Erosion on the Great Plains. — V[^hani. 39 
gic time by Dana, Walcott, the present writer, and others, have 
indicated, the mean rate of denudation on the I'lains through- 
out that era was approximately the same as now, or an average 
of one foot in three to four thousand years. 
Extensive Tertiary formations in the southern part of the 
Mississippi basin and along the Gulf border accord well with 
the foregoing estimate of erosion and resulting deposition. But 
northward, in the Hudson bay region, Tertiarv beds are want- 
ing, which, with the similar general absence of Tertiary marine 
strata about the northern Atlantic and Arctic shores of our 
continent, implies for that great land area an altitude through- 
out the Tertiary era above that of the present time. We may 
infer that the epeirogenic and orogenic movements originally 
forming the Great Plains and the Rocky mountains elevated 
this region much above its present hight ; that during Ter- 
tiary time the Plains were cut down and mainly base-leveled, 
having at last, in the Pliocene period, only a moderate hight 
above the sea, so that their vast expanse was mostly reduced 
by its streams to a mature peneplain ; that in the early part of 
the Quaternary era it was again greatly uplifted, by another 
grand but slow epeirogenic movement, attaining its present 
eastward slope; and that during the same time, and before tlie 
culmination of the Glacial period, the broad flat valley of 
the Red river of the North, and of the large lakes in Manitoba, 
was formed by stream erosion of the former eastern edge of 
the Plains, or, as we may better say, of the continuation of 
their Cretaceous area. 
Beneath the waters of Hudson bay and strait and of the 
North Atlantic lies the great tribute carried from the Rocky 
mountains and the Plains by the Tertiary and early Quaternary 
streams that now live anew, since the Ice age, as the Saskatche- 
wan, Red, and Nelson rivers. From the depths of fiords and 
of submarine valleys, as those of the St. Lawrence and Hud- 
son rivers, we know that this region was raised to a preglacial 
altitude of 3,000 feet, or more, higher than now, probably giv- 
ing the cold and snowy climate which induced glaciation. 
On the Cordilleran belt farther west, and along the Pacific 
border, far more complex conditions of erosion and marine de- 
position characterized these eras, which 1 hope to consider in a 
later paper of this series, dealing especially with the Puget 
Sound fiords. 
