Great Lake Basins of the St. Lawrence. 285 
supplemented by the shallows becoming marshes which in 
time will fill up with mould arising from the annual growth 
and decay of the reeds, rushes and grasses which flourish 
in profusion there. 
Leaving out of view the above subsequent changes in 
parts of its area, Lake Erie probably dates the outlining in 
a general way of its present limits back to the time when 
the Ontario Lake ridges were being formed, and when the 
clays and gravels were being piled up against the Niagara 
escarpment and had blocked the Dundas valley. The entire 
Ontario peninsula had been under water for a long period, 
and by the deposition of the clays over it, the courses of 
the pre-glacial rivers had been partly filled up. The united 
lakes, as their terraces show, had at first a high level, and 
their waters found here, as Newberry has shown, outlets 
to the southward through the gaps furnished by the river 
‘valleys in Ohio. On the elevation of the land, new drain- 
age channels had to be cut by the water. It was then that 
the outflow from Lake Huron began by the St. Clair and 
Detroit Rivers and of Lake Erie by the Niagara River, the 
channels of the old glacial river having been blocked and 
the waters being kept back, not merely by the superficial 
deposits, but probably by warpings of the strata beneath as 
well. It may be that the lake level was at first retained at 
a higher point than now, the escarpment at Lewiston being 
38 feet above Lake Erie. This would have prevented a 
separation then between that lake and Lake Huron. It is 
most probable, however, that the Niagara did not fall over 
the escarpment at Lewiston but found at this point, as at 
St. David’s, a great fracture in the cliff, affording it a 
natural gorge down which its waters ran, and which they 
gradually further eroded. Other such fractures are found 
in the escarpment both south of Lake Ontario and between 
it and the Georgian Bay, some of them forming great 
ravines several miles in length, and presently, in some cases, 
the beds of streams, Such fractures were a necessary con- 
sequence of the elevation of the escarpment and of the direc- 
tions which this elevation followed. 
