1870.] 201 [Shaler. 



access to the base of these hills, it is readily seen that such ^s in fact 

 the case, the ridges of rock are distinctly traceable on the flanks of 

 the drift hills of Somerville, Brookline and Brighton. Even where, 

 as is often the case, the wash from the upper part of the hills has 

 covered their flanks so that it is not possible to get access to the pro- 

 tecting buttresses of rock, there is generally other evidence to 

 show that the drift ridge has a rock ridge beneath it. Allusion has 

 already been made to the fact that while the drift material does not 

 indicate anything which can be properly called stratification, there 

 is nevertheless a certain obscure bedding near the top of the mass. 

 Now it is easily seen, as, for example, at the easternmost point of 

 Prospect Hill in Somerville, that this bed is not horizontal, as it 

 would naturally have been had the mass been laid down on a plain, 

 but curves over the hill in a gradual arch, precisely as it should if 

 the supposition of an internal ridge of rock be correct. 



The interpretation of the record of the events of the glacial and 

 post-glacial parts of the history of this region is difficult, and much 

 doubt must necessarily exist for a long time as to what is the true 

 reading. I cannot doubt, however, that we must acknowledge that 

 very great changes in the face of the surface as it was left by the 

 glacial sheet, have taken place. If the foregoing reasoning is correct, 

 and there originally lay all along our shore a sheet of glacial detritus 

 of which the ridges which we have been studying are only the wrecks, 

 then we must be prepared to admit long continued erosion to produce 

 such changes as have taken place. Nor will erosion, unattended by 

 other agents of change, have produced the result; we must suppose 

 that the present shore was at one time higher above the sea in this 

 neighborhood than it is now. When the ridges which remain in the 

 harbor were protected by their rock bases from erosion, they must 

 have stood at a higher level above the sea than at present, for they are 

 now without this protection at ordinary tide mark, and are all, except 

 when artificially guarded, giving way before the action of the sea. 

 The glacial period was evidently a time of great oscillations of level, 

 and not the least of the difficulties which the geologist finds in study- 

 ing its phenomena, is to determine the character of the changes in 

 the level of the land. The formation of the more conspicuous features 

 in the topography of the drift formations of eastern Massachusetts, 

 would require something like the following succession of events. 



1. The covering of the whole country with a glacial sheet, the ice 

 having a slow movement towards the shore, the direction of move- 



