

 At Solano Bay, for instance, we counted eight such beaches. 



The animals represented by our collections of bones 

 represent a very advanced and specialized fauna comparable 

 to none found on any other continent. It is from the 

 relationships of the land animals that conclusions as to 

 whether they have migrated from one continent to another 

 are formed, and thus as to what connections a continent 

 had. 



The geography of South America during the time when 



