Llewellyn Formation

Llewellyn Formation Stratigraphic range: Late Pennsylvanian, ~300 Ma PreЄ Є O S D C P T J K Pg N ↓
A hand sample of the bottom of the Llewellyn from St. Clair, Pennsylvania
Type	Geological formation
Unit of	none
Sub-units	none (most economic coal beds are named)
Thickness	up to 1,500 feet (460 m)
Lithology
Primary	Sandstone
Other	Shale, Conglomerate, Coal
Location
Region	Appalachian Basin of eastern North America
Extent	Anthracite fields of Pennsylvania
Type section
Named for	Llewellyn, Pennsylvania
Named by	G. Wood, 1964

The Llewellyn Formation is a mapped bedrock unit in eastern Pennsylvania. It was previously known as the "coal measures" and the post-Pottsville rocks. The formation is named for the community of Llewellyn in Schuylkill County.

The Llewellyn is defined as a gray, fine- to coarse-grained sandstone, siltstone, shale, conglomerate, and anthracite coal in repetitive sequences. Although gray (light to dark) is the dominant color, other colors described include: buff, dark to light brown, and black.

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. As a result, a large amount of organic matter was buried and eventually turned to coal. The repetitive nature of these sediments have been well documented and are often called cyclothems.

Nearly 100 fossil species have been identified in the Llewellyn; almost all of them plants. The major divisions of flora that have been recognized include: Lycopodiophyta, Arthrophyta, Pteridophyta, and Pteridospermatophyta. Fossils have been recovered from every rock type in the Llewellyn but are predominantly found in the siltstone, shale, and coal layers. The compression fossils from the Llewellyn formation are well known for the striking white color on the dark stone. An important location for these fossils is near St. Claire, Pennsylvania.

Relative and fossil age dating of the Llewellyn places it in the late-Middle to Late Pennsylvanian, being deposited between 308 to 300 (±1) million years ago. It was thought that the uppermost beds of the Llewellyn were of Permian in age, but no fossils have been found from that era. The bottom of the formation is placed either at the bottom of Buck Mountain (no. 5) coal bed or at the bottom of the underclay or shale bed below this coal measure. Since this is the last bed deposited in the Appalachian Basin in eastern Pennsylvania that still exists, only recent alluvial and colluvial deposits rest on top.