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and strike, their fissured and shattered condition, and their merging 

 in places into rubble breccia of small fragments due to the breaking 

 up of the blocks either by the force of the original slide or by that 

 of later movements. A diagnostic may be found in the inward dip 

 of the blocks, as noted by Russell. 1 Owing apparently to friction 

 with the surface, the sliding blocks tend to rotate backward on 

 axes normal to the slope and thus come to rest with a marked dip 

 toward the cliffs from which they were derived. On the other 

 hand, blocks creeping down talus maintain a dip parallel with the 

 slope. The upper surface of both rock-fall and rock-slide breccias 

 is hummocky, but this feature will hardly be preserved excepting 

 where the landslide fell into deep water. 



Bajada breccia. — -The rugged mountains of arid regions are 

 commonly fringed with wide slopes of rock-waste called bajada. 

 Like those of talus, the fragments of the bajada have been detached 

 by mechanical weathering. But unlike other subaerial breccias, 

 the bajada has been aggregated largely by intermittent streams. 

 Its fragments have been more or less water-worn. It forms an 

 imperfect breccia and yet is far from being a typical conglomerate 

 of well-rounded pebbles. The streams which build the bajada 

 have certain peculiarities which greatly lessen the wear on the 

 stones they carry. The long accumulated waste on the mountain 

 slopes swept down by spasmodic rains loads so heavily the tempo- 

 rary streams of the barrancas that they have been designated as 

 mud-flows. 2 In the washes and on the lower unchanneled slopes 

 of the bajada the viscosity of the flow is further increased by 

 absorption of the water by the thirsty sands. In the moving mass 

 of the mud-flow, stones such as are carried in mountain torrents 

 of humid climates as the bottom load and dashed against one 

 another and the stream bed are here intermingled with the finer 

 waste held in suspension, and are thus protected from mutual abra- 

 sion. Hence pebbles remain imperfectly rounded even to the outer 

 edge of the bajada slope. 3 



1 1. C. Russell, "Geology of the Cascade Mountains in Northern Washington," 

 U.S. Geol. Surv., 20th Ann. Re.pt., Part II, p. 19. 



2 For a graphic description of a mud-flow in the Himalayas see Sir W. M. Conway, 

 Geog. Jour., II (1893), 291. 



3 R. D. Oldham, Quar. Jour. Geol. Soc. London, L (1894), 469. 



