Fallingwater sits within the Valley and Ridge Physiographic province which is divided into the Allegheny Mountain Section and the Pittsburgh Low Plateau Section within the area. The Western slope of Chestnut Ridge is the boundary between the two sub-sections, which puts Fallingwater in the more eastern Allegheny Mountain Section. To the east is the Allegheny Structural Front where deformation styles change significantly and are capped by the horizontally tilted erosion-resistant Pottsville Formation.
Before the Allegheny Orogeny majority of Pennsylvania and the more western surrounding states were under the Appalachian Basin complex which acted like a delta/shoreline for an ocean (Catskill Delta). Sediment was placed in the marine environment from the eastern eroding Arcadian orogenic highlands to the southeast. This range was built up during the middle to late Devonian (393 to 359 million years ago) in Maryland, New York, and southeastern Pennsylvania.
To the northwest, the Laurentian Shield (Canadian Shield) was another source for eroding sediment. The shield today is a massive section of exposed Precambrian rock layers. The area is also a part of the North American Craton where very old polydeformed crystalline basement rock is located, this can only happen when the continental crust has been through profound degrees of metamorphism continually and now remains very stable.
The Last mountain building event to affect the Fallingwater area was the Allegheny Orogeny, this affected the Appalachians during the Permian period (299 - 251million years ago) as Laurentia and Gondwana (Africa) converged.
The Allegheny deformation was developed by a low angle-thrust that extended westward, it was mostly developed through the hanging wall of an allochthon as thrust faults and fold-to-thrust structures. An allochthon is a block of rock which has been moved from the original location of deposition and formation, this is usually formed from low-angle thrust faults. In the Piedmont zone, strike-slip faults trending towards the northeast along with dextral shear zones where the fault block moved horizontally are common and represent very early deformation from the Orogeny.
Massive mining operations within Southern Pennsylvania exist for the Bituminous Coal Fields present within several sedimentary formations, mining extends into 21 separate counties in Pennsylvania alone. The Lower Kittanning Coal Bed of the Millstone Run 'Formation' is usually surfaced mined and is the most widespread in the coalfield.
Fallingwater does not lie within the main Bituminous Coal Field but it is still surrounded by it, the thin gap in the field extends to the north into the next county over (Westmoreland County) as Fallingwater is located within Fayette County.
As stated in the above section, the sediment that was deposited in the Catskill Delta formed the two below formations the Pottsville, and the Allegheny formations.
The older Pottsville Formation was once called the Pottsville Group but that has since been abandoned and replaced with formation in all states but Pennsylvania, as its split into formations based on coal seams.
Grey sandstone and conglomerate are what dominate the formation but it also contains shale, claystone, limestone, and coal. The formation was deposited within the lower/middle Pennsylvanian age (323 to 307 million years ago) and is the youngest section of two in the Carboniferous.
Formation Names - Youngest to Oldest (Pennsylvania Only)
. Curwensville (Formation)
Mercer Coal Zone
. Elliot Park (Formation)
The Elliot Park Formation is mostly sandstone with discontinuous layers of conglomerate, shale, siltstone, clay and coal but they arent as dominate. Its base is an unconformity with the older Mississippian Mauch Chunk Formation (358-323 million years old).
The younger Allegheny Formation like the Pottsville Formation has also had a name change, by the abandonment of the word group which has now been replaced with Formation, in all states but Pennsylvania, as its split into formations based on coal seams.
The Allegheny Formation is represented by cyclic (repetitive sequences) of sandstone, shale, limestone, coal, and commercially valuable clay and Vanport Limestone. Several different coals are also commercially marketable, the Freeport, Kittanning and the base layer of the formation, the Brookville-Clarion Coal. The formation was also deposited in the middle/upper Pennsylvanian Age (307 to 299 million years ago).
Formation Names - Youngest to Oldest (Pennsylvania Only)
. Glen Richey (Formation)
Upper Freeport coal bed
Lower Freeport coal bed
. Laurel Run (Formation)
Upper Kittanning coal bed
. Mineral Springs (Formation)
The Mineral Springs Formation is mostly comprised of mixed Shale and Siltstone with large interbed's of sandstone, claystone capped by coal in thin beds, and sections of shale that aren't mixed with siltstone.
. Millstone Run (Formation)
Middle Kittanning coal bed
Lower Kittanning No. 3 coal bed
Lower Kittanning No. 4 coal bed
Lower Kittanning No. 1 coal bed
. Clearfield Creek (Formation)
Clarion No. 1 (Brookville) coal bed
McElroy, T. A., Shaulis, J. R., & Kuchinski, J. G. Pennsylvania Bureau of Topographic and Geologic Survey, (1988). Geologic map of Fayette County, Pennsylvania showing the locations of wells and springs. Pennsylvania State University Libraries.
Skema, V. W., Edmunds, W. E., & Flint, N. K. Part II, Stratigraphy and Sedimentary Tectonics, Chapter 10: Pennsylvanian, The Geology of Pennsylvania: Pennsylvania.
Bleeker, W., & Davis, B. W. (2004, May). What is a craton? How many are there? How do they relate? And how did they form? NASA/ADS.
U.S. Department of the Interior, U.S. Geological Survey, Milici, R. C., Freeman, P. A., Carlton, R. W., Bragg, L. J., Butler, D. T., & Tewalt, S. J. (2000). Geology and Geochemistry of the Middle Pennsylvanian Lower Kittanning Coal Bed, Allegheny Group, Northern Appalachian Basin Coal Region, U.S. Geological Survey Professional Paper, 1625–C. Northern and Central Appalachian Basin Coal Regions Assessment Team.
United States Geological Survey (USGS). (n.d.). Mineral Resources, Pottsville Formation. Retrieved September 3, 2020.
United States Geological Survey (USGS). (n.d.). Mineral Resources, Allegheny Formation. Retrieved September 3, 2020.
Western Pennsylvania Conservancy. (2000, July). Fayette County Natural Heritage Inventory.
Designing Fallingwater. (n.d.). Fallingwater.org.
DEP. (n.d.). PA Mining History. Department of Environmental Protection.
United States Geological Survey (USGS). (n.d.). Mineral Resources, Mauch Chunk Formation. Retrieved September 9, 2020.
Faill, R. T. & American Journal of Science. (1998, October 16). A geologic history of the north-central Appalachians, part 3. The Alleghany orogeny. United States Geological Survey (USGS).