McShan formation, here introduced, consists of more or less glauconitic cross-bedded sand, gravelly sand, and laminated clay --marine deposits in contrast to preponderantly nonmarine beds of the underlying Tuscaloosa group. Thickness about 25 ft. Present in relatively small outliers in mapped region. Beds were formerly included in the Eutaw formation.

Source: GNU records (USGS DDS-6; Reston GNULEX).

McShan formation unconformably overlies continental Gordo formation and unconformably underlies marine Eutaw formation. Basal bed does not necessarily contain principal conglomerate of formation. At many places basal 6 to 20 ft of formation, consisting of laminated and rippled very fine grained glauconitic sand lies abruptly on sand and gravel of Gordo formation and is in turn overlain by as much as 40 ft of gravelly sand. At designated type locality in Pickens Co., AL, beds are more or less lenticular and intergradational. Thickness in type area is about 100 ft; 240 ft in Warror River valley, but thinner to northwest; 57 ft in Itawamba Co., MS.

Source: GNU records (USGS DDS-6; Reston GNULEX).

McShan described in Prentiss Co., MS, where it is about 28 ft thick. Overlies Gordo formation and underlies Eutaw formation. In MS, the McShan occupies a somewhat narrow irregular outcrop area, which includes parts of Tishomingo, Prentiss, Itawamba, and Monroe Cos. In Prentiss, Co., it is at the surface only on lower slopes of creek branches in southeastern part of county.

Source: GNU records (USGS DDS-6; Reston GNULEX).

Typical McShan lithologies are composed of very thinly interbedded and interlaminated, very fine grained, glauconitic, micaceous sand and clay laminae and very thin beds often varicolored with shades of red, purple, and very light gray, or dark brownish gray where shale intervals occur. Bedding is generally horizontal with various forms of ripple bedding and ripple laminations. At many localities contains petrified wood. McShan Formation thins rapidly westward from 240 ft in western AL to 57 ft in roadcuts along US Hwy 78, 7 mi east of Fulton, Itawamba Co., MS. Downdip in the subsurface of Neshoba and Kemper Cos., MS, unit is over 200 ft thick. Northernmost exposures of the McShan occur along U.S. Rte 72 in SE1/4 sec. 34, T3S, R11E. Unit unconformably underlies Eutaw Formation and unconformably overlies Tuscaloosa Group. North of Clear Creek the McShan is absent and the Eutaw Formation directly overlies Tuscaloosa Group. At the surface of Tishomingo Co., the McShan consists of a narrow, generally north-trending belt of exposures in central and southern portions of the county.

Source: GNU records (USGS DDS-6; Reston GNULEX).

Gravels mapped as Tuscaloosa in Tishomingo Co., TN, and northwestern AL are actually two distinct lithic units of different age. Lower gravels are chert gravel in a quartz sand matrix that can be traced southeast into the Gordo Formation. Upper chert gravel, with rare lenses of chert sand and kaolinitic clay unconformably overlies the lower unit and is best developed to the north in Tishomingo Co. This upper gravel is probably equivalent to the McShan Formation.

Source: GNU records (USGS DDS-6; Reston GNULEX).