l64 BOTANICAL GAZETTE [September 



and is usually seen as a bold ridge parallel to the outer slope of 

 the foothills some distance to the east. The term ''hogback'' is 

 familiarly applied to the steep Dakota cuesta. 



A deep and wide trough usually extends between the Fountain 

 crags and the Dakota cuesta. The upper part of the east-facing 

 slope of this trough is the outcrop of a "creamy sandstone," which 

 in places forms prominent outcrops, or even strong ridges, as at 

 Morrison at the mouth of Bear Creek. Just east of and below the 

 creamy sandstone is an easily eroded shale, which gives its rich 

 red color to the deep soil of the valley. The west-facing slope, 

 below the Dakota crest, is the outcrop of a calcareous sandstone 

 stratum which is weathered so slowly as to be covered only by a 

 thin soil. In certain places this limy sandstone stratum is hard 

 enough to form a separate ridge or hogback crest. 



The Dakota hogback is one of the most constant and conspicu- 

 ous topographic features of the mountain-front, since it is practically 

 everywhere harder than the strata above and below. Its top is 

 usually quite even and straight, representing the level of a former 

 graded surface. Its crest is quite rocky; there is no soil except in 

 the crevices. 



The present graded slope to the plains begins usually with the 

 outer slope of the Dakota hogback, through first a layer of dark 

 shales, then a thin limestone overlaid by soft light-colored shales, 

 then clays and shales. Near every east-flowing stream, however, 

 the graded slope is likely to be cut beneath by side-gulches cutting 

 down into the dark shales, leaving a cut-off mesa with the limestone 

 at its high western end. 



Local distribution of vegetation in the mountain-front belt of 

 upturned sedimentary rocks presents a variability apparently 

 dependent almost entirely upon topography and soil texture, just 

 as in the area of granitic foothills. There seem to be few if any 

 perceptible differences in the floras of the different geological 

 formations which can be traced to chemical differences in the sub- 

 stratum. It is perhaps true that cedars are more frequent in the 

 limestone or calcareous sands of the stratum just below the Dakota, 

 where these are exposed in gulches which notch the Dakota hog- 

 backs, and that there are certain slight floristic differences between 



