228 GUIDEBOOK OF THE WESTERN UNITED STATES 
The valley fill here is thick, for borings 1,530 to 1,730 feet deep, for 
water, appear not to have reached bedrock, unless “hard beds” in 
the lower 550 feet are Tertiary or Cretaceous. In the surrounding 
region there is a succession of older beds of gravel and sand ® which 
are mostly tilted and in places faulted. They are overlapped uncon- 
formably by the later sand and gravel that floor the valley. There 
are excellent exposures of these relations on the slopes of the Gila 
Bend Mountains near Woolsey Well, 15 miles northwest of Gila Bend, 
and farther west at the north end of the Gila Bend Mountains west 
of Dome. 
As the Gila Valley below Mesa is filled with a thick mass of alluvium 
underlain in part by sandstone of Tertiary age, it is evident that the 
region was 1,000 feet or more higher when the valley was being ex- 
cavated than it is now, and it has sunk to its present level as the 
younger formations were deposited. Possibly this loading was the 
cause of the sinking, but more likely it was due to some widespread 
crustal movement. A notable feature revealed by the logs of deep 
borings in the valley is a deposit of clay of wide extent, with a maxi- 
mum thickness of 860 feet at Gila Bend. This clay must have been 
deposited in quiet waters, such as those of a lake or estuary that 
continued for a long period of time. The deposition of clay was 
followed by the accumulation of coarser material spread by streams, 
and since that time terraces higher than the present bottom lands 
have been developed. In places these later deposits were flooded by 
lavas, through which the present river trench has been excavated 
nearly 100 feet. From the historical record the Gila River channel 
has changed materially in a century or less. When it was originally 
discovered there was a well-defined channel with hard banks sustain- 
ing cottonwoods and other trees and plants. The current was swift 
and deep in places, so that the stream could be navigated by flat 
boats of moderate size, and it contained sufficient fish to be relied 
upon as food for many Indians. It was reported also that the water 
was clear and sea-green, very different from the present muddy stream. 
Now the Gila River is depositing sediment in its lower part, and its 
braided course follows many narrow sand-clogged channels. Possibly 
these changes may be due partly to diverting and damming the water 
and to an increase of silt caused by the removal of forest and increased 
grazing in the higher region. 
Irrigation has been practiced in this region for a very long time, 
for old Indian ditches are found near the Painted Rock Mountains 
below Gila Bend and at other places along the river flats. Irrigation 
was again started in a small way by settlers who came soon after the 
_ eas Sing a per ae Na aes and enh: 
% These older beds are in general cor- 
Telated with the Temple Bar conglom- 
ae _ erate of Lee and the Gila conglomerate 
of Gilbert. In places they include lava 
flows (basalt) which are tilted and 
