74 GEOLOGY OF OHIO. 



indisputable evidence of ice action ; proving that these basins were filled 

 by moving glaciers in the last ice period, if never before, and that part, 

 at least, of the erosion by which they were formed is due to these gla- 

 ciers. 



No other agent than glacial ice, as it seems to me, is capable of exca- 

 vating broad, deep, boat-shaped basins, like those which hold our lakes. 



If the elevation of temperature and retreat northward of the glaciers 

 of the lake basins were not uniform and continuous, but alternated with 

 periods of repose, we should find these periods marked by excavated 

 basins, each of which would serve to measure the reach of the glacier at 

 the time of its formation ; the lowest basin being the oldest, the others 

 formed in succession afterwards. Such a cause would be sufficient to 

 account for any local expansions of the troughs of the old ice rivers. 



Where glaciers flow down from highlands on to a warmer plain, the 

 excavating action of each ice mass must terminate somewhat abruptly in 

 the formation of a basin-like cavity, beyond which would be a rim of rock, 

 with whatever of debris the glacier has brought down to form a terminal 

 moraine. 



When glaciers reach the sea, the great weight of the ice masses plows 

 up the sea bottom out to the point where the greater gravity of water 

 lifts the ice from its bed and bears it away as an iceberg. 



If it is true, as the facts I have cited indicate, that our lakes are but 

 portions of great excavated channels locally filled with Drift material, 

 the fiords of the northern Atlantic and Pacific coast present remarkable 

 parallels to them ; and I would suggest Puget's Sound, Hood's Canal, and 

 other portions of that wonderful system of navigable channels about 

 Vancouver's Island, as affording interesting and instructive subjects for 

 comparison. Like our lakes, these channels are for the most part exca- 

 vated from sedimentary strata which form a low and comparatively level 

 margin to the bases of mountain chains and peaks. They, too, have their 

 depths and shallows, their basins and bars, and probably all who have 

 seen them will assent to Prof. Dana's view, that they are the " result of 

 subserial excavation," in which glaciers performed an important part. 



There can be no doubt that the basin of each of the great lakes has 

 been produced by a local glacier, and that the great ice-sheet which ex- 

 isted during the period of intensest cold, moving as a solid, continuous 

 mass of great thickness, from north to south, would have the effect to 

 obliterate rather than form such local troughs. Our lake basins must, 

 therefore, have been formed before or after the continental glacier, or both 

 before and after. Probably the latter is the true statement of the case. 

 We find on the south shores of Lake Erie and Lake Ontario proof that 



