42 UNIVERSITY OF COLORADO STUDIES 



wind and from slight thawing and freezing at the surface, and to 

 become covered with wind-blown or other debris, is probably sufficient 

 to mark off the old deposit from the fresh deposit above. Thus, 

 under some circumstances, a few days of warm winds may serve to 

 produce hardened dust-covered layers, and thus separate the snow- 

 falls of successive storms, dividing the aggregate snowfall of the year 

 into thin laminae. Then the more prolonged rest from deposition 

 in the summer, with its active melting and continued dusting, may 

 separate the groups of laminae representing annual snowfalls from 

 each other by a more pronounced Hne of demarkation. Then cycles 

 of years of excessive dust-storms and reduced precipitation, alternat- 

 ing with cycles of reverse conditions, may give to the broad zones 

 containing the snows of the one class of cycles a much larger propor- 

 tion of debris, which at a distance would give it a decidedly different 

 color. In Arapahoe Glacier, and probably in most mountain glaciers, 

 another well-known type of dirt-band exists, formed by the accumula- 

 tion of dust in the mel ted-back lips of "pinched crevasses" — that is, 

 crevasses that have closed as the ice moved forward beyond the change 

 of slope which produced them. Such pinched-crevasse dirt-bands are 

 sometimes parallel with the stratification and sometimes cut the 

 latter at considerable angle, as would be expected. Sherzer also 

 illustrates a type of fine dust laminae quite superficial and only show- 

 ing at close view. 



Icebergs. — Icebergs in the small lakelets which occur at the foot 

 of the ice when a glacier or other ice-field has rapidly shrunk away 

 from the old moraine, have been taken sometimes to indicate an active 

 glacier, the breaking-away of the ice being attributed to its forward 

 movement into the lake, as occurs in case of the northern glaciers 

 which discharge into the ocean. It is very doubtful whether any of 

 the Colorado instances noticed are due to such a cause. It seems 

 impossible, considering the depth and slope of the ice-fields and 

 the shallowness of the lakes. I have found such icebergs in lakes at 

 the foot of consolidated snow, which showed no evidence whatever 

 of motion, as well as at the foot of neve-^e\ds where the motion was 

 surely not sufficient to produce such results. In the winter the waters 



