Information on which specifications were originally drawn for drilling the Chattanooga shale was obtained largely from the TVA, whose geologists and driller laid great stress on the difficulties of maintaining circulation in their ho;es. The stated that the shale itself was not particularly difficult to core, the trouble being in the overburden. They did not use deep casing, depending on cementing to hold the holes open. On this basis, the Survey's specifications called for mid casing only, it being assumed that solid rock would be encountered at relatively shallow depths. This belief was borne out by examination of such road cuts and other exposures as were available.