Last week I mentioned, in passing, an event that occurred very late in the life of the current drainage system in southwestern Pennsylvania – the development of Lake Monongahela – and glossed over its influence on the Chartiers Valley, while promising myself to discuss it in detail this week. This remarkable event occurred in recent (geologically speaking) time, yet long before the arrival of the human species.
Scientists believe that the drainage system here one million years ago was strictly to the north, into a predecessor of the St. Lawrence River. An early version of the Monongahela River began in southern West Virginia, followed its current path north to Pittsburgh where it met the “lower Allegheny” (primarily the current Kiskiminetas/Conemaugh system). It then flowed up the Ohio Valley to Beaver where it joined a northerly flowing stream in what is now the Ohio River Valley, a stream whose headwaters were also in southern West Virginia. The ancestral “Monongahela” then proceeded up the Beaver Valley into Canada, eventually reaching the ancestral St. Lawrence. The “middle Allegheny” (primarily the Clarion River) flowed north from Emlenton to Franklin, then up the French Creek valley and to Canada. The “upper Allegheny” flowed north from Warren into New York State and on to the ancestral St. Lawrence. We believe Chartiers Creek generally followed its current route even in those days, joining the Monongahela at McKees Rocks.
When the Laurentide Ice Sheet came south (about 900,000 BC), it eventually covered most of northwestern Pennsylvania and nearly all of Ohio. A few miles north of Beaver the leading edge of the glacier was perhaps three thousand feet thick, a massive dam terminating flow to the north. Water began to back up in the “Mon”, all the way to Weston, West Virginia, two hundred miles to the south. Eventually it reached a level 1,140 feet above sea level. For reference, where I sit in my living room, I am very close to that elevation. The resulting lake, Lake Monongahela, would be lapping at my front steps. Based on current topographical maps, the bay reaching back through Chartiers Valley would cover every inch of Bridgeville. A narrow peninsula would run up Bower Hill Road in Scott Township beginning just below Kelso. Two islands on Cook School Road would lead to another peninsula on Fort Couch Road. To the southeast would be another pair of tiny islands just beyond Lesnett Road. The “Cow Hollow Bay” would extend almost to Mayview where there was another narrow peninsula. A peninsula beginning at Fairview and National Hill would be pierced by “Coal Pit Bay”, half a mile wide where Alpine crosses it today. “Millers Run Bay” would extend west within four miles of Hickory and be intersected from the northwest by a series of narrow, steep-walled bays, beginning with “Thoms Run”.
This was a major geological event that lasted several hundred thousand years, terminating only when the glacier receded, leaving massive terminal moraines as permanent barriers to drainage to the north. In the interim the lake found an outlet near Moundsville and began to carve out a new riverbed, the current Ohio, reversing the flow from Beaver to the south. The “upper” and “middle” ancestral Allegheny Rivers joined their “lower” compatriot to form the current Allegheny River. Its puzzling zig-zag path from Port Allegany north into New York then back to Pittsburgh via Warren, Oil City, Franklin, Emlenton, and Kittanning makes much more sense knowing this history. Nonetheless, through all of this we believe the Chartiers watershed retained most of its heritage.
However, I have always been puzzled by the fact that Gould City Hill is on the wrong (inner) edge of a major Chartiers Creek meander near Presto. Currently I speculate that, before Lake Monongahela, Chartiers Creek flowed down Washington Avenue to Lower End and from there on to Woodville. The pre-Ice Age Millers Run skirted the north side of Gould City Hill, meandered to Presto, then back to Lower End where it joined Chartiers Creek. When Lake Monongahela finally drained and emptied itself into the brand-new Ohio River, it left behind enough lacustrine deposits to persuade Chartiers Creek to abandon Washington Avenue and proceed straight ahead to Millers Run and then appropriate its streambed out through Presto and then back to Lower End. This speculation is supported by a careful examination of topography across downtown Bridgeville – shallow slope on the inside of the meander (Chestnut to downtown) and steep cliff (Gould City Hill) on the outside. Granted this is pure speculation, but it seems as logical as many of the scientists’ theories.
A similar mystery has always been the saddle between Cook’s Hill and Fryer’s Hill, the current route of Bower Hill Road. What stream carved out that feature? Did McLaughlin Run originally follow Bower Hill Road to the northeast and meet Painter’s Run at the “Panhandle” before joining Chartier’s Creek? Or did Painter’s Run ignore its propinquity to Chartier’s Creek at the “Panhandle” and flow to the southwest and join McLaughlin Run at the other end of Bower Hill Road? One clue is the fact that the saddle does slope gently (fifty feet in half a mile) to the southwest. I am not as confident about this speculation as my earlier one, but I give the Painter’s Run to McLaughlin Run speculation a sixty/forty probability and suggest that it too existed before Lake Monongahela arrived and terminated when the Lake subsided.
It is intriguing to imagine Lake Monongahela filling up and the edge of the Chartiers Creek Bay creeping up Bank Street, slowly flooding Gregg Avenue, then Elizabeth and Elm, then rapidly covering the relatively flat land above it before reaching Lesnett Hill, before completely engulfing even it. Equally intriguing is imagining the massive Lake receding ever so slowly, probably leaving behind a landscape somewhat different from the one it found millennia earlier.