The physiography of the arid lands of the Great Basin and of the open basins south of it, stretching across Arizona into Sonora, Mexico, early attracted attention. The casual observer gains an impression of scattered mountain ranges buried beneath the products of their own disintegration which, in the form of great alluvial fans and aprons, seem to slope away in all directions toward the playa flats of the intermont basins.

To some of the early explorers it came as a great surprise to discover that in many instances what appeared to be alluvial fans and slopes were, in reality, rock surfaces only thinly veneered with waste, or even quite bare. These had the shapes of alluvial fans or aprons, but were floored by rock.

Gilbert1 was one of the first to describe such features, which are well developed around the base of the . . .

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