What’s along the rail trail?

Posted by Jeri Jones on October 27, 2011 in Scenic Features | 1 comment

If asked what the No. 1 spectator sport in the world is, could you answer the question?

The answer is looking at rocks. Can’t figure it out? Think about the number of people who visit attractions where rocks are seen. Sites including the Grand Canyon, Niagara Falls, Devil’s Tower, Yosemite National Park and Meteor Crater come to my mind quickly. Millions of people enjoy vacations where rocks are visible and, yes, maybe I am exaggerating a bit, but it is true. On a much smaller scale is the York County Heritage Rail Trail, where more than 300,000 park users enjoy the 22-mile corridor every year. Do people come out to the trail to view the rocks? To be honest, I doubt it, but many users soak in the beauty of the trail which includes the rock outcrops.

Let’s take a look at the rocks along this popular recreational trail. The old North Central Railroad was built from the south to the north finding the easiest route with the lowest slope and still passing through the towns that would use the rail for transportation of people and goods. These towns include New Freedom, Railroad, Glen Rock, Seven Valleys and York. Interestingly enough, the rail trail crosses the county in a near north-to-south direction. The rock strata crosses York County in a southwest-to-northeast direction, which means as you travel the rail trail you are passing through numerous rock units, all telling a piece of history.

Starting in York, you will not see many exposures of limestone, but believe me — this sedimentary rock underlies the city. The rock is so soft and erodes away easily. That is why York lies in a valley compared to the harder rocks of the surrounding ridges. This limestone formed on an ancient continental shelf 600 million to 480 million years ago. Our continent at that time was not known as North America, but geologists know it as Laurentia. This continent at this point in time was located in the Southern Hemisphere. The climate was tropical, as told by the composition of the limestone and fossils found in the rocks.

Traveling out of York and going through the first ridge at Hyde, you will notice the rock has changed to metamorphic rocks. In the long rock cut south of Hyde, the rocks are quartzite and phyllite. These rocks were once sandstone and shale, again formed on the continental shelf similar to the limestone. During a large collision between Africa and Laurentia about 420 million years ago, these rocks underwent heat, pressure and movement. Look closely and you will see that the rock is bent or folded and contains numerous quartz veins telling the story of this great mountain-building event known as the Alleghenian Orogeny. In fact, all of the rocks along the rail trail have been affected by this orogeny.

Folding in quartzite and phyllite at Hyde.

Our next section of interest lies about one-half mile south of Brillhart Station. A continuous rock outcrop of phyllite (metamorphosed shale) is seen on the east side of the trail. This rock belongs to what geologists call the Harpers formation, a rock unit named for its excellent exposures at Harpers Ferry, West Virginia. This rock originally was formed in a shallow ocean as mud or clay. Similar to the rocks at Hyde, examples of folding and quartz veins can be seen here. Also, some fractures in the rocks could be small faults, suggesting movement. Notice also how tree roots grab hold of the rocks to stabilize themselves and grow using the nutrients directly from the rocks.

Howard Tunnel viewed from the south. Evidence of faulting is seen on wall on left side.

Farther south is one of the historic “gems” of the rail trail. Howard Tunnel was completed in 1840 after a 2-year construction period. Imagine digging through solid rock in the mid-1800s to accomplish this goal. Just thinking about it makes my hands hurt!

On some of the rock at either end of the tunnel, you will notice semi-circled vertical grooves. These are remnants of drilling where black powder was installed, lit and detonated, loosening the rock for easier removal. The drill holes were produced by two people working together to hammer and chisel this void. Remember, there was no power equipment available in the mid-1800s.

At the rock cut at the south end of Howard Tunnel, can you see the vertical lines in the rock? These lines were produced when another rock slid over the bedrock, producing lines known as “slickensides.” These lines are evidence that you are standing on a fault — a crack in the earth’s surface where movement has taken place. This fault, passing through this spot in westerly direction, is known as the Reynolds Mill Fault.

Drill core along the Rail Trail.

