MODIFIED DRIFT IN NEW HAMPSHIRE, 15 



Plains and Terraces. The extensive level plains and high terraces 

 which border our rivers, constituting the most conspicuous and by far 

 the largest portion of our modified drift, were also deposited in the 

 Champlain period. The open valleys became gradually filled with great 

 depths of horizontally stratified gravel, sand, and clay, which were brought 

 down by the glacial rivers from the melting ice-sheet, or washed from 

 the till after the ice had retreated, and which were deposited in the same 

 way as by high floods at the present time. The departing ice-sheet was 

 the principal 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 hori- 

 zontal stratification of these deposits shows that they were spread over 

 large areas by the current of the floods which held them in suspension. 

 The modified drift thus increased 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. 



The Terrace Period. 



During the recent or terrace period \h.Q rivers have been at work exca- 

 vating 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 insuf- 

 ficient 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, 

 undermining 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-depos- 

 ited 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 the Connecticut river, along the greater 

 part of its course on the west border of New Hampshire, has excavated 

 its ancient high flood-plain of the Champlain period to a depth of from 

 one hundred and fifty to two hundred feet for a width varying from one 



