 |  | GEOLOGY / PHYSIOGRAPHY / HYDROLOGY
SOIL & VEGETATION / HISTORIC VEGETATION
THE PRESENT FOREST - IMPACTS & TRENDS |
Geology/Physiography
This particular landscape has been shaped largely by water and rock. The great inland sea that covered southwestern Pennsylvania some 600 million years ago left layers of sediment that gradually hardened into the sedimentary rocks seen today at Fallingwater. These cyclical beds of Pottsville sandstone, shale, and limestone were pushed up into long parallel ridges when the continental plate containing North and South America collided with the continental plate containing Africa, about 300 million years ago. Since then, water falling and forming streams has eroded the softer limestone and shales. The stream at Bear Run dropped from the more resistant sandstone ledges to create a series of waterfalls. It is at the most dramatic of these falls, on top of the stream, just above the dropping water, that Wright sited the Kaufmann house. The large, fractured sandstone ledges visible at these falls are faithfully echoed in Wright’s design of the cantilevered terraces.
Bear Run - and almost all of southwestern Pennsylvania - lie within the Appalachian Plateaus and Moutains. This long and relatively narrow physiographic region stretches from northern Georgia to southern portions of New England. The terrain consists of ridges, plateaus, steep slopes, and dramatic gorges - landform variations that provide a range of orientation, elevation, and moisture that in turn govern the different expressions of the forest.
Commercially valuable clays and coals underlie most of our region. The combination of these minerals, timber, and the abundant streams and rivers for water power and transportation fueled an explosion of industry in the 19th century that left a lasting effect on the regional landscape. Seams of soft coal lying just beneath the land’s surface were readily stripped away, gouging huge holes in the hillsides, dismantling forests, removing soil, altering and polluting waterways. These industries encouraged the expansion of the railroads across the Allegheny Mountains which in turn fueled further commercial exploitation of the region.
Scars from coal mining and its related industries are still visible throughout southwestern Pennsylvania. At Bear Run, thanks to the careful consideration given the site first by the Kaufmanns, then by the WPC, small surface mines have been closed up, and timbering has been stopped, allowing the forests to grow dense again.
Hydrology
Bear Run is a swift stream, fed by numerous mountain springs, falling 1,430 feet in four miles from the western slops of Laurel Hill ridge to join the Youghiogheny River. Continuing downstream, the Youghiogheny flows into the Monongahela River, which joins the Allegheny River at Pittsburgh to form the Ohio River. Eventually, the Ohio winds its way to the Mississippi River, which flows to the Gulf of Mexico.
Small waterfalls, or cascades, dot the length of Bear Run. Streams with waterfalls represent especially dynamic interactions between the geology and hydrology of a site. Waterfalls develop where streams cross resistant ledges of rock. In one type, the stream crosses a relatively flat-lying, resistant stratum and excavates a deep channel in the easily eroded, underlying formations. A series of cascades such as those of Bear Run develop in similar fashion, but because the resistant beds and more erodible beds are relatively thin and cyclically interbedded, the falls develop a characteristic "stair-step" profile. Such falls retreat upstream without appreciably losing their height.
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