CORRASION BY GRAVITY STREAMS. 289 



decrease in corrasive power. At such points one could not 

 expect to find pronounced interruptions of channel slope. 



Similarly the apparent failure of the ice sheet to corrade 

 the Lockport limestone of the Niagara escarpment, while 

 it deepened the Cayuga and Seneca Lake Basins may be 

 explained. (See Part III of this Series.) 



The greater the volume and velocity of the Ice Sheet? 

 the more pronounced will be the increase of velocity in the 

 basal ice when passing from the broad flaring valley into 

 the canon with much reduced cross-sectional area. But 

 over the senile, or old age topography, there is little or no 

 opportunity for differential velocity to be set up locally in 

 the Ice Sheet. In proportion then as the topography 

 approaches the plain stage, the more will the ice act as a 

 static body. 



(g) Drumlins. — Another very important point for con- 

 sideration is that of the further history of the morainic 

 debris during and after the readjustment of channel grades 

 attendant on decrease of stream volume. 



As the ice sheet passes over regions which have been 

 dissected to the senile stage, its transporting and corrasive 

 power will experience but little variation in individual 

 southern zones equidistant from the ideal glacial centre. 

 In the flood stage then, the ice tends to carry its ground 

 moraine in even manner over these regions of gentle relief. 

 Even at the flood stage, however, as the ice sheet enters 

 on to areas of gentle relief after leaving hilly couutry, it 

 will be characterised by some aggradation. But with a 

 decrease in the ice volume, its power of transportation of 

 the infra-glacial load will be greatly decreased. The great 

 ground moraines of the flood stages are now too large to 

 be carried along bodily with evenly distributed corrasion 

 of the area of gentle relief, such as we have been consider- 

 ing. The reduced ice sheet therefore at this stage seeks 



