392 HISTORICAL GEOLOGY, 



wider and deeper old river-bed, R R, which is filled up 

 often several hundred feet deep with river-silt, b b, and 

 into this the river is now cutting its bed. The great river- 

 bed, R R, was cut out during the epoch of elevation (Gla- 

 cial) and previous periods. They are preglacial river- 

 beds. The filling was' done during this epoch of subsi- 

 dence (Champlain). The river since then has again cut 

 down, but not so deeply. All the rivers in the drift- 

 region, therefore, are bordered on each side by a wide 

 area of old silt, usually much above the present flood- 

 level, and therefore forming high bluffs or terraces, 

 sometimes one, sometimes many, on each side. 



The Cause of the flooded condition was primarily the 

 great water-supply from melting of the ice-sheet. But it is 

 evident that the subsidence of the land would cause the 

 sea to enter the mouths of many rivers, forming great 

 estuaries ; and also, by diminishing the slope of the river- 

 bed, would tend to increase their floods. 



From this subsided condition the land gradually rose 

 again, by successive stages, to the present condition. 

 These successive stages are marked by a succession of 

 sea-beaches, lake-terraces, and river-terraces, below the 

 highest just described. As the land rose, successive sea- 

 margins were left ; the outlet of the lakes also cut deeper 

 and deeper, and drained the lakes to lower and lower 

 levels. Also, all the rivers cut deeper and deeper into 

 the old Champlain silts, leaving them as bluffs and ter- 

 races high above the present flood-line (Fig. 348). Some- 

 times there is but one great bluff on each side, as in the 

 Mississippi River. Sometimes there are several terraces, 

 one above the other, as in the case of the Connecticut 

 River. It is evident that when Lake Champlain was first 

 cut off from the sea by elevation it was a salt lake. It 

 was freshened in the manner explained on page 79. 



