Maryland Geology Day Trip

We had just gotten a new copy of The Roadside Geology of Maryland, Delaware and Washington, D.C., so I knew what she had in mind. She wanted to see some clues to how the environs we know so well came to look the way they do. And if I could throw in some fine scenery and fall colors, so much the better.

After perusing the RGMDW for some suitable roadside destinations, we decided to start at an old favorite: Sideling Hill.

Sideling Hill is one of the long parallel ridges that make up the ridge and valley region of the Appalachian Mountains. Starting somewhere in central Pennsylvania, it slants southwest across the Maryland panhandle, just west of Hancock, and continues on halfway across West Virginia.

The ridge is bisected at the West Virginia line by the Potomac watergap, where, during the course of 100 million years, streams on either side of the ridge eroded back into the ridge until they met and the stream on the east side captured the stream on the west side. That was roughly 300 million years ago.

In 1984, just a few miles north of the watergap, the engineers building Interstate 68 accomplished roughly the same thing in a matter of months. Driving west on old Highway 40, the route I-68 replaced, the road took a sharp left at Sideling Hill, contouring up the east side of the ridge to a low spot five miles south. At the top, the road reversed course, descending diagonally down the west side of the ridge until reaching the spot opposite its first detour and veering west again. On the map, this bit of engineering created a deep, V-shaped detour in the old National Highway.

Modern Interstate Highway designers, though, don’t tolerate such detours. Instead, the I-68 engineers simply blasted through Sideling Hill, carving a notch in the ridge that’s 340-foot deep and wide enough for a six-lane highway to pass through. The Great Pyramid of Giza, turned upside down, would fit nicely in the gap.

In addition to speeding the east/west passage of traffic, the beautifully excised Sideling Hill road cut has become a Mecca for geologists and geologist wannabe’s like me and Dawn. That’s because the facing escarpments of the roadcut expose a spectacular textbook version of a syncline, one of the principal features of the ridge and valley geology of the Appalachian Mountains.

The Appalachians were created about 340 million years ago when the African tectonic plate slammed into the East Coast of North America. The force of this collision caused the land to the west to fold up like a table cloth pushed from both ends of the table. Geologists call places where these folds point up anticlines. Synclines, like the one at Sideling Hill, mark the places where those folds dipped downward, bending the layers of rock into a U-shape.

Part of the reason the road cut through Sideling Hill is so mesmerizing is that the U-shape of the syncline seems to defy logic. The upside-down U of an anticline simulates the slope of a hill. But it’s hard to understand how a syncline could create a mountain as long and steep as Sideling Hill.

As with so many geological questions, though, the answer is simple: erosion.

Sideling Hill exists because the layer of Purslane sandstone at the top of the ridge is less susceptible to erosion than the layers of shale that were exposed on either side. 300 million years ago, the ridge was a lowspot in the Acadian Mountains, one of the antecedants to the Appalachian chain. But as the softer layers of the Acadians eroded away through the millenia, the obdurate sandstone atop Sideling Hill and the other ridges of the ridge and valley region remained.

If you look closely at the walls of the roadcut, you can see this process in action. The syncline is composed of alternating layers of hard sandstone and softer layers of shale. These layers have weathered at different rates so that the sandstone layers now stand out in bold relief against the more quickly eroded shale layers.

But you don’t need to be a geologist to appreciate the roadcut at Sideling Hill. The remarkable cross section of rock created by I-68 is so popular with spectators that the engineers had to build a vistors center at its base. The center was closed down several years ago due to budget cutting, but the parking lot is still open and visitors can still hike up the staircase to a viewing platform about halfway up the north face of the roadcut.

There is also a pedestrian bridge linking the parking lots on the north and south sides of the highway. Small window frames have been left in the chainlink fence so you can get good pictures from the center of the bridge.

After you’ve had your fill of Sideling Hill, there are plenty of other sights to see in the area. In fact, if you’re anything like me and Dawn, geology is really just a pretext for sightseeing.

Town Hill, the ridge just west of Sideling Hill, has its own roadcut. Although it’s not as spectacular as the one at Sideling Hill, it’s worth a detour.

Even more amazing is an exposure of paper-thin layers of Brailler shale on 15-Mile Creek Road, just south of I-68 near the entrance to Green Ridge State Forest.

And if you don’t mind driving on a well-maintained gravel road, you can take 15-Mile Creek Road to Green Ridge Road, which runs along a high ridge all the way to the Potomac and the C&O Canal. Dawn and I did this drive in mid-October and the fall colors rivaled those of Skyline Drive, with this distinction: We were just about the only ones on the road.

Other sights not to be missed in Green Ridge State Forest include Paw Paw Tunnel, where the C&O Canal bores 3,118 feet through aptly named Tunnel Hill. If you have a flashlight, you can hike through this marvel of 19th Century engineering.  In full fall foliage, it’s hard to imagine a better view. And you’ll probably have the overlook all to yourselves.