i62 BOTANICAL GAZETTE [September 



narrow zone of older and lower strata contains alternating resistant 

 and soft members, giving rise to the hogback ridges and intervening 

 valleys already mentioned, while the newer rocks are mostly soft 

 shales and sandstones, giving a flat or rolling topography over the 

 surface of the plains, with occasional escarpments at the edges of 

 stream-valleys. Both angle of dip and hardness of rock, therefore, 

 contribute to a differentiation, in the sedimentary area outside 

 the foothills, of a relatively narrow ridge-valley mountain-front 

 zone from the very broad and mostly flat plains region. 



Just outside the ridge-and-valley zone is the graded slope to the 

 plains, covered with rock-debris and dissected into terraces or 

 mesas of varying level. In places along the mountain-front the 

 ridge-and-valley topography is absent or poorly developed, either 

 because the troughs are not yet carved beneath the slope from the 

 granitic hills, or because the ridges are already planed (locally) to a 

 graded floor. The terraces are also missing from certain parts of 

 the mountain-front. The topographic complexes of the ridge 

 country and of the mesa country may now be described separately. 



The hogback ridges (cuestas) and intervening troughs 

 'tigs. 4, 5). — Two of the numerous sedimentary strata overlying 

 the crystalline rocks are so resistant as to form ridges over great 

 lengths of the mountain-front. These two strata are of such 

 conspicuous geographic importance that they merit distinctive 

 names and since many persons know them by their geological 

 names, these will be used here in a geographic capacity. The 

 Fountain sandstone, which in most places lies directly upon the 

 granites, is very thick, and is composed of dark red, rough arkose 

 n^aterials, variable in texture. It is in places more resistant than 

 the granites, so that side-gulches tributary to the east-flowing 

 streams of the foothills are common in the granite just beneath 

 the Fountain. Continuous troughs between the Fountain and the 

 granite are not frequent. In many places the hard red sandstones 

 form broad smooth-faced crags lying upon the outer foothill slope, 

 reaching maximum size in the well known "flat-irons" south of 

 Boulder (fig. 6). The other hard stratum is the massive gray 

 sandstone known as the Dakota. It is separated from the Fountain 

 by several less resistant strata of considerable aggregate thickness. 



