THE CENTRAL REGION OF NORTH AMERICA. 609 



Drift Plateau of Northern Minnesota and Eastern Manitoba. 



The great plateau of Northern Minnesota, which stretches south- 

 ward from the Lake of the "Woods, shows only drift materials, and 

 is composed of them to a great depth (see Map and Section PL 

 XXXIL, a). Its general surface is remarkably uniform, and its 

 slopes almost imperceptibly slight. It is, however, diversified on a 

 small scale, being thiekly strewn with shallow hollows, which are 

 filled by little lakes or the almost impassable " muskegs" of the 

 region. There are also low flat-topped ridges of sand and gravel of 

 the nature of kames or eskers, and in many localities traces of larger 

 lakes than those now existing, which have been drained by the gradual 

 wearing down of the beds of their outfall streams. 



The drift-deposits of this region rest on the gently sloping foot of 

 the Laurentian axis, and are, where I have seen them, composed to a 

 depth of 60 feet or more of fine sands and arenaceous clays, with 

 occasional beds of gravel and small boulders. The finer deposits are 

 generally very evidently false-bedded, and sometimes quite hard. 

 The gravelly layers, as a rule, are found resting on the finer material 

 between it and its surface-soil, and sometimes lie on the denuded 

 edges of the curved sand-beds below. In one place only did I find 

 any trace of organic remains. On the Roseau River, about 30 feet 

 from the top of the bank, a piece of wood protruded from a cliff of 

 hard sandy clay, and, on microscopic examination, appeared to be a 

 fragment of the common cedar (Tlmja occidentalis). I have no 

 doubt that these distinctly -bedded deposits of the plateau repose 

 throughout on boulder-clay. I have observed them to do so in the 

 southern part of the Lake of the Woods ; and, on the Eoseau River, 

 also, indications of the underlying boulder-clay are found. In general, 

 however, the few sections which exist do not penetrate sufficiently 

 deep to show this deposit. 



An interesting confirmation of the general direction already stated 

 for the glacial action, is found in the composition of the materials of 

 this plateau. Its eastern side, fronting on Lake Superior, is very 

 abrupt, and seems to be held up by a ridge of hard old rocks, which 

 here and there appears from beneath it. Ascending to the plateau- 

 level from the extreme western point of Lake Superior by the 

 Northern Pacific Railway, the drift is seen to have a reddish-purple 

 colour, which continues, though gradually becoming less marked, for 

 some distance after attaining the summit. The colour then changes 

 tb the pale yellowish grey which is generally characteristic of the 

 drift of this plateau. The red drift is derived from the red rocks 

 of the border of the lake, and is found along its whole southern side. 

 It is here bounded by a line lying a short distance back from the 

 north-western shore and nearly parallel to it. This western edge 

 of the red drift has been already noticed by Whittlesey in his paper 

 in the Smithsonian Contributions. The surface of the plateau is 

 very generally strewn with erratics ; and some of them are of great 

 size. They are chiefly derived from the Laurentian and Huronian 

 to the north ; but there are also many of white limestone. Dr. Bigsby 



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