334 THE CANADIAN NATURALIST. [Vol. viii. 



in the Champlain period. The departing ice-sheet was the prin- 

 cipal source both of the vast amount of material and of water 

 for transporting it into the valleys, which appear in most cases 

 to have been filled to the level of the highest terraces or plains. 

 The prevailing horizontal stratification of these deposits show that 

 they were spread over large areas by the current of the floods 

 which held them in suspension. The modified drift thus in- 

 creased in depth in the principal valleys through a long period, 

 which may have continued until the last of the ice at the head of 

 the valley and of its tributaries had disappeared. 



During the recent or terrace period, the rivers have been at 

 work, excavating deep and wide channels in this alluvium. The 

 terraces mark heights at which, in this work of erosion, they 

 have left portions of their successive flood plains. As soon as 

 the supply of material became insufficient to fill the place of that 

 excavated by the river, a deep channel was gradually formed in 

 the broad flood-plain. The process was very slow, allowing the 

 river to continue for a long time at nearly the same level, under- 

 mining and wearing away its bank on one side, and depositing 

 the material on the opposite side, till a wide and nearly level 

 lower flood-plain would be formed, bordered on both sides by 

 steep terraces. When the current became turned, to wear away 

 the bank in the opposite direction, a large portion of this new 

 flood-plain would be undermined and re-deposited at a lower 

 level ; but the direction of the current's wear might be again 

 reversed in season to leave a narrow strip, which would then 

 form a lower terrace. In this way we often see the highest plain 

 on our large rivers, and the lower terraces very frequently, being 

 now undermined by the wear of the current, forming steep bluffs 

 and banks. The fine character of the materials which compose 

 the lowest terraces and the interval, or present flood plain, is due 

 to this wearing away and re- deposition by the river, which have 

 been many times repeated, till what may have been at first gravel 

 becomes very fine sand or silt. 



Neither the deposition nor terracing of the modified drift 

 requires any submergence, as by lakes or the sea. These deposits 

 have the form which they must naturally take, in being rapidly 

 brought into the valley by floods, and in afterward being partly 

 excavated by rivers in the process of deepening their channels. 



Along Connecticut river, for a distance of 120 miles south 

 from Fifteen-miles falls, and thence extending south into Massa 



