i 9 i 9 ] VESTAL— PHYTOGEOGRAPIIY OF COLORADO 157 



The outer slope to the plains has been described by Johnson (5) 

 as a debris-apron or composite of alluvial fans, of which the profile 

 is that of a stream-grade, rapidly flattening into the very slight 

 and uniform incline of the Great Plains. The graded surface is 

 covered by unassorted rock-waste from the hills, which thins out 

 and becomes finer in texture toward the east; it is absent from most 

 of the surface of the plains, which is of fine grained residual soil. 

 This grade is that of the High Plains; 1 the streams have very gener- 

 ally cut below it, especially near the mountains. The Platte and 

 Arkansas rivers, the trunk-streams, have cut very broad valleys 

 in the soft shales of the plains. The north-south valleys of their 

 tributaries which parallel the mountain-front are bordered on the 

 east by escarpments of considerable height and are notable geo- 

 graphic features. This recent downcutting, where working in soft 

 shales just outside the foothills, leaves many terraces, remnants 

 of the older and higher stream-grade levels; their covering of rock- 

 waste preserves their # flat tops. They are generally known as 

 "mesas"; although not true mesas, the term is convenient. 2 

 Where the upper sedimentary beds consist of sandstone or limestone, 

 extensive plateau areas with deep canyons, buttes which may be 

 numerous or scattered, or simple escarpments may be encountered. 

 In a few places igneous intrusions are seen as dikes or as basaltic 

 layers capping large mesas (true mesas in this case). From these 

 features the mountain-front zone derives its varied character; the 

 mountain upland on the west, and the plains extending far to the 

 east, are of less irregular structure. 



Arrangement of the component ranges and smaller ridges en 

 echelon is a notable feature of the easternmost line of mountains. 

 Ranges which are in general north and south of each other are 

 themselves oriented with the northern end a little to the west. 

 Marvine writes (7, p. 132): 



1 The distribution of the remnants of the High Plains may be seen in a map by 

 Johnson in the article mentioned. 



2 A true mesa is a tableland 'capped by a more resistant stratum which keeps 

 the top flat by retarding erosion except on the sides. The debris-covered terraces 

 flanking the mountains are like a true mesa in that the rock-waste layer acts as a more 

 resistant cap. 



