312 ELIOT BLACKW ELDER 



probably reduced very nearly to its present remnants, although its 

 relative elevation has since been greatly increased. 



Black Rock cycle : The next cycle has been named after Black 

 Rock Meadows, near the head of one branch of Snake River, which 

 at that point has excavated a flat-bottomed valley with gentle 

 slopes chiefly on Mesozoic strata. Farther downstream, the flat 

 is trenched and appears as a series of flat-topped spurs. These 

 features lie several hundred feet lower than the valleys representing 

 the Union Pass cycle in the vicinity. More or less generally 

 throughout the district, the Black Rock valleys and remnants are 

 500-800 feet above the present stream. Along the Gros Ventre 

 River, the cycle is probably represented by the long flat-topped 

 spurs from one to eight miles below the mouth of Fish Creek. The 

 surfaces of these spurs trunkate Cretaceous beds which dip from 

 15° to 25°, and if the intervening gulches were filled there would 

 be a continuous graded valley floor several miles wide. On one 

 of the remnants a thin capping of water-worn gravel is still pre- 

 served. Corresponding features may be found in the tabular areas 

 and flat gravelly spurs which obstruct the Snake River valley below 

 Jackson Hole at elevations of 500-600 feet above the river, and the 

 similar forms in the lower Snake River valley near Irwin, Idaho. 

 These forms are apparently remnants of a wider, flat-bottomed 

 valley much shallower than the present canyon of Snake River. 

 In the Teton Range the wide shallow valleys of similar nature, 

 from 500 to 1,000 feet above the bottoms of the modern canyons, 

 suggest the same cycle. 



Returning eastward to the north side of the Wind River basin, 

 we find around Black Mountain (see Fig. 24), and westward along 

 the mountains to DuNoir River, a high plain, distinctly lower than 

 the Union Pass cycle, and now much dissected. Most of these 

 remnants are veneered with weathered stream-gravel which plainly 

 indicates the origin of the plain. At the mouth of each ravine in 

 the mountain side, an old alluvial fan is still to be seen although 

 nearly all such fans have been dissected. In some places in this 

 district there is evidence that there are really two plains separated 

 by a terrace 50-100 feet high, but in general there is not sufficient 

 basis available for resolving the Black Rock cycle into two. In the 



