Llewellyn Formation
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The Llewellyn Formation is a mapped bedrock unit in eastern
Pennsylvania Pennsylvania, officially the Commonwealth of Pennsylvania, is a U.S. state, state spanning the Mid-Atlantic (United States), Mid-Atlantic, Northeastern United States, Northeastern, Appalachian, and Great Lakes region, Great Lakes regions o ...
. 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.Wood, G.H., Trexler, J.P., Kehn, T.M., (1964). Geology of the West-Central Part of the Southern Anthracite Field and Adjoining Areas, Pennsylvania. United States Geological Survey, C-46.


Description

The Llewellyn is defined as a gray, fine- to coarse-grained
sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
,
siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...
,
shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...
, conglomerate, and
anthracite coal Anthracite, also known as hard coal and black coal, is a hard, compact variety of coal that has a submetallic lustre. It has the highest carbon content, the fewest impurities, and the highest energy density of all types of coal and is the highe ...
in repetitive sequences. Although gray (light to dark) is the dominant color, other colors described include: buff, dark to light brown, and black. It contains the worlds thickest anthracite coal bed, the Mammoth vein.


Depositional environment

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 A forest is an ecosystem characterized by a dense community of trees. Hundreds of definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing, and ecological functio ...
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 In geology, cyclothems are alternating stratigraphic sequences of marine and non-marine sediments, sometimes interbedded with coal seams. The cyclothems consist of repeated sequences, each typically several meters thick, of sandstone resting upon ...
.Edmunds, W.E., (2002), Coal in Pennsylvania (2nd ed.): Pennsylvania Geological Survey, Educational Series 7.


Fossils

Nearly 100 fossil species have been identified in the Llewellyn; almost all of them plants. The major
divisions Division may refer to: Mathematics *Division (mathematics), the inverse of multiplication * Division algorithm, a method for computing the result of mathematical division Military *Division (military), a formation typically consisting of 10,000 t ...
of flora that have been recognized include: ''
Lycopodiophyta The lycophytes, when broadly circumscribed, are a group of vascular plants that include the clubmosses. They are sometimes placed in a division Lycopodiophyta or Lycophyta or in a subdivision Lycopodiophytina. They are one of the oldest lineag ...
'', '' Arthrophyta'', ''
Pteridophyta A pteridophyte is a vascular plant (with xylem and phloem) that reproduces by means of spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as " cryptogams", meaning that their means of reproduction is h ...
'', and ''
Pteridospermatophyta Pteridospermatophyta, also called pteridosperms or seed ferns, are a polyphyletic grouping of extinct seed-producing plants. The earliest fossil evidence for plants of this type are the lyginopterids of late Devonian age. They flourished partic ...
''. 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. Clair, Pennsylvania.


Notable exposures

* Bear Valley Strip Mine, located west of Shamokin in
Northumberland County, Pennsylvania Northumberland County is a County (United States), county in the Commonwealth (U.S. state), Commonwealth of Pennsylvania. As of the 2020 United States census, 2020 census, the population was 91,647. Its county seat is Sunbury, Pennsylvania, Sunb ...


Age

Relative and fossil age dating of the Llewellyn places it in the late-Middle to Late
Pennsylvanian Pennsylvanian may refer to: * A person or thing from Pennsylvania * Pennsylvanian (geology) The Pennsylvanian ( , also known as Upper Carboniferous or Late Carboniferous) is, on the International Commission on Stratigraphy, ICS geologic timesc ...
, being deposited between 308 and 300 (±1) million years ago. It was thought that the uppermost beds of the Llewellyn were of
Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...
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 The geology of the Appalachians dates back more than 1.2 billion years to the Mesoproterozoic era when two continental cratons collided to form the supercontinent Rodinia, 500 million years prior to the development of the range during the form ...
in eastern Pennsylvania that still exists, only recent
alluvial Alluvium (, ) is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluvium is also sometimes called alluvial deposit. Alluvium is ...
and
colluvial Colluvium (also colluvial material or colluvial soil) is a general name for loose, unconsolidated sediments that have been deposited at the base of hillslopes by either rainwash, sheetwash, slow continuous downslope creep, or a variable combina ...
deposits rest on top.Berg, T.M., et al., (1983). Stratigraphic Correlation Chart of Pennsylvania: G75, Pennsylvania Geologic Survey, Harrisburg, Pennsylvania.


Economic use

The economic gains of this formation have been exploited to their greatest potential in years past. The peak year of anthracite production was in 1917 when nearly 100 million tons were mined. Anthracite production has declined every year since then. Recently, the old spoils of the mining operations (culm) have been refined to extract more coal for power production. There are 40 coal seams in the Llewellyn the largest is the Mammoth vein which is nearly 20 feet thick.


References


See also

*
Geology of Pennsylvania The Geology of Pennsylvania consists of six distinct physiographic provinces, three of which are subdivided into different sections. Each province has its own economic advantages and geologic hazards and plays an important role in shaping every ...
{{Clear Sandstone formations of the United States Siltstone formations of the United States Shale formations of the United States Geologic formations of Pennsylvania Paleontology in Pennsylvania Carboniferous geology of Pennsylvania Carboniferous southern paleotropical deposits Conglomerate formations of the United States Coal formations