A GEOLOGIC SPKING TIME. 277 



posits were partially washed away. The moraine of the first 

 glacial epoch, farther south, was now subjected to the action 

 of a second flood. It suffered greater erosion than the second 

 moraine, and hence remains to us a less conspicuous feature 

 than the second. 



The ice-sheet had laid down an unstratified bed of "till" 

 a compact mass of clay, pebbles, and bowlders; the glacial 

 flood transported vast quantities of material, and left them in 

 a state of torrential stratification overspreading the till. There 

 is much reason to believe that the materials thus transported 

 were borne beyond the limits reached by the glacier. In 

 this way, the action of the glacial expedient for renovating 

 the surface of the north was extended to the southern states. 

 There has certainly been a southward transportation of peb- 

 bles and sand throughout all the Gulf states. It was an event 

 synchronous with the dissolution of the great glacier. But 

 we must bear in mind that the south had not been visited by 

 an agency which plowed up the disintegrated rocks accumu- 

 lated during preglacial ages. The flooding of the south ex- 

 erted only a surface action. 



Between the glacier and the floods, the surface of the 

 whole country east of the Great Plains with the exception 

 of a few small isolated areas underwent a process of thorough 

 repair. The sharp river gorges were filled even an ancient 

 gorge of the Niagara Kiver and a fresh bed of subsoil mate- 

 rials was spread over the land. The larger rivers sought out 

 the drainage valleys which they had occupied before the in- 

 vasion of glaciers. The fundamental features of the drainage 

 were everywhere determined by the underlying rocky struc- 

 ture. But many of the smaller streams which now sprang 

 into existence, selected for the first time their winding chan- 

 nels among the inequalities of the Drift-covered surface. From 

 that epoch to the present, all the streams have employed 

 themselves in effecting an ever deepening erosion. Of the 

 greater arteries of the continental drainage, the ancient pre- 

 glacial bounding walls may sometimes still be traced. The 

 high cliffs of the Upper Mississippi show where the great river 



