Small amounts of clay within sandy Coastal Plain sediments of northern Delaware often hinder successful well development. Clays usually show relatively higher gamma radiation than do sands. Thus it was thought that gamma radiation logging techniques could be refined to determine the amount of clay within a given depth interval and thus predict the chances for successful well development. Test holes were augered to various depths in selected formations and cores were taken every five feet. A gamma log was run through the hollow stem augers in the completed hole and again in the open hole after the augers were pulled. The output of the gamma probe was calibrated in counts per second. The core samples were analyzed for clay percentages and the clay percentages compared against the gamma readings for the corresponding depths.