Llewellyn Formation
Encyclopedia
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
.
, 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.
.
, Arthrophyta, Pteridophyta, and Pteridospermatophyta
. Fossils have been recovered from evey rock type in the Llewellyn but are predominantly found in the siltstone, shale, and coal layers.
, 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.
Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
. It was previously known as the "coal measures" and the post-Pottsville rocks. The formation is named for the community of Llewellyn
Llewellyn, Pennsylvania
Llewellyn is an unincorporated community in Branch Township, Schuylkill County, Pennsylvania. The community is located about 1.5 miles southwest of Minersville, Pennsylvania, along the West West Branch of the Schuylkill River....
in Schuylkill County
Schuylkill County, Pennsylvania
-Notable people:*Boxing heavyweight great Muhammad Ali had his training camp in Deer Lake.*Charles Justin Bailey, commanding general of the 81st Division in World War I, was born in Tamaqua on June 21, 1859....
.
Description
The Llewellyn is defined as a gray, fine to coarse grained sandstoneSandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
, siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
, shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
, conglomerate
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
, and anthracite coal
Anthracite coal
Anthracite is a hard, compact variety of mineral coal that has a high luster...
in repetitive sequences. Although gray (light to dark) is the dominant color, other colors described include: buff, dark to light brown, and black.
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 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 cyclothemsCyclothems
In geology, cyclothems are alternating stratigraphic sequences of marine and non-marine sediments, sometimes interbedded with coal seams. Historically, the term was defined by the European coal geologists that worked in coal basins formed during the Carboniferous and earliest Permian periods...
.
Fossils
Nearly 100 fossils have been identified in the Llewellyn; almost all of them have been plants. The major divisions of flora that have been recognized include: LycopodiophytaLycopodiophyta
The Division Lycopodiophyta is a tracheophyte subdivision of the Kingdom Plantae. It is the oldest extant vascular plant division at around 410 million years old, and includes some of the most "primitive" extant species...
, Arthrophyta, Pteridophyta, and Pteridospermatophyta
Pteridospermatophyta
The term Pteridospermatophyta refers to several distinct groups of extinct seed-bearing plants . The oldest fossil evidence of plants of this type is of late Devonian age, and they flourished particularly during the Carboniferous and Permian periods...
. Fossils have been recovered from evey rock type in the Llewellyn but are predominantly found in the siltstone, shale, and coal layers.
Notable Exposures
- Bear Valley Strip MineBear Valley Strip MineThe Bear Valley Strip Mine is an abandoned coal strip mine located in Coal Township, Northumberland County, to the southwest of the town of Shamokin, Pennsylvania. It lies in the Western Middle Field of the Anthracite belt in the Ridge-and-Valley Appalachians, where the Pennsylvanian Llewellyn...
, located west of ShamokinShamokin, PennsylvaniaShamokin is a city in Northumberland County, Pennsylvania, at the western edge of the Anthracite Coal Region. At the 2000 census the population was 8,009 residents...
in Northumberland County, PennsylvaniaNorthumberland County, PennsylvaniaThere were 38,835 households out of which 27.30% had children under the age of 18 living with them, 52.40% were married couples living together, 9.60% had a female householder with no husband present, and 34.10% were non-families. 30.20% of all households were made up of individuals and 15.50% had...
Age
Relative and fossil age dating of the Llewellyn places it in the late-Middle to Late PennsylvanianPennsylvanian
The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...
, being deposited between 308 to 300 (±1) million years ago. It was thought that the uppermost beds of the Llewellyn were of Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "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
Colluvium
Colluvium is the name for loose bodies of sediment that have been deposited or built up at the bottom of a low-grade slope or against a barrier on that slope, transported by gravity. The deposits that collect at the foot of a steep slope or cliff are also known by the same name. Colluvium often...
deposits rest on top.