170 



BOTANICAL GAZETTE 



[SEPTEMBER 



although the much older Dakota is at the surface over consider- 

 able areas in the plains drained by the southern tributaries of the 

 Arkansas. For a considerable thickness above the Dakota the 

 strata are mostly soft shales, which erode too readily to give table- 

 land topography. 



The larger through streams and their tributaries have cut below 

 the level of the High Plains, leaving escarpments which are par- 

 ticularly notable near the Platte-Arkansas divide. Plum and 



p IG r j — Buttes and plateau areas: divide between East Plum and West Plum 

 creeks in Castle Rock area, showing some of rhyolite buttes; one of the most 

 imposing of these, Dawson Butte, shown in fig. 4. 



Cherry creeks, running north into the Platte from the divide, and 

 Monument, running into Fountain Creek, south to the Arkansas, 

 have eroded deep valleys parallel to the mountain-front. Away 

 from the mountain-front proper these valleys are bounded by lines 

 of steep cliffs, but the west border of Monument and West Plum 

 Creek valleys is the graded slope from the foothills, with its debris- 

 covered terraces. Isolated buttes are present within these valleys, 

 some of them protected by caps of igneous rocks from local outflows. 

 The southern part of the Sangre de Cristo Range (sometimes 

 considered as a separate mountain chain, the Culebra Range) is 

 flanked on the east by a sedimentary plateau which rises abruptly 

 above the plains in a steep line of bluffs. The plateau is of sand- 

 stones mostly, of slight dip, and is much dissected by the eastward 



