<P> This is one of the best examples of its kind . The strata here are Cretaceous or early Tertiary, lying nearly horizontal . The surface is shown to be a plain of degradation by a gradual ascent here and there to the crest of a ragged escarpment, the escarpment - remnant of a resistant stratum . There are also the occasional lava - capped mesas and dike formed ridges, surmounting the general level by 500 ft (150 m) or more and manifestly demonstrating the widespread erosion of the surrounding plains . All these reliefs are more plentiful towards the mountains in central Montana . The peneplain is no longer in the cycle of erosion that witnessed its production . It appears to have suffered a regional uplift or increase in elevation, for the upper Missouri River and its branches no longer flow on the surface of the plain, but in well graded, maturely opened valleys, several hundred feet below the general level . A significant exception to the rule of mature valleys occurs, however, in the case of the Missouri, the largest river, which is broken by several falls on hard sandstones about 50 miles (80 km) east of the mountains . This peculiar feature is explained as the result of displacement of the river from a better graded preglacial valley by the Pleistocene ice sheet . Here, the ice sheet overspread the plains from the moderately elevated Canadian highlands far on the north - east, instead of from the much higher mountains near by on the west . The present altitude of the plains near the mountain base is 4,000 ft (1,200 m). </P> <P> The northern plains are interrupted by several small mountain areas . The Black Hills, chiefly in western South Dakota, are the largest group . They rise like a large island from the sea, occupying an oval area of about 100 miles (160 km) north - south by 50 miles (80 km) east - west . At Black Elk Peak, they reach an altitude of 7,216 feet (2,199 m) and have an effective relief over the plains of 2000 or 3,000 ft (910 m) This mountain mass is of flat - arched, dome - like structure, now well dissected by radiating consequent streams . The weaker uppermost strata have been eroded down to the level of the plains where their upturned edges are evenly truncated . The next following harder strata have been sufficiently eroded to disclose the core of underlying igneous and metamorphic crystalline rocks in about half of the domed area . </P> <P> In the intermediate section of the plains, between latitudes 44 ° and 42 °, including southern South Dakota and northern Nebraska, the erosion of certain large districts is peculiarly elaborate . Known as the Badlands, it is a minutely dissected form with a relief of a few hundred feet . This is due to several causes: </P> <Ul> <Li> the dry climate, which prevents the growth of a grassy turf </Li> <Li> the fine texture of the Tertiary strata in the badland districts </Li> <Li> every little rill, at times of rain, carves its own little valley . </Li> </Ul>

Who gave one of the earliest accounts of the topography and tribes of the great plains