CHARACTERISTICS AND DISTRIBUTION OF LAKES 623 
and therefore had been subject to subaerial erosion. This old land- 
surface had a well-developed drainage system with many rivers flowing 
across it to the sea through broad valleys, in which the advancing ice- 
sheet subsequently deposited detritus that obstructed the flow so as 
to form important lakes. 
The problem of the origin of the Great Lakes of the Laurentian River 
basin is not completely solved, but there are many facts that lead to 
the supposition that they lie in old valleys clogged by drift, and that 
glacial erosion played a comparatively minor part in their formation. 
Before the glacial epoch there was a system of river-drainage different 
from the present one,^ but the lake-troughs were empty except along 
the deepest bottom line. The position of these troughs was determined 
by that of the more easily eroded rocks, which they follow with re- 
markable closeness, and their recent conversion into lakes has been 
accomplished by local concentration of drift in deep narrow valleys, 
where it could act effectually as a barrier. As the ice retreated from 
the region the sheets of water found one line of discharge and then 
another, leaving their record in the immense deposits of gravel dropped 
by the overloaded glacial streams, and in the numerous water-worn 
channels, too large for the streams which now occupy them, and with- 
out catchment areas commensurate with their size. During all the 
remarkable changes that ensued, the land to the north-east was slowly 
rising, and this change of level had much influence in determining 
the various lake-outlets. An examination of a number of authentic 
records by Gilbert ^ has shown that this rising still continues, and that 
there is a tilting of 0"42 foot in a hundred miles in a century. If 
continued, the banking or backing up of the waters at the southern 
end of Lake Michigan will go on much faster than the lowering con- 
sequent on the work of the Niagara River in wearing down the falls, 
and in two or three thousand years all the lakes but Ontario will be 
tributary to the Mississippi River, as they were during the period of 
the retreat of the ice-sheet. The history of the Laurentian basin, as 
read from the hard-rock topography of the region, has received 
different renderings, but the facts seem to go to prove that the basins 
of the Great Lakes were formed by a combination of erosion, warping, 
and obstruction. 
A survey of the Laurentian lakes made by the Corps of Engineers, 
U.S. Army, between 1841 and 1881, is the basis of nearly all the 
accurate information now accessible concerning the physical features ; 
1 See W. M. Davis, "Classification of Lake Basins," Proc. Boston Soc. Nat. 
Hist., vol. xxi. p. 362, 1883. 
2 " Modification of the Great Lakes by Earth Movement," Nat. Geogr. Mag., 
vol. viii. p. 245, 1897. 
