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Geology Field Trip (The Pottsville Formation)

The Pottsville Formation in Pennsylvania is a bedrock unit that is mapped within Pennsylvania, Ohio, West Virginia and Western Maryland. This formation is recognized within Alabama, as well. It could be viewed as a main former of ridge within the Appalachians of Ridge-and-Valley located in the East of the United States. This Formation is found to be conspicuous at several sites of the Front of Allegheny, the Appalachian Plateau or the Allegheny’s eastern escarpment. The Pottsville Formation has a conglomerate that is gray in color and fine to sandstone that is coarse grained. The Formation contains shale, limestone and siltstone including bituminous coal and anthracite. The Formation is considered to be orogenic classic molasses (Robinson & Prave, 2001). The Pottsville Formation was first described from a cut of railroad that is in the south of Pottsville, Pennsylvania. Though the Formation is predominantly made up of conglomerate and gray sandstone; it has thin beds of clay-stone, coal, shale and limestone. Also included are Sharon and Olean conglomerates of the northwestern Pennsylvania, high-alumina clays that are commercially valuable and present locally; as well as thin marine limestone that is present in Mercer, Beaver, and Lawrence Counties (Robinson & Prave, 2001).

Regarding the coming to be of the Pottsville Formation, the orogenic molasse was formed as deposits of the terrestrial (in this case, likely to be a braided stream) next to a chain of a rising mountain that is deposited in the foreland basin. Shale became graded into sandstone leading to fluvial. The crossbeds are present, however, there are no climbing ripples or rather obvious finding sequence leading to braided stream. This is of great tectonic significance as there was the 315 Ma Early Pennsylvanian, the Alleghanian Orogeny, including the infilling of sediment as a result of the collision between the Euramerica and Gondwana. Moreover, there is the deposit of the Pottsville conglomerate along the basin’s tectonically active margin (Robinson & Prave, 2001).

Layer Composition

The Pottsville Formation is regarded as the most significant feature in Pennsylvania. It creates the capping in Sand, Fox Mountains and Lookout. On the sandstone plateaus, it is taken as the only exposed formation except certain deep gorges. Within these gorges, it appears as though the streams have cut through the shale and sand-stone layers into the limestone underlying formation. Pottsville Formation consists of sandstone beds that alternate with shale beds. The thickness and uniformity of these beds tend to vary greatly. In certain areas, the beds of sandstone appear to be relatively thin while the shale beds appear to be thick. This is, however, different from the others, where the beds of sandstone appear to be as thick as those of shale. Shale lenses may occur within the sandstone, or sandstone lenses may occur within the shale (Robinson & Prave, 2001).

The Pottsville Formation’s sandstone elements seem resistant to weathering. The available rock is mainly composed of medium to fine-grained, massive or thick-bedded sandstone. However, in certain areas, there is a fairly thin-bedded hard rock, which is commonly called a flagstone. In other areas, it mainly consists of the conglomerates. In certain places, the rock seems to have a rather loose structure. It is composed of a nearly unconsolidated sandy material which can get readily broken into a consistence of a single grain. From a local perspective, the lenses or interbedded layers of the soft clay shale tend to occur within the sandstone (Robinson & Prave, 2001). These layers are partly indurated as opposed to the common shallow beds of the varicolored stratified or the extremely thinly bedded silt, clay, as well as extremely fine sands. These beds are almost thick, about two to six feet and are underlain by the sandy shale generally. A part of the Pottsville beds of shale that separate the strata of sandstone are of almost pure clay whereas others are more or less sandier. In certain areas, the clay shale beds contain a single or more mineable coal beds. Shale including the sandstone of the Pottsville Formation has to a large extent contributed to parent materials for several of the soils within its locality. From them, there has been the development of the parent materials around Pottsville, Muskingum, Hartsells, Crossville, Linker and Apison series, the associated colluvial and alluvial series, as well as miscellaneous types of land. Several of the soils within valleys of limestone are modified by both the clayey and sandy materials which derive from the Pottsville Formation (Robinson & Prave, 2001).

When observing at close range and making an analysis in terms of measurement, one can describe the layers as arranged from top to bottom. There is an 8.5 cm silty shale which gets muddy at the top and is loosely compacted. This is followed by a 4 cm wack stone medium grain, plant fossil, sub-rounded and not well sorted with some lithic fragment with quartz and mica grains. Then there is 50cm muddy shale, which is loosely compacted followed by a 40 cm wack stone, sub-angular medium grain, which is medium sorted. Then there is a 50 cm shale, 30 cm wacke stone and 70cm muddy shale with plant fossil that is loosely compacted. Finally, there is a 3 cm muddy shale that is loosely compacted with a 5cm wacke stone trough bedding, and a tool mark, conglomeratic at the bottom. The bedding size changes from fine to medium grain. Nevertheless, there are a tool mark and a flat mark cutting bottom layers.


Within the lower Pottsville Formation of Pennsylvania, there is the extraction of coal-bed methane from a naturally fractured stratum. Major systems of fracture include the orthogonal fractures that consist of joints that are systematic in a siliciclastic stratum. Calcite, including minor amounts of pyrite usually fills the joints within the shale and sandstone and, tends to less commonly, cleat within the coal. The joint-fill calcite postdates most pyrite and seems to be a ferroan that is weak. Moreover, it is coarse-crystalline variety, which formed during the uplift period and erosion later in the history of burial. Pyrite forms fine to coarse crystals of the euhedral which line the joint walls or seem to be complexly inter-grown with calcite (Robinson & Prave, 2001). With the temperature of 41 degrees Celsius, negative values of carbon can become attributed as 13C depleted CO2, which is derived from organic matter oxidation and moderately to carbon values that are highly positive. It can be attributed to the methanogenesis of bacteria. There are unroofing and uplift as the Mesozoic brought about a meteoric recharge of the Pottsville strata. Fresh water influxes into the existing basin via the coalbeds and faults facilitated the late stage of bacterial methanogenesis. This accounts for the coal’s high gas content including the carbonate cementation of the cleats and the joints (Robinson & Prave, 2001).

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