540 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



In another way, though rarely, the till may simulate the bedding of 

 sedimentary deposits, where it is thickened into dome-shajied drumlins, and 

 it will be seen later on that these are common in the valley. I have once 

 or twice seen a rude separation into thick, irregular layers molded into each 

 other and distinguishable only in a view of a broad surface at a distance. 

 This occurs at the section mentioned at the bridge in Leeds. It would seem 

 that the ice pushed one layer after another into the accumulating mass and 

 so gradually built it up. 



In the foregoing discussion of the various phenomena of the valley 

 drift I have assumed its subglaeial origin, though many of the details per- 

 haps would fit equally well with the idea that the mass was pushed out 

 from the front of the ice as it retreated northward with various oscillations. 

 The great compactness of the whole and the pressure cleavage would hardly 



tYsf-er/eifeJ be/oyv c/am ', continuec/onf/ie5urf3ceoff/ief//la-^b. 

 '6round moraine lof^^T. 



-G. 31 — Section on tlie left bank of the Mill River .it tlie hoe f.ictory, Northampton, taken after the washout of 

 1878, which carried the dam away, showing the atriio on the surface of sandstone continued on the surface of the till below. 



be explicable upon this assumption, and I have now to describe two sections 

 which render it certain that the whole mass is of subglaeial formation. 



On the night of December 10, 1878, the Mill River, flooded by the 

 very abundant rains and liy the breaking awav of several dams on its head- 

 waters, rose in Northam})ton to a height greater than on the occasion of the 

 flood of May 1(5, 1874, which caused so great a loss of property and life, 

 and was less destructive only because the earlier flood had done its work 

 so thoroughly. It carried away the western part of the dam at the hoe 

 factory in Northampton and wore deeply into the western bank, ex})osing 

 the section seen in fig. 3 1 . 



The dam had been built on a reef of coarse red sandstone which ran 

 diagonally across the stream from northwest to southeast, the stream flowing 

 here from north to south, and the section runs in the latter direction. The 



