14. 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 
subeerial 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 
