during the melting of the great Glacier. 179 



(a) Such obstructions would have been of all grades, from 

 that which could simply impede the free flow of the waters, to 

 the nearly perfect dam. 



Q)) The obstructions in particular cases might have existed 

 for a very long era, instead of for a few weeks such as hap- 

 pens after a modern winter. 



(c) Again, the slackened or suspended flow of the water, 

 caused by such ice-obstructions, would have favored the depo- 

 sition and accumulation about them of drift, and some may 

 have thus been converted into complete dams. This process 

 might occasionally have wholly filled with earthy material a 

 gorge or narrow valley, so as to block up and divert the course 

 of the stream.— The well-known case of Niagara River may be 

 an example of this. 



In view of these possible results, or rather these probable con- 

 ditions of many river-valleys in the era of the Glacial flood, we 

 are required to consider whether the height of the upper terraces 

 ahove the narrows on the several rivers, — the Thames below 

 Norwich, the Connecticut below Middletown, the Housatonic 

 below Derby, Westfield River below Westfield, and Farmington 

 River east of Tarifville — was not partly owing, in each case, to 

 the existence of ice-obstructions at the narrows. 



It seems to be very probable that this was so. The height of 

 modern spring floo(is in the Connecticut at Middletown and 

 Hartford is now often due in part to this very cause. 



It appears to be certain, that if such obstructions existed in 

 the Thames, Connecticut and Housatonic valleys, they were 

 only partial obstructions ; for, in the case of each, the terrace of 

 the valley below the narrows declines quite gradually in height 

 from the level above the narrows, instead of abruptly. Had 

 the waters been held back, up to the height of the high upper 

 terrace, by a close dam, they would have fallen over the dam 

 with a plunge to a lower level ; and this abrupt fall would have 

 been registered by means of an abrupt fall in the level of the 

 terrace. Instead of this, the terrrace on passing the narrows 

 southward falls ofl" at a rate not exceeding 10 feet a mile, vary- 

 ing in rate only with the varying width of the valley: a fact 

 that seems to testify to the vastness of the flood as its cause, 

 and not mainly to obstructions. Moreover, the material of the 

 terraces below the narrows is like that above : 

 prevalence of sands below and i 

 naving the latter of greatei 

 rapid flow of the stream along a narrower valley. 



Further evidence with reference to the existence of such ice- 

 barriers is to be looked for in a distribution of gravel and large 

 bowlders across the valley just above the gorge or narrows, where 

 the ice-masses had been brought to a stop and piled together; 



