410 



LAKE SUPERIOR. 



accunmlated by the agency of strictly terrestrial glaciers ; such is, 

 at least, the case everywhere far from the sea-shore. But we may 

 conclude that these ancient glaciers reached, upon various points, the 

 sea-shore at the time of their greatest extension, just as they do at 

 present in Spitzbergen and other arctic shores ; and that therefore, 

 in such proximity, phenomena of contact should be observed, indi- 

 cating the onward movement of glacial material into the ocean, such 

 as the accumulation within these materials of marine fossil remains, 

 and also the influence of the tidal movements upon them. And now 

 such is really the case. Nearer the sea-shores we observe distinctly, 

 in some accumulations of the drift, faint indications of the action of 

 the tide reaching the lower surface of glaciers, and the remodeling, 

 to some extent, of the materials which there poured into the sea. A 

 beautiful example of the kind may be observed near Cambridge, 

 along Charles River, not far from Mount Auburn, where the unstrati- 



fied glacial drift (a) presents in its upper masses strictly the charac- 

 ters of true terrestrial glacial accumulation, but shows underneath 

 faint indications (6) of the action of tides. Above, regular tidal 

 strata (c) are observed, formed probably after the masses below had 

 subsided. The surface of this accumulation is covered with soil (c?). 

 The period at which these phenomena took place cannot be fully 

 determined, nor is it easy to ascertain whether all glacial drift is 

 contemporaneous. It would seem, however, as if the extensive accu- 

 mulation of drift all around the northern pole in Europe, Asia and 

 America was of the same age as the erratics of the Alps. The cli- 

 matic circumstances capable of accumulating such large masses of 



