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10 <br />7 <br />Draw, Lake Bench, Starvation Valley, and Warm Springs Cedars. Outwash of <br />rubble from these areas produces constrictions in the river channel which <br />may result in the formation of gravel islands, riffles, or rapids. <br />4. Round Valley Formation (Lower Pennsylvanian) <br />This is a cherty limestone which underlies the Morgan Formation. <br />Exposures of Round Valley Limestone are limited to the upper half of the <br />canyon where it is often covered by landslide or talus slope deposits. Good <br />exposures are found intermittently between miles 30.3 and 23.5. <br />5. Doughnut and Humbug Formations (Upper Mississippian) <br />These two minor formations are frequently treated as one functional <br />unit. They consist of clayey shale (Doughnut) and very fine-grained <br />sandstone interbedded with shale and limestone (Humbug). Exposures of these <br />formations are limited to a short reach of river between miles 30 and 25.4. <br />[where exposed, the Doughnut forms slopes and the Humbug forms ledgy slopes <br />or cliffs. <br />6. Madison Limestone (Lower Mississippian) <br />The oldest rock exposed by the Yampa River in the Monument is this <br />fine- to medium-grained cherty or dolomitic limestone. It is the least <br />important formation in the canyon, being exposed in only a few places <br />between Little Joe Rapids (mile 29.5) and Five Springs Draw (mile 26). <br />At river level, Quaternary alluvial and colluvial deposits comprise the <br />most important substrates for vegetation. Talus slopes are the principal <br />geomorphic structures along the river's edge. These are usually arnored <br />with gravel or stone along the flood zone. where the current is swift, bare <br />bedrock forms the river banks. Sand deposits are found where the current is <br />