Beds composed of oölites or of shell fragments are more soluble than beds of fine-grained or earthy limestone. Vertical crevices are most likely to develop where joints traverse the more soluble beds; bedding-plane crevices in the same beds are likely to form directly above shale partings that are somewhat thicker than average. In the Hopkinsville quadrangle 80 per cent of the wells drilled in limestone of Mississippian (Carboniferous) age obtain adequate water supplies for home or farm needs, because of the extensive development of the bedding-plane crevices.
Data from 146 wells reveal a fairly uniform distribution of crevices to an average depth of about 80 feet; the frequency of crevices declines rapidly at greater depth. A rapid increase in mineral content of water below 100 feet signals a decrease in rate of circulation that must be attributed to a decrease either in number of or size of crevices, or to a combination of both.
Ratio between the lowering of water level in observation wells and the gaged stream discharge during rainless periods (base flow) gives a storage coefficient of 0.005—that is, 0.5 per cent of drainable open space in creviced limestone. The average seasonal range of fluctuation of the water table is about 13 feet; thus the ground-water reservoir stores less than an inch of water in the interval between high and low water levels.
