The surficial material of the Crescent City-Deland Ridge is composed of sand and shell. The ridge overlies the Hawthorn Group or in places directly overlies the Ocala Limestone (Brooks, 1981). Johnson (1986) describes a very thin Hawthorn Group (<3 m or 10 ft) at minus 1.5 m (5 ft) NGVD in a well about 5 km (3 mi) northwest of the lake. Natural gamma logs from wells depicted on the gamma log profile sheet (Index Map D, wells P-0410, P-0146, P-0011) show logs with sufficient counts per second to characterize the Hawthorn Group. In some areas during deposition Hawthorn sediments have been reworked with the surficial Plio-Pleistocene sands of the Crescent City-Deland Ridge. The gamma response from these sediments may drop significantly as in well V-0283 located to the south. This situation makes delineating the Hawthorn Group more difficult. The top of the Floridan Aquifer was contoured by Rutledge (1982). For this area he identified this surface between -12 to -15 m (-40 to -50 ft) NGVD. The natural gamma log profiles also show this contact at -15 m (-50 ft) NGVD in wells P-0410 and P-0146, but it is not identifiable in P-0011 and P-0495 from the gamma logs alone.