
GLACIAL PHENOMENA AND SUPERFICIAL DEPOSITS. 213 
the shore of Lake Superior, the summit of the plateau is reached, and 
the same leyel maintained, with an extreme variation in altitude of not 
much more than 200 feet, for 250 miles westward, when a compara- 
tively rapid descent is made to the Red River Valley. As in the northern 
region of the plateau, the surface is covered, to a great extent, with lake 
and swamp, but the railway cuttings give occasional opportunities for the 
study of the underlying drift deposits. Near the western edge of the 
plateau, at Thomson Junction; green slaty rocks, apparently Huronian, 
project above the drift deposits, but beyond this point the underlying 
rocks are nowhere visible. The cuttings, which are chiefly through 
banks more or less closely resembling eskers, show sandy and gravelly 
material, generally stratified, and not so much of the nature of till or 
boulder clay, as of beds resulting from its re-arrangement, in shallow, 
rapidly moving waters. 
483. A line drawn north-east and south-west, nearly parallel with 
the north-western shore of Lake Superior, but lying a short distance back 
from it, and cutting the Northern Pacific Railway some miles west of 
Thomson, in this part of Minnesota separates superficial deposits of 
different aspects. North-west of this line, the prevailing tint of the drift 
material is pale yellowish-grey, or drab; south-east of it, reddish tints 
are almost universal, and become specially prominent on the northern 
part of the line of the Lake Superior and Mississippi Railway, and continue 
to St. Paul. The junction of these two varieties of drift, cannot of course 
be exactly defined, but is interesting as an indication of the direction of 
transport of material in this region; the reddish matter being derived 
from the red rocks of the lake shore.* 
484, The average heizht of the Plateau above the sea, on the line of 
the Northern Pacific Railway, may be estimated at about 1,350 feet. 
Where crossed by the forty-ninth parallel, it has a gemeral altitude of 
rather under 1,000 feet, showing a decline northward. The highest part 
of the plateau appears, however, to lie about midway between the Lake 
of the Woods and the line of the Northern Pacific, the elevation of the 
sources of the Mississippi being, according to Nicollet, at an altitude of 
1,680 feet. 


*The junction of the red drift of Lake Superior with the lighter-coloured western material, of 
northern Minnesota, has, I find, been already noted by Whittlesey, in his valuable memoir on the “‘ Fresh 
water Glacial Drift of the North-western States.” (1864.) ‘‘In descending the St. Louis River, the ash- 
coloured drift clay of the Embarras and Savannah rivers, assumes a more purple hue near the mouth of 
the Savannah.. The purple graduates into red, between this point and the Knife Rapids, and becomes 
entirely red on the Grand Portage. The red extends along the shores of Lake Superior to St. Mary’s 
and to Lake Huron. It is found on all the tributaries of Lake Superior which flow into it from the south 
up to their sources, and beyond the summit, on the streams that run southerly into Lake Michigan.” p. 9, 
