In the Appalachian Mountains there is a region known as the ridge-and-valley province. In Pennsylvania, Maryland, and West Virginia, this physiographic province is dominated by the Allegheny Mountains. This area is characterized by long and narrow alternating sequences of ridges and valleys that are generally parallel.
The (simplified) geologic history of this area began over 400 million years ago during the Devonian Period. For millions of years this once-low region was relatively flat and stable. Rising and falling sea levels over millions of years laid down alternating horizontal beds of sandstone, shale, siltstone, limestone, and coal depending on the depositional environment at a given time.
Between 350 and 300 million years ago during the Mississippian and Pennyslvanian Periods, the Alleghenian orogeny distorted these sedimentary rock beds. During that time the North American and African continents collided from the force of plate tectonics, compressing the existing bedding and folding it.
Folding of sedimentary bedding resulted in two main geologic structures known as anticlines and synclines. Anticlines are raised folds while synclines are depressed folds. As these beds were compressed, the folding of anticlines lead to stress fractures deep into the bedding. Over the last 300 million years, these folds have been continually eroded by geologic processes. The anticlines in particular, with their deep cracks, were especially susceptible to erosion.
Today it’s the synclines that form the resistant ridges of the ridge-and-valley province, while the breached anticlines have been largely removed by erosional forces. This fact is evident in part thanks to exposures like the Sideling Hill road cut on Interstate 68 west of Hancock Maryland.
This road cut was constructed in the early 1980s as this section of I-68 was completed. At the time it was of great interest to geologists, and it’s one of the premier geologic features in the eastern United States. In 1991 a visitor’s center was opened to provide interpretation of this feature to the tens of thousands of people who visited annually. Sadly this visitor’s center was closed in 2009 due to state budget cuts, but the highway overpass and walkways remain open.