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4 ".., ,. physiographic sections. Geologically, the area is characterized by north-northwest ~ountainous blocks (Figure A-2) separated with broad basins filled with-Cenozoic clastic deposits as m~ch as 10,000 feet thick. It is believed the mountains and basins are separated by faults. Early deformation occurred in Laramide time; tectonic disturbance and regional subsidence has continued intermit- tently to Recent time. The Algodones fault trends north\'lesterly across the Yuma Valley area. This major fault is believed to be an extension of the San Andreas fault system in the Salton trough. The groundwater reservoir in the Yuma area comprises the deltaic sediments deposited by the ancestral Colorado River. These sediments are bordered by impermeable bedrock on the north and east the mountain blocks (Figures A-3 and A-4). The sediments have been i I i I I ; arei I I except where the Gila and Colorado Rivers have carved narrow gaps in dissected into river terraces and mesas. These present land forms remnants of the former extensive Colorado River valley. The pre- dominant source of water for recharge to the subsurface reservoir has been from the Colorado River system. ~ne Older alluvlum comprlses tne major sequence of deltaic sedi- rnents and it attains a maximum thickness of more than 2,000 feet in the sQuthwestern part of Arizona. These strata comprise a heteroge- neous mixture; ranging from clay to sand, cobbles and boulder gravel (Figure A-S). The younger alluvium consists of river deposits of sand, silt and clay. It is about 100 feet thick beneath Yuma Valley and is less than 200 feet thick beneath Yuma Mesa. The principal groundwater reservoir is the older alluvium which has been divided into two parts; the lower one is the "wedge zorte" and the upper is the "coarse gravel strata" which inter-fingers with the lower part