Forest physiography; physiography of the United States and principles of soils in relation to forestry . anging dip and hardness, con-sequently neither the structure of the rock nor alluviation can beappealed to in explanation of the general topographic uniformity. The 1 W. Upham, Tertiary and Early Quaternary Base-leveling in Minnesota, Manitoba, andNorthwestward, (Abstract) Geol. Soc. Am., vol. 6, pp. 17-20. See also the American Geolo-gist, vol. 14, 1S94, pp. 235-246. 2 W. M. Davis, The United States in Mills International Geography, 1900, p. 756. 412 FOREST PHYSIOGRAPHY interstream spaces
Forest physiography; physiography of the United States and principles of soils in relation to forestry . anging dip and hardness, con-sequently neither the structure of the rock nor alluviation can beappealed to in explanation of the general topographic uniformity. The 1 W. Upham, Tertiary and Early Quaternary Base-leveling in Minnesota, Manitoba, andNorthwestward, (Abstract) Geol. Soc. Am., vol. 6, pp. 17-20. See also the American Geolo-gist, vol. 14, 1S94, pp. 235-246. 2 W. M. Davis, The United States in Mills International Geography, 1900, p. 756. 412 FOREST PHYSIOGRAPHY interstream spaces are rather flat and give no indication of the markeddepth and steep walls of the intrenched streams which are practicallyinvisible until one is almost at the canyon brink. The narrow valleyof the Missouri is from 300 to 600 feet below the general level of theplains, and that of the Yellowstone and many others have been cut tocomparable levels. GLACIAL FEATURES The map, Fig. 148, shows how small a portion of the Great Plains hasbeen glaciated. Besides glacial features similar to those of the northern. Fig. 148. — Note the large proglacial lake west of the Highwood Mountains, caused by the glacial dammingof the Missouri River. Its discharge across the northern border of the Highwood Mountains formedShonkin Sag, a temporary outlet. (After Calhoun, U. S. Geol. Surv.) part of the Prairie Plains may be noted the two terminal moraineswhich cover the country north of the Little Rocky Mountains. Thesurface is a rolling plain with rounded flat-topped ridges and broadand low intervening hollows. The larger drift is chiefly Laurentian,but the bulk of the material is made up of quartzite drift from theRocky Mountains, and consists of well-rounded pebbles and smallquartzite bowlders of different colors.^ The Keewatin ice sheet extends southward into the northern GreatPlains as far as the Highwood, Bear Paw, and Little Rocky elevations stopped its southward progress locally,
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