C. R. Keyes — Ejphemeral Lakes in Arid Regions. 377 



Art. XXX YII. — Ephemeral Lakes in Arid Regions ; by 

 Charles R. Keyes. 



The bolson plains — those broad mountain-locked basins 

 which occupy so large a part of the high plateau regions of 

 both Old and New Mexico — present certain geographic pecu- 

 liarities that many writers have taken as evidence of their 

 lacustrine origin. Their Spanish name signifies a purse. This 

 they certainly are. From the immediate piedmont zone there 

 is a gentle slope from all sides towards the middle. Drainage- 

 ways there are none. And the central depression has no out- 

 let. The reported remnants of old terraces on the mountain 

 slopes, so far as personal examination has gone, all seem to be 

 mistaken interpretations. 



Since the flat-bottomed intermontane basins assumed their 

 present attitude as slightly warped surfaces of an old destruc- 

 tional plain, they have been modified by fluviatile rather than 

 lacustrine conditions. The old surface, formed on the beveled 

 edges of Paleozoic and Mesozoic strata, has been covered by 

 deposits which, though tliey are stratified gravels, sands and 

 clays, must be now regarded as of subaerial origin. 



The coarse materials covering the bolsons are readily seen to 

 be composed of the same kinds of rocks that are found in the 

 adjoining mountains. The arroyas in flood time carry out into 

 the plain immense quantities of bowlders, gravel and finer 

 materials. These produce great alluvial fans which become 

 confluent and form marginal conglomerates of great thickness. 

 For example, in the Sandoval bolson at the foot of the Ortiz 

 mountains, the conglomerates are 150 or more feet in thickness. 

 Shumard"^ reports their thickness in the Jornada del Muerto at 

 500 to 600 feet. Powell,t Dutton,:}: Hill,§ and others have 

 described these bolson gravel deposits and consider them to 

 have a subaerial genesis. This evidence is sufiicient to clearly 

 indicate that for the great part at least the later deposits of the 

 bolsons cannot have ascribed to them a lacustrine origin. 



There are some of the bolson plains in which limited deposits 

 occur giving undoubted evidences of the existence of old lakes. 

 The Sandoval bolson, south of Santa Fe, contains traces of a 

 comparatively recent lake of considerable size. At the present 

 time the remnants are found in a group as small salt ponds, 

 the chief of which is Laguna del Perro. 



There is a class of lakes which occur in connection with the 

 bolsons which appear to have escaped the notice of writers on 

 the arid regions. For want of a better title they are here 

 called ephemeral lakes. They originate under abnormal though 

 frequently recurring conditions. 



* Trans. St. Louis Acad. Sci., i, p. 341, 1858. 



f Geology of Unita Mts., p. 170, 1876. 



X Geology Higli Plateaus, p. 219, 1880. 



§ Occurrence of Artesian and Underground Waters, etc., p. 140, 1892. 



