Up to 10 meters (33 ft) of black, fine-grained, thick-bedded dolomite fill two large, prehistoric sinkholes and grade upward almost imperceptibly into Lenoir Limestone.

[5] At Douglas Dam, the Douglas Lake Member consists of three-part sequence of volcanic ash, conglomerate and shale and / or shaly dolomite that fill one of these ancient sinkholes developed in the Mascot Dolomite of the Knox Group.

[2][4] The upper unit consisted of about 11 meters (36 ft) of thin-bedded, slabby reworked volcanic ash and impure dolomite with green shale partings.

[1][4] The 33 beds were completely excavated for construction of Douglas Dam by the Tennessee Valley Authority in 1942, but yielded articulated arthropod fossils including Chasmataspis and Douglasocaris.

[16] This is agreed in later review as well, considered to lack sufficient characters to be unequivocally assigned to land plants.

This unconformity represents periods of falling relative sea level starting in latter part of the Floian Stage, which halted the accumulation marine sediments of the Knox Group and exposed the region to terrestrial erosion and karstification.

It is not until late in the Darriwilian Stage that rises in relative sea level drowned the Douglas Lake region and initiated the accumulation of the marine sediments that now comprise the Lenoir Limestone.