[1] The Llewellyn is defined as a gray, fine- to coarse-grained sandstone, siltstone, shale, conglomerate, and anthracite coal in repetitive sequences.

[3] The Llewellyn was deposited upon a broad flat plain with sediment-choked rivers delivering detritus from the eroding uplands that were located to the southeast.

Fluctuations in sea level, coupled with the shifting nature of the rivers and highlands, allowed dense forests to grow on the broad plain.

[5] Fossils have been recovered from every rock type in the Llewellyn but are predominantly found in the siltstone, shale, and coal layers.

Relative and fossil age dating of the Llewellyn places it in the late-Middle to Late Pennsylvanian, being deposited between 308 and 300 (±1) million years ago.