lower-than-normal Mountain Lake in June, 2002
Mountain Lake's origin is has its own mystery to it, even though Virginia Tech geologists have finally deciphered the plumbing and origin of the lake.
From 1998-2001, the surface area shrank by half, down to 25 acres. As of June 2008, the lake level remained far lower than average, despite record-breaking rainfall in the first half of 2008. The same shrinkage happened in the 1950's.
Mountain Lake shrinks, even disappears at times. When Christopher Gist came through the area in 1751 on his way to explore lands claimed by the Ohio Company, he found a lake. In 1768, settlers recorded just a spring in a valley with a grassy meadow. It was a good location for watering cattle and providing them salt as a dietary supplement, and the settlers named the location Salt Pond Mountain... while questioning the honesty of Gist's records of his explorations. Later, however, the lake reappeared.
Mountain Lake has a leak. There's a natural hole in the bottom of Mountain Lake, 106 feet deep when the lake is full (and covers about 50 acres). When silt plugs the hole, rainfall exceeds evaporation and the lake level rises. Once the lake reaches the level of the outlet to Little Stony Creek, any excess inflow from rain and springs will just flow down to the New River. In drought years, the outflow may be reduced - but drought alone does not explain the lake's dramatic shrinkage every 50 years or so.
Silt and leaf litter is continually washing into Mountain Lake, but the water draining through the natural crevice in the rocks at the bottom of lake erode away the silt plug occasionally. When that happens, more lake water flows underground to a nearby spring and the surface level drops. Occasionally, the lake may dry up conpletely.
The rock at the bottom is presumed to be sandstone; Salt Pond Mountain itself is not limestone, so the lake is not related to the nearby karst topography. The cracks in the sandstone at the lake bottom open and close because of earthquakes, primarily. A fault runs right through the hole. Apparently one of the constant minor earthquakes in Giles County is also required to adjust the sandstone at the bottom, realigning the rocks so the natural silt can seal the crevices between them again. When that occurs, it takes only a few years for the lake to rise to the level of the outlet again and start the process once more.
Because the underground passage was wide enought to carry sediment, no organic material accumulated in the stream bottom. . The soil in that small watershed washed away, and a deep little valley was etched into the side of Salt Pond Mountain - until the crevices are sealed by rock movement during an earthquake, and a lake fills the valley.
If for some reason the hole at the bottom of the lake were to be sealed completely, then sediments would accumulate and Mountain Lake would gradually become a bog. Soil would develop in the bog, as bedrock decayed and organic nutrients were deposited each Fall. Trees would grow in the soil, and a forest would replace the bog. That's what happened to all the other depresssions in the mountains that did not have a discharge outlet, draining water to the Atlantic Ocean/Gulf of Mexico.
Until recently, the traditional story of the formation of Mountain Lake was that it was caused by a large rockslide. When the layered sediments on Salt Pond Mountain eroded, a layer of soft shale eroded faster underneath a ledge of hard sandstone. The overhanging sandstone ledge grew too large, and finally collapsed in a landslide that blocked Doe Creek. Water piled up behind the landslide, forming Mountain Lake.
The landslide was so large that the new dam was higher than the surrounding edges of the lake. The rising water did not flow over the dam back into Doe Creek, but instead was diverted to a side drainage - Little Stony Creek. That's why the dam did not erode away, as typically happens after Blue Ridge and Appalachian landslides, and create a small canyon through remains of the old landslide. Instead, the waters of Mountain Lake flow down Little Stony Creek now, over the large Cascades waterfall (gorgeous spot!), past Tangent Outfitters on US460 at Pembroke, and on to the New River in Giles County.
But that's not the full story. Turns out the Clinch Sandstone has a crack in it, and the lake may have formed in part because the sandstone eroded into the crack. As fast as overhanging ledges or even a cave may have formed in the sandstone, however, the rock collapsed into the crack and was carried away down the hole. The depression filled by the lake was formed in part by a landslide damming up the valley, but also in part by the collapsed sandstone cavity.
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