Camp Polk and North Camp Polk are underlain by poorly consolidated fluviatile and brackish-water sediments of the Miocene series, which are at or near the surface throughout central and northern Vernon Parish. These formations, which dip southward at a rate of 50 to 100 feet per mile, consist chiefly of alternating loose sands, sandy shales, and tough clays with a few hard sandstone and lime-nodule horizons. The most important water-bearing sands, referred to in this article as the "A," "B," "C," and "D" sands or sand-zones, are found at depths of 600 to 1,450 feet. The "B" sand is the only water sand developed at both cantonments. The "A" and "B" sands are found at depths of 600 and 900 feet, respectively, at the south camp; the "B," "C," and "D" sands are found at 600, ,200, and 1,450 feet, respectively, at the north camp. These sands are recharged principally by rainfall in the hills of northern Vernon Parish. The hardness of the water ranges from very soft to medium hard.

A total of nine supply wells are pumped part-time at the south camp. Two of these wells draw from the "A" sand, the remainder from the "B" sand. At the north camp there are also nine supply wells that are used part-time. Five of these wells draw from the "B" sand, three from the "C" sand, and one from the "D" sand. The combined pumpage at the two camps is several million gallons a day. Pumping tests of wells at both camps indicate that both the "C" and "D" sands in the existing wells are better aquifers than the "A" and "B" sands.