EROSION IN ARIZONA BOLSON REGION 139 
often controlled by a mosaic of small fault-blocks, showing a 
remarkably close relation of hills and gullies to blocks and faults. 
Toward the south and southwest, the fault-blocks develop into 
parallel strips; ridge after ridge of Paleozoic limestones, and successive 
sheets of rhyolitic and andesitic lavas rear their heads above the 
desert-filled wastes. These fault-strips, rather than fault-blocks, 
are well developed southwest from Casa Grande, the steep scarps 
facing northeast, and the faults striking northwest, the most important 
escarpment being that of the Vekol Mountains where several thou- 
sand feet of Carboniferous strata are exposed, with an unusual 
development of upper arenaceous members. The waste between 
the hills consists of wash deposits, volcanic deposits, playa deposits, 
and occasional lake deposits, this complicated series dating back 
through the Quaternary and probably the late Tertiary. Sufficient 
data have been collected by scattered observations to indicate that in 
their structure they show the effect of various conditions of climate 
and topography, and give color to the belief that the history of the 
Quaternary at least will be discovered by a well-directed reconnais- 
sance of the region. 
DISCUSSION OF THE TERM ‘‘ BOLSON’”’ 
With the above description in mind, it is evident that southern 
Arizona is a region of bolsons, as defined by Hill. He states: 
The bolson plains . . . . are newer and later features consisting of structural 
valleys between mountains or plateau plains, which have been partially filled with 
débris derived from the adjacent eminences. * 
Keyes strenuously objects to the application of the term so defined 
to the New Mexican examples. He states that the Jornado del 
Muerto, and Estancia plains are valleys showing erosion bevelment, 
and are not structural, and that they have but a thin veneer of desert 
waste.? Going farther, he states that New Mexican plains are in 
general destructional planes, in soft Cretaceous strata with only a 
thin capping of desert waste, and by assumption these conditions are 
extended to cover Arizona bolsons. ‘Tight has taken exception to 
these conclusions as applied to New Mexico, and certainly in the 
structurally broken country of southern Arizona, the deeply filled, 
t Op. cit., p. 8. 2 Op. cit., pp. 66, 67. 
