12 UNIVERSITY OF COLORADO STUDIES 



East of the last foothill ridges the slope to the plains may be continuous or may 

 be broken by a sharp step, leaving a high terrace adjacent to the foothills. This 

 terrace is called a mesa, or locally "the mesa." It occurs in fragments of unequal 

 height abutting against the most easterly foothill ridge or separated from it by a 

 trough due to recent erosion. These fragments are commonly separated from one 

 another by ravines. The mesas have a height of from one hundred to three hundred 

 feet above the lower plains, and a west-east width in this vicinity varying from a 

 fraction of a mile to three miles. 



The eastward slope of the terraces may be as great as io° on the foothill side, 

 but it diminishes rapidly toward the plains. At one mile east from the foothills a slope 

 of 3° or 4° is common, and at three miles the slope is i°. An east-west cross section 

 shows an even curve like the profile of a stream. In addition to the eastward slope 

 there is generally a distinct northward or southward inclination toward a lower bench 

 quite similar to the higher, but separated from it by a steep bluff, or more commonly 

 by a ravine. In a general way the highest mesas are farthest from the larger streams 

 and the height diminishes as these streams are approached. 



Structure and Covering. — These mesas are essentially rock benches covered ten 

 or twenty feet deep with unassorted rock waste. The rocks, chiefly Pierre shales, 

 are steeply upturned, being affected by the mountain uplift. They have been 

 smoothly planed off, not to a flat, but to an inclined surface. The debris covering 

 may have a fairly uniform thickness of twenty feet on the highest benches. Generally 

 speaking, the thickness is less as the mesa surface is lower. This thoroughly unas- 

 sorted debris comprises fragments of all sizes from sand grains to boulders more than 

 ten feet in diameter. The heaviest boulders are close to the foothills, but fragments 

 a foot or more in diameter are found some miles to the east. 



It is the opinion of Fenneman and most other recent writers that 

 these mesas are, in their origin, river terraces. 



Horse Mesa and Long Mesa 



Location and Surroundings of the Mesas. — As already stated, these 

 two mesas are located between Boulder Creek and Bear Canyon, some 

 two miles south of the city of Boulder. Here the Dakota "Hogback," 

 commonly the outermost foothill ridge, is wanting, and the slope of the 

 mesas extends unbroken with gradually increasing steepness, westward 

 to the ridge formed by the Lyons and Fountain formations. Here the 

 slope is very abrupt. In this locality the conglomerates of the Fountain 

 formation are exposed in huge masses dipping at an angle of about 50 

 to the east. Locally these rocks are known as the "Flatirons" (Fig. 1). 

 A mile to the west of the mesas, the summit of Green Mountain rises 