Our next point of interest is about one-half mile south of Glatfelter Station. On the west side of the rail trail sits a beautifully renovated stone farmhouse. This house is the original homestead of the Glatfelter family, who immigrated here from Switzerland in 1850. The house is composed of sandstone blocks quarried directly from the hillside behind the house. Although not well-exposed along the rail trail, this rock was formed as sand naturally cemented itself together from possible barrier islands or at the beach some 550 million years ago.

The orginal Glatfelter House composed of Antietam Formation sandstone.

From this meadow southward to Seven Valleys, the trail is in a broad valley. If you picture this valley having a “U” shape, with the top of the “U” being the ridges to your left and right, this is how the rocks are deformed here. The same rocks that are in the ridge to your right are the same rocks that make the ridge to the left. This is another example of the Alleghenian Orogeny.

Halfway between Seven Valleys and Hanover Junction near the barn of the east side of the trail marks the Martic Line. The Martic Line is named after Marticville, Lancaster County, and is believed to be a major fault that crosses southeastern Pennsylvania. To the south of this line, the rocks are heavily metamorphosed, while to the north the rock is lightly to moderately changed. The Martic Line begins in Maryland and runs to the metro Philadelphia area where the fault joins another fault system.

About 4.5 miles south of this point on the trail is Glen Rock. The town was named by railroad construction workers who encountered some very resistant rock south of town that actually delayed the construction of the Northern Central Railroad. At the north end of the borough directly across from the rail trail parking area is a small exposure of quartzite. This rock was once sand in an ancient ocean known as the Iapetus some 500 million years ago. The sandstone underwent intense heat and pressure, forming larger quartz grains and producing this metamorphic rock. This rock is very hard and can be seen here holding up a small ridge over which state Route 616 travels. You can see the layering in the rock as it tilts toward the trail. As the rock was fractured during the Alleghenian Orogeny, the quartzite broke into rectangular pieces.

From this point southward, we pass through a large area of metamorphic rocks known as schists. These rocks originally were shales and sandstones formed in deeper water of the Iapetus Ocean. During the metamorphism, for example, the clay in the shale was changed to mica, forming a new rock type. The color of these schists ranges from light green to greenish-black and varies almost on a foot-to-foot basis.

Shortly after the first overpass with Route 616 heading toward Railroad, check out one of the best folding examples in York County and certainly along the rail trail. York County parks have placed an interpretative sign at this site to explain the folding. Here the schist displays the intensity of the earth’s movement during the Alleghenian Orogeny. During metamorphism, the rock was heated enough that it became like a semi-done Jell-O and flowed and folded. It is hard to imagine the earth’s crust sliding 50 miles to the northwest during the African-Laurentian collision and causing this deformation.

The best example of folding along the corridor is found south of Glen Rock.

As you continue south, just before the small picnic area north of Taylor Hill Road, look at the rock outcrop on the west side of the trail. You will notice this rock has small cavities in it. The rock formed from lava and cooled into basalt about 420 to 470 million years ago. Actually, small, somewhat hidden bands of basalt crossing the rail trail between Glen Rock and New Freedom. The cavities represent gas bubbles in the lava, which never filled in with other minerals. A modern-day picture of the forming of the basalt is the Mid-Atlantic Ridge. Today tremendous volcanic activity occurs along this divergent plate boundary in the Atlantic Ocean. This basalt represents the same picture, only millions of years sooner.

Cavities in metabasalt north of Taylor Hill Road.

From this point south to New Freedom, you will see much of the same between the schists and basalts. Small examples of folding and quartz veins also are seen.

Our excursion ends today just south of New Freedom at Summit Grove, the highest point on the rail trail in Pennsylvania. Here the elevation is 920 feet above sea level. From our starting point at the Colonial Courthouse in York, the trail drops 520 feet. As biker riders can tell you, the steepest portion of the recreational trail is between Glen Rock and New Freedom. Riding toward York is easy, but wait until your return trip to New Freedom.