and Deposits of Flooded Rivers. 35 



branches of trees, layers of peat, or patches of the old surface 

 soil, enclosed between deposits of the till or unmodified glacial 

 drift, and by numerous roughly parallel and interlocking belts 

 of hilly and knolly drift, amassed as terminal moraines at the 

 margin of the ice during its interrupted and oscillating final 

 retreat. 



Extensive and thick beds of gravel, sand, and clay or fine 

 silt, called stratified or modified drift, were deposited in valleys 

 which received the drainage from the glacial melting, especially 

 during its comparatively rapid progress in the Champlain period. 

 The dissolution of the ice, with accompanying rains, produced 

 extraordinary floods along all the rivers flowing away from the 

 waning ice-sheet ; and these were heavily laden with detritus 

 set free from the ice in which it had been held, and brought 

 down by the rills and small and large streams formed on the 

 melting ice surface. Other portions of the englacial drift were 

 let down as till in a loose unstratified mass upon the subglacial 

 till or ground moraine, making the Champlain epoch, as Pro- 

 fessor Dana has shown, preeminently one of abundant deposi- 

 tion, both of stratified and unstratified drift. 



This epoch was immediately succeeded by one of rapid 

 erosion of the valley deposits, as soon as the continued glacial 

 recession beyond the drainage areas of the rivers cut off the 

 supply of water and of drift that had been derived from the 

 melting ice. The resulting excavation of the glacial flood- 

 plains has left remnants of those deposits in conspicuous 

 terraces along all our river valleys which lead southward 

 within the glaciated region or on its southern border ; and 

 postglacial time, extending to the present day, is therefore 

 named by Dana the Recent or Terrace period. It is to be 

 remarked, however, that much of the terracing of the valley 

 drift was doubtless done speedily after the retreat of the ice 

 from any basin, while yet adjacent drainage areas on the north 

 were receiving from it thick flood-plain deposits. The Glacial, 

 Champlain, and Terrace periods thus overlap, the second being 

 wholly, and the third partially included within the Glacial or 

 Pleistocene period, if continental areas are considered ; but for 

 any limited district, as a single river basin, the sculpturing of 

 the terraces took place chiefly after the departure of the ice 

 beyond its watershed. 



Deposition and erosion by rivers are determined by their 

 rate of descent, their volume, and the amount of detritus which 

 they hold in suspension, received along the higher portions of 

 their course. All these conditions have been subject to im- 

 portant changes during the Quaternary era, not only for the 

 regions that became ice-covered in the Glacial period, but also 

 for many other parts of the earth's surface. The efficiency of 



