1 68 STUDIES FOR STUDENTS 



Unlike all other subaerial breccias, the bajada is stratified, and 

 unlike that of marine and ordinary fluviatile conglomerates, the 

 stratification is imperfect. The bedding is often local and ill- 

 defined. Unsorted beds pass both vertically and horizontally 

 into those well sorted. All this follows from the nature of the 

 building-streams. Leaving the barrancas of the mountains and 

 debouching on the gentler slopes, they suffer a sudden arrest of 

 velocity as well as diminution of volume by the absorption of their 

 waters. These checks are often enough to cause them to throw 

 down their load, fine and coarse alike, with little or no sorting. 

 The attitude of unequiaxed fragments of mud-flows on bajadas has 

 not been sufficiently observed. It may be inferred from the method 

 of their carriage that they will not always take the position of 

 repose and that even flat stones may be left at any angle. 



A bajada breccia is wedge-shaped, but on a larger scale than that 

 of talus. Furthermore, there is a gradual decrease in size and 

 angularity of the material in passing from the thick to the thin end. 



The matrix of the bajada consists of the finer stream-wash and 

 of dust and sand contributed by the wind. Beds of rubble breccia 

 with interstitial matrix pass into endostratic breccias of predomi- 

 nant matrix arid sporadic fragments. A characteristic matrix is 

 a chemical deposit of lime carbonate by calcareous evaporating 

 water. As in the Tintic mining district, Utah, 1 the material may 

 thus be cemented into compact rock in which roofs and walls 

 of deserted tunnels remain standing for years untimbered. The 

 limy matrix may whitewash the pebbles and, by filling the inter- 

 stices, form beds of caliche. 



Dry climate conditions are indicated further by the absence 

 of carbonaceous deposits and by the complete oxidation of iron 

 compounds, the interfingering of playa clays about the outer 

 margin, and, under extreme aridity, the association of beds of salt 

 and gypsum in the centers of the bolsons. Wind-carved pebbles 

 may be found, and beds of the millet-seed sand of the desert. The 

 presence of well-rounded sand grains in a breccia, however, is a 

 criterion to be used with intelligent caution. Desert sand once 



1 G. W. Tower and G. O. Smith, U.S. Geol. Surv., igth Ann. Rept., Part III, 

 pp. 668-69. 



