The Carlisle Deluge, 1779, Revisited

We report here that evidence for the 1779 Carlisle Deluge still exists. In the Summer, 1996 issue of Cumberland County History, Whitfield J. Bell described what he called the Carlisle Deluge.¹ Bell used primary sources, mainly a letter from David Rittenhouse to Benjamin Franklin,² to describe how, on the night of August 19, 1779, a thunderstorm with copious rain opened a gash on the south side of North (Blue) Mountain east of Flat Rock and northwest of the present Bloserville. Rocks and trees were carried down the mountain. A channel four to ten feet deep and 30 to 100 feet wide was carved into the mountainside from near its top to the base. According to a later report by Benjamin Lincoln³, mud rose in the channel to as much as 30 feet deep, as determined by marks on trees left standing. The scar could be seen from Carlisle, ten miles away. The water caused Conodoguinet Creek to rise ten feet in a very short time. The feature attracted the curious for some decades afterward.

We were particularly interested in Bell's account of what was surely a debris flow or debris avalanche because this may be one of the earliest accounts of such an event in North America. We reasoned that if we could find the deluge scar, over 200 years after a documented event, we could study how the evidence of these catastrophic features changes with time. We knew what to look for because we have seen the effects of recent debris flows elsewhere in the Appalachians shortly after they occurred.

Debris flows, or avalanches, occur when soil on steep slopes becomes so saturated with water that it loses strength, and a mass of soil, rocks, and water moves downhill. An extreme rainfall or snowmelt is usually required to supply the large amounts of water. The flowing material resembles wet concrete, and can move slowly or very rapidly. The velocity of the flow depends on the consistency of the debris and the steepness of the slope. Large rocks up to several feet in diameter are carried away in the soupy mass, and trees are uprooted. At the head or uppermost part of the avalanche, soil is often stripped to the underlying bedrock. A channel is carved in soil by the avalanche as it moves downhill. Excess debris spills out along the margins of the channel. This spilled muddy debris, often consisting mainly of rocks with a small amount of mud, creates rims known as levees. Downhill where the slope decreases, the flow slows, and much of the debris is deposited as large piles and tongue-shaped masses of mud and boulders. Most of the water moves on downstream. Within years to decades after the event, the channel becomes revegetated and evidence of the scar becomes harder to find. Bell reported that during a helicopter flight over the area on March 30, 1996, no evidence of the deluge could be seen.⁴

We decided to attempt a search for the deluge on the ground. On April 10, 1997, before the trees had leafed out, we went to the site, described as about a half mile east of Flat Rock. We studied the mountain front with binoculars from near Pa. Route 944 for signs of the deluge and could see nothing identifiable. We then drove up onto the mountain as far as we could on an unimproved road among many small parcels of private property. There, thanks to the kindness of a local resident, we obtained access to Tuscarora State Forest land about half way up the mountain. We traversed the mountain parallel to the ridge. We were looking for a nearly straight channel with boulder-strewn levees, and found what is undoubtedly the channel of the Carlisle Deluge about 3/4 mile east of Flat Rock. Today the channel is overgrown with mature trees similar to those in the forest on either side of the channel. Because nearly all forests in the area have been extensively logged, it is unlikely any trees survive from the time of the deluge. We followed the channel part way both up and down the mountain. It has well-developed boulder levees everywhere except at its upper end. Boulders in the levees are commonly up to two fee t in diameter, and we found several up to six feet across. The channel varies in width from 20 to 80 feet. Depth below the tops of the levees is 4 to 12 or more feet. Downslope, the slope is less steep, the channel is wider and shallower, and boulders more than one foot in diameter are spread over an area at least 200 feet wide.

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