32 CONTRIBUTIONS TO THE 



these were attacked, re-transported and the materials re-arranged 

 through the agency of water, still flowing in great volume from 

 the receding ice to the northward. So that when we contem- 

 plate the fact that the bowlder clay and in fact the great bulk 

 of all unstratified drift was used over and over again, the prob- 

 lem of the origin of the great modern clay beds does not seem 

 obscure. 



Prof. Dana says the melting of the great ice-sheet was the 

 cause of mighty floods in the valleys, so vast as not to be com- 

 pared with those resulting from the breaking up of the ordinary 

 winter. He adds that with the melting of the lower one thous- 

 and feet of ice came the principal deposition of the coarser gravel 

 and stones, the material being " heaped pell-mell over the land," 

 This happy phrase accurately describes the condition which we 

 find prevailing to-day in the fields, pastures and plains about us. 

 A map of our farm-lands, drawn upon a scale to give the stone 

 wall division lines, would show an almost inconceivable bulk of 

 this material in single and double walls, while thousands of fields 

 dotted with the familiar rock-heaps, and numberless ravines, by- 

 places and road-side ways serving as unloading places for name- 

 less millions of tons of this "pell-mell" material, yet represent 

 but a very small fraction of the original deposit. These modest 

 monuments of New England thrift and industry give us but a 

 faint conception of the operation of the beneficent forces of na- 

 ture, which, while they seemed destructive, were making Earth 

 a fit abiding-place for man. We should add that most of the 

 material was at first left unstratified, while that which found its 

 way to'lake basins or to shoals and bars in flowing streams would 

 have become stratified, and that is precisely what is found in 

 the region under consideration. 



Dana also remarks the coarsely stony character of the upper 

 part of the terrace formation, and concludes that the glacial 

 flood was greatly and suddenly augmented in depth and violence 

 toward the close of the melting period. 



