i68 



BOTANICAL GAZETTE 



[SEPTEMBER 



The debris-cover, where it has not been removed by recent 

 erosion, extends far out into the plains. Its removal from the 

 extensive areas of soft shales and clays marks a change from the 

 flat terrace level to the easily eroded, gently rolling surface of much 

 of the plains. The High Plains are extensive remnants of the old 

 graded surface, away from the mountains. 



The north-south distribution of the terraces is practically that 

 of the mountain-front, although as conspicuous topographic forms 

 the mesas are not so extensive. So far as effects on distribution of 

 vegetation are concerned, the presence of the coarse mixed soil of 



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Fig. S. — Ta'ble Mesa, about 7 miles north of Boulder; outlines of hills sketched 

 in with ink; ridge DDD is Dakota hogback; Boulder mesas may be seen in figs. 6 

 and 7. 



the detrital surfaces is the important physical condition. It 

 permits the growth in the same small area of a rich variety of plants, 

 representing numerous vegetation- types and different geographic 

 elements. 



Plateau areas, buttes, and escarpments (tigs. 9-ti). — 

 Where the sedimentary strata are horizontal or of rather slight dip 

 the harder layers protect the softer rocks beneath, and extensive 

 plateau surfaces are left above the grade established by present 

 erosion. These can be invaded only at the edge and by ravines 

 which eat their way headward into the bluff's. Smaller elevated 

 areas or buttes, recently or long ago cut away from plateaus by 



