Formation of the Falls Run Gorge

Waterfalls, such as Howard Falls, are dynamic formations but can maintain their vertical faces over long periods of time. A classic example of such a waterfall is Niagara Falls, where over 100,000 cubic feet of water flow over the escarpment every second, yet the waterfall remains vertical because of a hard sandstone layer at the top, called dolostone. Howard Falls too probably has a harder sandstone layer at the top and softer shale beneath. The water of Falls Run erodes into the softer shale, undercutting the hard top layer, which eventually collapses, and the process repeats over the years.

In doing so, a waterfall remains vertical but recedes over time. The retreat of Niagara is well documented, currently receding at about 1 foot/year. In doing so, it has created the gorge below, which flows into Lake Ontario. By analyzing that gorge, Niagara has receded about 3 feet/year in the years of its existence.

All natural features in Northwestern Pennsylvania have only existed for about 12,000 years. Before then, the Wisconsin glaciation which began about 22,000 years ago sent the Laurentide ice sheet across Canada and into the northern United States. The ice sheet, up to 3,000 feet thick in northwestern PA, obliterated everything in its path, while the sheer weight of it compressed the earth surface too, depressing the landscape in northwestern PA by 100 feet or more.

The maximum extent of glacial ice in the north polar area during the Pleistocene period included the vast Laurentide ice sheet in eastern North America. (via wikipedia)

The ice sheet began to melt about 12,000 years ago, creating prodigious amounts of water. The St Lawrence Divide crosses Erie County, PA east-west several miles south of Lake Erie. Water south of this Divide flows into the Ohio River watershed and eventually the Gulf of Mexico, while water north of this Divide flows into Lake Erie, and eventually the Atlantic Ocean.

Principal hydrological divides of North America (via Wikipedia)

Falls Run had its origins at the St Lawrence Divide in southern Franklin Township. Now, a bog there collects surface water and Falls Run begins. Flowing northward toward Howard Falls, other small tributaries join to create a larger aggregate flow, eventually flowing over Howard Falls and into the Falls Run Gorge.

As the Laurentide ice sheet began to melt and expose the landscape, Falls Run would have been a raging torrent of water for long periods of time. As the ice receded, and the depressed land began to rebound, the flow would have slowed, eventually reaching the flow rate seen today.

Could Howard Falls have had its origin at Elk Creek, also formed during the same period when the ice sheet melted? The Falls Run Gorge flows northward to its confluence with Elk Creek, and the circuitous route it takes is about 3 miles long. If Howard Falls eroded up the Gorge to its present location, it would have had to recede about 1.3 feet/year for those 12,000 years, a plausible rate of recession.

Should this hypothesis be true, Howard Falls itself is responsible for the formation of the Falls Run Gorge. Starting at the high escarpment at its confluence with Elk Creek, it would have incessantly marched backward up what is now Falls Run Gorge, carving out about 60 million cubic yards of material as it cut through the soft Devonian shale in the landscape over the past 12,000 years.

It would give new meaning to Howard Falls. No longer would it just be a stationary artifact at the southern end of the Falls Run Gorge. Instead, it would be thought of as a living entity, dynamic in its movement over time, and the creator of the Falls Run Gorge itself; a completely new, dramatic role for Howard Falls.

The following graph describes various scenarios as to this gorge-creation hypothesis.

As the graph shows, Howard Falls would have had to recede about 1.3 feet/year over 12,000 years to create the Falls Run Gorge. However, assuming that the flow rate of Falls Run was greater as the Laurentide ice sheet was melting, the graph depicts the recession rate at two other flow rates. Assuming Falls Run flow rate was high early in its life, it might have begun its life at a recession rate of 4 feet/year, decreasing uniformly over the millennia to a recession rate of about 1 inch/year today. Another scenario shows a more moderate initial flow rate of 2 feet/yr, decreasing uniformly to a current recession rate of 6 inches/year.

While we will never precisely know the flow rates of Falls Run during those 12,000 years, the modeling shows that a higher initial flow rate results in a lower than average recession rate today. The current recession rate of Howard Falls needs to be quantified. Examination of photographs of the Falls over the past 100 years is inconclusive, although some erosion is evident. It is possible that the current recession rate of Howard Falls is a few inches per year.

Interesting research proposals have been suggested as a result of this recession hypothesis.

  • The Gorge, nearer to its confluence with Elk Creek, is wider and deeper than the Gorge closer to the Falls. This is consistent with what would be expected if the recession of Howard Falls created the Gorge.
  • The face of the Falls needs to be LIDAR surveyed to create a topographic map, useful to precisely track its erosion and movement over time.
  • The flow rate of Falls Run should be monitored. As the land continues to rebound from its compression by the Laurentide ice sheet and the climate in the region changes, the flow will be affected.

The Falls Run Gorge is likened to a living organism of co-dependent flora and fauna. It may be time to give Howard Falls its rightful role in that splendid ecosystem, as it may be the very reason for the existence of that Gorge.


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