226 
GEOLOOy OF THE FIRST DISTRICT. 
Massachusetts, where the inflence of the western and northern currents were also felt, the 
results are very obvious. 
The diluvial elevations and depressions of Prof. Hitchcock, or round-backed hills with 
bowl-shaped valleys, remain to be explained. I must confess my inability to do it satisfac¬ 
torily. Effects somewhat similar are produced where sand and snow are drifted along a 
surface on which there are slight obstacles, as gravel, pebbles or grass. Eddy currents of 
wind or water pile up materials drifted by them in a manner somewhat analogous, but the 
particles in all these cases are nearly uniform in size; but in the drift hills, the materials are 
of all sizes from large blocks and boulders to sand and clay. 
Circumstances were related by Capt.-, of the effects produced by icebergs grounding, 
and which would seem to offer a satisfactory explanation of the drift hills and valleys.* An 
ice island, when aground, grinds upon the bottom ; the earth and rocks frozen in its mass are 
gradually disengaged, and fall or are washed off, and sink to the bottom beneath and around 
it; and as the upper parts, where exposed to the warmer surface waters, melt most rapidly, 
it occasionally tumbles over. The materials sinking around it, and the grinding of the mass 
of ice in its bed by the action of currents, or by gyrations from changes of the centre of gra¬ 
vity, seem a plausible explanation of the phenomenon of the drift hills and valleys. A strong 
objection may be urged against it, and it applies with equal force to the transport of the drift 
deposits generally. The objection is, that if this be the explanation, we must infer that either 
the drift deposits of the diluvial elevations and depressions are composed of rocks from far 
to the north, where icebergs are supposed to have been formed ; or, that icebergs and glaciers 
have been formed on all the elevated lands of the country. 
Observations have shown that the greater portion of the drift is composed of such materials 
as exist in place at no great distance. Local ice, such as we now see formed and transporting 
detritus, although sufficient to afford a partial explanation of the transport of some of the 
drift, yet is not capable of explaining the phenomena of diluvial elevations and depressions as 
formed by icebergs ; neither will local ice such as we now see formed on our shores, suffice 
to have caused the scratches at great depths below the surface, as it is evident from the facts 
observed, must have been effected. 
Glaciers and icebergs, if they had been formed on our coasts, would not, in fact, be suffi¬ 
cient to account for all the phenomena; for the drift deposits are composed of rocks existing 
at all elevations, from the present tide-water level, to the tops of the high mountains; and 
glaciers and icebergs formed on the coasts as they then existed, could not have been formed 
as they are at present supposed to he formed, so as to enclose rocks at all these various levels, 
when most of the present land was deep below the level of the ocean. 
The formation of ground ice is the only supposition we are authorized to make, and on 
this subject there is little positive information. It is known to form where the water is more 
than one hundred feet in depth, and to raise from the bottom great quantities of earth and 
* American Journal of Science, Vol. 41 or 42. I have not the volume, and there is no library near that has a copy; so I cannot 
give the language or name of the author of the observations. 
