Abstract Mesas and buttes of the central Colorado Piedmont are composed of at least twodistinct rock types, which differ in their cohesiveness and resistance to erosion. Thelower parts of the exposed stratigraphic section are poorly cemented, Upper Cretaceousto Middle Eocene sandstones of the Dawson Formation. The caprocks arecomposed of one or more resistant formations of Late Eocene age: the Castle RockConglomerate, Wall Mountain Tuff, and the conglomerate of Larkspur Butte. Theseformations were originally deposited in topographic lows, but due to their resistance,they now cap prominent buttes and mesas of the Colorado Piedmont. Erosion of thecaprock through progressive retreat of the butte scarp produces colluvium that has ahigher resistance to erosion than the poorly cemented underlying sandstone. Once the caprock of a butte has been removed by erosion, the underlying weaklycemented Dawson Formation is readily eroded. Ultimately, the armored lower slopesof the former butte remain as a circular ridge standing as much as 100 m above thesurrounding topography. This process produces a topographic low surrounded byrelict faceted slopes where the flat top of the butte once stood. Prominent alluvial fans are associated with some of these annular features, andthey record the main phases of butte removal and excavation of the central part of thearmored slopes. Multiple generations of alluvial fans contain coarse- and fine-grainedfacies that represent changes in effective stream power and record alternating phasesof aggradation and erosion. The degree of soil development in the fan alluvium andheight of the fan surfaces above streams indicates the oldest preserved gravel fandeposit is of late-middle Pleistocene age. The youngest luminescence (optically stimulatedluminescence) dated alluvial fans were deposited during the late Pleistoceneabout the time of the Pinedale glacial maximum in Colorado, ca. 21,000 yr B.P. Keywords: Colorado Piedmont, talus flatiron, talus flatiron ring, inverted topography.

Abstract This chapter summarizes the current state of late Cenozoic stratigraphic knowledge in some Rocky Mountain basins (here defined as the structurally low portions of major drainage basins) that have been studied in detail since Scott’s (1965) summary on the nonglacial history of the southern and middle Rocky Mountains. The Quaternary history of few of these basins has been studied as intensively as that of the surrounding mountain ranges, despite the wealth of fluvial and other types of deposits present in the basins and the potential for dating them. The areas discussed here include the Bighorn Basin in Wyoming and Montana, the Laramie Basin and part of the Sweetwater River Basin in Wyoming, the Yampa River Basin in Colorado, and the Uinta Basin in Utah (Fig. 1). Small parts of the Great Plains and Colorado Plateau physiographic provinces are included in this discussion. Correlations of deposits within and among some of the areas can now be made more confidently than in 1965 because of detailed mapping and the discovery of many more localities with dated volcanic ashes.

Abstract The Great Plains physiographic province lies east of the Rocky Mountains and extends from southern Alberta and Saskatchewan nearly to the United States-Mexico border. This chapter covers only the northern part of the unglaciated portion of this huge region, from Oklahoma almost to the United States-Canada border, a portion that herein will be referred to simply as the Northern Great Plains (Fig. 1). This region is in the rain shadow of the Rocky Mountains. Isoheyets are roughly longitudinal, and mean annual precipitation decreases from about 750 mm at the southeastern margin to less than 380 mm in the western and northern parts (Fig. 2). Winters typically are cold with relatively little precipitation, mostly as snow; summers are hot with increased precipitation, chiefly associated with movement of Pacific and Arctic air masses into warm, humid air masses from the Gulf of Mexico. Vegetation is almost wholly prairie grassland, due to the semiarid, markedly seasonal climate. The Northern Great Plains is a large region of generally low relief sloping eastward from the Rocky Mountains toward the Missouri and Mississippi Rivers. Its basic bedrock structure is a broad syncline, punctuated by the Black Hills and a few smaller uplifts, and by structural basins such as the Williston, Powder River, and Denver-Julesburg Basins (Fig. 3). Its “surface” bedrock is chiefly Cretaceous and Tertiary sediments, with small areas of older rocks in the Black Hills, central Montana, and eastern parts of Wyoming, Kansas, and Oklahoma. During the Laramide orogeny.

Abstract The Osage Plains are that part of the Central Interior Lowlands extending from the glacial limit southwestward through eastern Kansas, central Oklahoma, and north-central Texas (Fig. 1). The Interior Highlands adjoin the Osage Plains on the east and comprise two distinctly different highland areas, the domed Ozark Plateaus and the tightly folded mountains of the Ouachita Province (Fig. 2). The Osage Plains and Interior Highlands have little in common, physiographically or stratigraphically, and are included in the same chapter for convenience because of their proximity to one another. Situated near the center of the conterminous United States, the Osage Plains are a place of transition between regions of contrasting character. They lie between the humid east and the semiarid west and the glaciated Interior Lowlands on the north and the Coastal Plain of the Gulf of Mexico on the south. The ecotonal boundaries between forest and grassland lie within the Osage Plains, as does the boundary between calcic and noncalcic soils. The Osage Plains differ markedly from the Interior Highlands in many respects, including topography, geology, vegetation, and soils. The Interior Highlands can be thought of as western outliers of the Appalachian Highlands. The plateaus overlying the Ozark dome are similar in topography, rock types, and structure to the Interior Low Plateaus overlying the Nashville dome and to the Appalachian Plateaus farther east and north. Likewise, the tightly folded and faulted Ouachita Mountains are comparable to the Appalachian Mountains. The Quaternary stratigraphy of the Interior Highlands

Abstract The Rocky Mountain region is one of the most topographically distinct and impressive parts of North America. The Rocky Mountains rise abruptly above the bordering regions, particularly on the east and northeast where they are flanked by plains, less so on the west and southwest where they are bounded by high plateaus. The Rocky Mountains comprise more than 100 individually named ranges that form a belt extending for slightly more than 5,000 km, from near Santa Fe, New Mexico, on the south to the Bering Sea on the north (Fig. 1). The belt varies in width from less than 100 km in the Canadian Rockies to nearly 600 km in the Middle Rockies of Wyoming and northeast Utah. The summits of the ranges rise 1,500 to 2,100 m above adjacent lowlands, to heights 1,800 to 4,400 m above sea level. The Southern Rockies of Colorado have the greatest amount of area, between 3,300 and 4,400 m, and the highest peak, Mount Elbert (4,400 m). The largest area of low mountains is in the Northern Rockies of Idaho and Montana, where summits are commonly only 2,100 to 2,400 m above sea level. A substantial part of the Rocky Mountain region consists of lowlands, in the form of basins and fault-bounded troughs and trenches that lie between ranges. The Rocky Mountain Trench is perhaps the most spectacular fault-bounded lowland, even if it is not the most representative. It extends north from Flathead Lake, Montana, more than 1,500 km, and forms