THE HOT SPRINGS AT THERMOPOLIS, WYOMING' 



N. H. DARTON 



At the southern end of the Bighorn Basin there is a great hot 

 spring which presents some notable geologic features and an inter- 

 esting question as to the source of the hot water. The spring is 

 at the town of Thermopolis, a village and health resort which owes 

 its existence largely to the reputed therapeutic value of the water. 

 The locality is on the bank of Bighorn River, a few miles north of 

 a high range which may be regarded as the southwestern continua- 

 tion of the Bighorn Mountains. There are several springs, but 

 one of them has by far the greatest volume. They issue from the 

 red beds, here brought to the surface by a prominent local anti- 

 cline. The present springs and their predecessors — for the region 

 has been one of thermal activity for many centuries — have built 

 extensive terraces of travertine or hot spring deposits similar to 

 some of those in the Yellowstone National Park. 



The geologic structure at Thermopolis is relatively simple and, 

 owing to the extensive exposures of the formations, it is perfectly 

 plain. The cross-section (Fig. i) shows the relations from the 

 crest of the Bighorn upHft in the mountain summit lo miles south 

 to a point a few miles north of the springs. 



South of the springs there is the long monocHne constituting 

 the north slope of the antichne of Bighorn Mountains, and in the 

 vicinity of Thermopolis this monocline is crenulated by a sharp 

 antichne. The axis of this flexure crosses Bighorn River a short 

 distance north of Thermopohs, and along it there are exposed the 

 Chugwater red beds, while on either side is a succession consisting 

 of Sundance (Jurassic), Morrison, Cloverly ("Dakota"), and Benton 

 formations. The dips on the south hmb of the antichne are steep, 

 50° to 65°, while those on the north side are gentle. The Cloverly 

 sandstone on either side gives rise to a prominent "hogback ridge," 



I Published with permission of the Director of the U. S. Geological Survey. 



194 



