The following description was published in RI67 The Cat Hill Formation and Bethany Formation of Delaware, Andres, A.S., 2004:
The composition, thickness, and geophysical log signature of the Bethany Formation vary with location and depth. In general, the Bethany Formation is a sequence of clayey and silty beds with discontinuous lenses of sand (Andres, 1986; Ramsey, 2003). The most common lithologies are silty, clayey fine sand; sandy, silty clay; clayey, sandy silt; fine to medium sand; sandy, clayey silt, and medium to coarse sand with granule and pebble layers. Thin gravel layers occur most frequently in updip areas and are rarer in downdip areas. Sands are typically quartzose. Lignite, plant remains, and mica are common, grains of glauconite are rare. In the Lewes area, Ramsey (2003) describes the Bethany Formation as consisting of gray, olive gray, bluish-gray clay to clayey silt interbedded with fine to very coarse sand. Lignitic and gravelly beds are common.
Variations in thickness reflect spatial changes in depositional environments during filling of the sedimentary basin and post-depositional erosional truncation (Andres, 1986). The age of the Bethany Formation is reported to range from late middle Miocene (Owens and Denny, 1979; Hansen, 1981; Benson, 1990) to perhaps Pliocene (Miller et al., 2003), although the age estimates are poorly constrained because of a general lack of diagnostic fossils or other materials that can be age-dated.