180 J. D. Dana—Damming of Streams hy drift ice. 



for imich of the floating ice would have been loaded with 

 bowlders. I have as yet observed no satisfactory evidence of 

 this kind, but think the question needs more investigation. 

 Even if this evidence fails, we can hardly assert that no aid was 

 afforded by ice in producing the great height of the flood-waters 

 above the narrows, or doubt that ice-barriers made of drift ice 

 had much to do with the height and extent of the upper ter- 

 races in portions of many other valleys. 



There are two questions which should have here a word. 



1. May not the obstructions or dams have been made by the 

 Glacier itself? On this point we observe that the extent of the 

 terrace formations along the valleys, — sometimes a score of 

 miles in width even in New England— show that water swept in 

 immense streams over the surface ; and thus they seem to prove 

 that the glacier was already out of the lower part of the valleys, 

 and hence too far away to have obstructed the flow except 

 through the pieces set afloat by its dissolution. 



2. Were not the dams due to rocky barriers at the narrows^ or to 

 the non-excavation of the valley from the narrows southward f The 

 features of the region about the narrows on each of the rivers 

 mentioned, and of the valleys below, suggest decidedly that 

 the valleys had nearly the same depth and extent then as now. 

 The gradual decline in the height of the terrace on going from 

 the narrows southward to the Sound shows that all was one 

 valley, the part above the narrows and its continuation below. 

 The terraces below the narrows, moreover, are built up in gen- 

 eral from i\\Q present bottom of the valley, or from a lower depth, 

 and this points to a depth for the valley as great as now or 

 greater. It cannot be urged that the lower portions of the ter- 

 races were made after the upper Wherever the hills on one 

 side, at the narrows, retreat so as to give a chance for high ter- 

 race deposits, there these deposits are usually found, and some- 

 times the beds rise abruptly from the water's edge to the level 

 of the highest terrace ; and on the Connecticut, in a place of 

 this kind above Middle Haddam, the bottom layers are of clay — 

 like the lower layers in much of the stratified drift on the river. 



In fact, the conditions of the terrace deposits of the valley, as 

 well as the features of the valley itself, are explicable only on 

 the view that the part of each valley below the narrows, like 

 the rest of it, the narrows included, had been made before the 

 Champlain period opened. The Glacial period was the era of 

 valley excavation rather than the Champlain period. 



