North of latitude 43° the Great Plains province is in large part upraised peneplain, though perhaps not all of the same cycle. Second in areal importance are the extensive gravel plains, some of them perhaps being remnants of a sheet similar in origin to that which covers the high plains. Others are simple stream terraces of Pleistocene age. Some of the streams, especially near the mountains, have valleys hundreds of feet deep and their tributaries have deeply dissected the upland. Isolated mountains are dome-like uplifts, the largest of which is the Black Hills, almost loo m. long and so m. wide, crossing the Dakota-Wyo ming border. The structural uplift approximates 10,000 ft. but erosion has truncated the dome leaving the actual height less than 4,00o ft. above the plain on the east. The core of granite has thus been exposed at the top, while the strata outcrop in concentric ellipses, the stronger ones making ridges.
The northern part of the Great Plains province and a margin on the east (13a) was covered by the Pleistocene ice-cap leav ing a strongly morainic topography in the Dakotas. The present course of Missouri river results from the obstruction of former drainage north-eastward to Hudson bay. The ponded waters overflowing from basin to basin found the present south-east ward course. Its relative youth is shown by the small width of its valley (I to 3 m. between bluffs) as compared with that of the Yellowstone and other tributaries.
tracts is South Park, 9,00o to io,000 ft. high, west of Pikes peak. The same surface farther east and at many other places is carved into mountains whose summits approximate the same level. Looking across these crests, or over the undissected up land, Pikes peak is seen to rise as an isolated eminence 5,000 f t. above the level. It was a monadnock on the South Park pene plain. North of the Park and extending almost to Wyoming is the main crest of the Front range with many peaks 12,000 to 14,00o f t. high. Along this entire line residual mountains rise clearly above a dissected plateau. Only a minor part of what is called "mountains" in the southern Rockies consists of these residual eminences. The greater part are carved from the up lifted peneplain. In the entire system, both north and south, most of the residual mountains have "alpine" features due to former glaciers. These are cirques and U-shaped troughs, often separated by steep and narrow crags. The San Juan mountains in south-western Colorado consist largely of volcanic rocks which once covered the area in sheets of relatively small relief. Later they were carved by water and ice into their present forms. The Middle Rocky mountains, between the Wyoming basin and Yellowstone National park, consist of definite ranges in which structures and trends are clearly related. The axes radiate from Yellowstone park to the south, south-east and east. A long sickle-shaped arm whose eastern side is the Bighorn range almost surrounds the Bighorn basin. Of the group of ranges trending south, the Wasatch in northern Utah is outlined on the west by a great fault, the range on the upthrown side overlooking Great Salt lake on the downthrow. Meeting this range at right angles is the Uinta range which extends eastward forming the southern rim of the Wyoming basin.
Yellowstone park is a plateau of rhyolitic lava with an aver age height of about 8,000 ft., enclosed by mountains on three sides but open to the west where a rapid descent leads down to the Snake river (basalt) plateau. A single narrow belt of moun tains north and east of the plateau connects those to the north ind those to the south. The lava fills a basin probably 2,000 ft,. deep and appears to have conserved the heat revealed by gey sers and hot springs. The hot springs are among the most exten sive in the world and the geysers have no rival.