44 NEW YORK STATE MUSEUM 



consequent on such action that the manner of this be stated. 

 As the ice first passed over the country from the north it picked 

 up, and incorporated in its bottom layers, all the loose pebbles, 

 boulders and sand and soil particles that covered the preglacial sur- 

 face and moved these along southward. Moreover it pried loose 

 projecting blocks of the bedrock material, where such occurred, and 

 added these to its burden. Thus its bottom shortly became a rock- 

 and-earth-studded ice mass, pressed down on the floor of the coun- 

 try over which it moved forward by all the thickness of the overly- 

 ing ice. Since the ice eventually overtopped the highest of the Adi- 

 rondack summits it must have ultimately attained a thickness of sev- 

 eral thousand feet. After the unattached and projecting rock stuff had 

 been removed, the later advances of thicker ice wedged and plucked 

 loose fragments of the fresh bedrock; thus its bottom was continu- 

 ously shod with boulders and also with scouring material of a finer 

 sort. Aside then from its mass action as a plucking agent it is 

 apparent that this boulder and sand furnished bottom-ice, pressed 

 down by all the great thickness of the glacial mass above, must have 

 been a grinding and scouring agency of immense effectiveness and 

 hence been competent to carry away great quantities of rock mate- 

 rial during the thousands of years that the ice occupied the country. 

 There was, however, a south limit to the ice advance, and there, 

 as the ice melted, all the debris that had been picked up enroute was 

 necessarily dumped. Some of it was heaped up in ridges along the 

 front of the ice as it melted out; much of the finer material was 

 carried away from the immediate front by the streams created by 

 the ice melting. The heaped-up ridges of rock rubbish dropped by 

 the glacier are termed moraines; the material carried forward from 

 the front and distributed by streams is called outzvash. As the gla- 

 cial climate moderated and the supply of ice from the north was no 

 longer sufficiently great to maintain the front at the southernmost 

 latitudes reached at its maximum extension, the ice front itself 

 receded, not by any movement backward, but simply by melting back 

 faster than it could be supplied by flow forward. The interesting 

 fact in regard to this amelioration of the glacial climate is that it 

 apparently was not continuous, but occurred rather in distinct steps, 

 so that there were successive halts and stands of the ice front east 

 and west across the country. At each of these lines, where an 

 eqUjilibrium was established for a time between supply and melting, 

 the rock rubbish was heaped up in moraines, and outwash deposits 

 were formed, just as at the farthest front of advance; the mass of 

 such accumulations depending on the length of time that the halt 



