^86 



G. F. AVRIGHT POSTGLACIAL EROSION AND OXIDATION 



shed and occupy wide preglacial channels about 300 feet below the gen- 

 eral level of the country. In both cases the bottom of the troughs are 

 about a mile in width and as level as a floodplain, but in both cases the 

 trough is bordered upon the west side by gravel terraces from 100 to 200 

 feet above the present level of the stream. The readiest explanation of 

 these terraces is that they are composed of gravel deposited by torrents 

 from the melting ice, which flowed at that level between stagnant ice 

 which filled the valley and the sides of the preglacial rock gorge. Russell 

 has published photographs of a stream similarly situated, held at a level 

 2,000 feet above the sea, between the Malaspina Glacier and the flanks of 

 Mount Saint Elias. Emerson^ has thus given a rational explanation of 



m' 



Figure 4. — Oravel Accumulations along Section N-M in Figure S 

 A, bj c, (1, e, t, marks the contour of the preglacial valley 



many of the high-level terraces bordering the Connecticut River, which 

 had formerly been explained by Professor Dana as the results of incredi- 

 ble floods in the open trough of the river. An interesting feature in the 

 Killbuck, near Wooster, Ohio, is the occurrence of a deposit of till from 

 15 to 20 feet in thickness which has been pushed over this terrace without 

 disturbing its bedding. The elevation is here 200 feet above the flood- 

 plain of the river. It is interesting to remark, also, that both in the 

 "esker" terrace of the Styx and in that of the Killbuck, paleolithic im- 

 plements have been reported, but on evidence less exact than is necessary 

 to make them available for public discussion (see plate 11, figures 1 

 and 2 ) . 



^ B. K. Emerson : Geology of Old Hampshire County, Massachusetts. 

 Survey, monograph xxix. 



U. S. Q«oloKlcal 



