Pittsburgh Coal Seam

Pittsburgh Coal Seam

The Legacy of the Pittsburgh Coal Seam

The Pittsburgh Coal Seam holds a significant place in the annals of American coal mining history. Its remarkable thickness and widespread distribution have made it a crucial economic asset in the eastern United States’ Appalachian Basin.

Origins of the Pittsburgh Coal Seam

The Pittsburgh Seam, named after its surface exposure on Mount Washington’s sheer north face in Pittsburgh, Pennsylvania, forms the base of the upper coal measures of the Allegheny Plateau, also known as the Monongahela Group.

Geological Formation

The coal bed’s formation traces back to the Pennsylvanian and Permian eras. It was formed in a subsiding foreland basin filled with sediments eroded from an ancient landmass located to the east.

Peat Mire Development

The Pittsburgh coal bed was created during a pause in active clastic deposition that allowed for the growth of a vast peat mire. The resulting peat deposit eventually became one of the world’s most valuable energy resources.

Underlying Erosion Surface

Significant portions of the clay layer immediately below the Pittsburgh coal rest on an unconformity, an old eroded surface. This makes the Pittsburgh seam the basal member of the Monongahela Group, while the underlying erosion surface is considered the top of the Conemaugh Group.


The Monongahela Group, primarily made up of sandstone, limestone, dolomite, and coal, comprises a series of up to ten cyclothems. Each cyclothem represents a period when the land was flooded, allowing the accumulation of marine deposits such as shale, limestone, and sandstone. When the sea level fell, coal formed from the remnants of swampland.

Exploitation of the Pittsburgh Seam

The Pittsburgh Coal Seam has played an integral role in the region’s industrial growth. Its exploitation began in 1760, with coal extracted from drift mine entries into the seam.

Initial Mining Activities

The coal, mined from Coal Hill (now known as Mount Washington) across the Monongahela River from Fort Pitt, was used to fuel the military garrison.

Mining Expansion

By 1796, coal mines extended along the face of Mount Washington, centering across the Monongahela from Wood Street. By 1814, there were at least 40 coal mines in the Pittsburgh region, worked from adits in the face of the coal seam using room and pillar methods.

Pittsburgh Coal Seam

The Coal Boom

The Pittsburgh seam was America’s primary seam of coal production during the late nineteenth and early twentieth century. The Pittsburgh-seam coal was ideally suited for making coke, particularly for iron blast furnaces, fostering the development of southwestern Pennsylvania.

Transition to Coke

During the early nineteenth century, entrepreneurs in western Pennsylvania adapted British coke-making practices to produce coke for local iron works. To make coke, coal is burned under controlled conditions to drive off volatile matter, leaving carbon and ash from the coal fused together in the form of coke.

Rapid Growth

The mining of Pittsburgh-seam coal boomed after 1860 as the rapidly expanding iron and steel industry adopted coke. The output of the industry burgeoned during the late nineteenth and early twentieth century as demand for steel products surged.

Pittsburgh Coal Seam

Significant Areas of the Pittsburgh Coal Seam

The Pittsburgh Seam extends over large areas. Two prominent regions are the Georges Creek Valley of Western Maryland, known as The Big Vein, and the Irwin Basin.

The Big Vein

The Big Vein was discovered in 1804, in an outcrop east of Frostburg, Maryland. However, it wasn’t until 1824 that small-scale shipment to Georgetown began.

Development of the Big Vein

In 1842, the Baltimore and Ohio Railroad reached Cumberland, followed in 1850 by the Chesapeake and Ohio Canal. These transport links allowed large-scale exploitation of the Big Vein.

The Irwin Basin

While the Irwin Basin is a significant part of the Pittsburgh Coal Seam, the specific details about this region require further expansion.

Pittsburgh Coal Seam

Despite two centuries of mining, the Pittsburgh Coal Seam still holds about 16 billion short tons of resources, with the largest remaining block in southwestern Pennsylvania and adjacent areas of West Virginia. However, much of the remaining resource to the south of Marion County, W. Va., and west through much of Ohio is high in ash and sulfur, and is not likely to be extensively mined in the near future given current economic trends.

The legacy of the Pittsburgh Seam, its formation, and subsequent exploitation, remains an integral part of America’s coal mining history. Its contribution to the growth of the region and the country at large cannot be overstated.

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