HISTORY OF DERRYFlELD. I7 



to the east front, ranging north and south, the valley of the 

 Merrimack, and between these the lesser valley of Black Brook. 

 From the point of time we have chosen — a matter of seventy 

 or eighty thousand years ago — these little resemble the peace- 

 ful landscapes with which we are now acquainted. 



Three powerful, ice-fed streams, terrible in their energy, are 

 forcing their way southward, carving channels as they move ; 

 bursting their banks, assaulting rocky barriers, raging, roaring, 

 eroding ; with counter and cross-currents, eddies, whirlpools, 

 horrible, precipitous narrows, and tremendous rapids, forerun- 

 ners of still more tremendous cataracts. Borne along and 

 whirled hither and yon in the midst of these frightful torrents 

 we see indistinguishable masses of debris and angular blocks of 

 frozen clay, with an interminable procession of rifted fragments 

 of inland icebergs, accompanied with stones and rocks of differ- 

 ing dimensions, from the pebble to the bowlder. Add to this 

 the gloom of a cloudy sky, the ceaseless fall of rain, the riot of 

 winds, the song of the tempest. Try to picture the indescriba- 

 ble, continuous rush and turmoil of the elements, the intermit- 

 tent thunder of the pounding ice and bowlders, then turn to the 

 shrunken rivers of to-day. 



The figures of the transporting power of water are startling. 

 We know the force is as the sixth power of the velocity ; that 

 is, by doubling the rate we increase the power sixty four times. 

 To give concrete examples : A stream running at the rate of 

 three inches per second will wear away fine, tough clay ; with 

 a velocity of thirty-six inches per second the current will remove 

 angular fragments of rock from two to three inches in diameter. 

 The latter rate is quite moderate — a little more than two miles 

 an hour — and presents but a picture in little of the rapidity of 

 our earlier floods. We have taken no account of the influence 

 of gravity operating on descending slopes, and we may also call 

 to mind the fact that rocks lose nearly one-third of their weight 

 in water. 

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