The Stoney Brook Railroad cut – a glimpse into the past

Posted by on August 4, 2011 in Scenic Features | 0 comments

Compared to spectacular, massive rock exposures in southwestern United States, York County supports only small rock outcrops, unless you are in the area of one of our quarries.  This makes interpreting the geology of the area a bit more complicated.

For example, for those of you that have visited the world famous Grand Canyon in Arizona, it is easy to look at the various rock layers, most of which are horizontal and colorful.  In our area, the rocks have been greatly distorted and literally mixed up so that the geology is much more complex.

Let’s take a look at one of the most interesting small rock outcrops that yields some good information. We are going to visit the Stoney Brook railroad cut, located just west of the bridge on Pleasant Acres Road in Springettsbury Township.  This has been a classic site for those studying geology for many years.

To make this story a bit more interesting, let’s walk the railroad tracks from the east to west.  On both sides of the Pleasant Acres Road overpass, you will find limestone belonging to the Conestoga Formation.  This rock unit underlies much of the York Valley from Wrightsville to Hanover, following U.S. Route 30 and Pa. Route 116.  The limestone is a calcium carbonate sedimentary rock and is laid down in relatively thin layers.  The rock is a light bluish-gray with calcite being the dominant mineral in the rock.

To test for limestone, scratch the surface with a sharp implement and place some vinegar on it – the rock will bubble as it reacts to the calcite.  You may also drop a drop of muriatic or hydrochloric acid on the limestone for slightly more dramatic reaction.
The limestone is about 450 million years old belonging to the Cambrian and Ordovician periods.  The Conestoga Formation rocks and associated older limestone and dolomite (a calcium magnesium rock) make up a thick sequence in the York Valley.  The presence of these rocks in the area is why we have numerous active quarrying operations using the limestone and dolomite for various uses.

Geologists have interpreted that these rocks were formed from sediment forming on an continental shelf along an ancient North America.  Look closely on the top and bottom surfaces of the limestone here and you might see mud cracks and ripple marks.  Exactly what do these two features tell us?

Mud cracks show evidence that at times, the sediment along this continental shelf was exposed to the air.  The sun had an opportunity to dry out the saturated sediment, thus cracking the sediment.  Think about the infield of a baseball field.  After rain water evaporates and the sun dries the dirt out, the material cracks due to differential drying.

Ripple marks are present today in the shallow water at the beach.  As wave action comes up onto the beach, the sand takes on a bumpy look.  Ripple marks present in the Conestoga Formation tells us that some layers of the limestone were formed either close to the beach or at least in very shallow water.

Ripples marks similar to these have been seen in the Conestoga Formation at Stoney Brook.

Notice which way the rock is tilting into the Earth.  The rock
is tilting toward the south at about 20°.

Layers of sediment that eventually become rock are usually formed near a horizontal position.  If the rock is tilted in some direction other than close to horizontal; a geologist will have to determine what occurred to cause the layers to shift.

Now continue to walk west along the railroad track.  On the north side of the track, you will notice that the limestone takes on a totally different appearance. The rock changes color to a brownish, denser rock.  The layering in the rock has disappeared as the rock takes on a blocky appearance.  Before stopping and looking at this rock, continue to walk west and you will come upon a very dense grayish rock, protruding outward toward the track more than the limestone.  This massive rock represents another part of the geologic history seen here.


Remembering that the Conestoga Formation limestone formed during the Cambrian and Ordovician periods, this rock was actually formed about 170 million years ago.
This rock is known as diabase.  Diabase is a very popular rock found on the Gettysburg National Military Park and forming Devil’s Den and Little Round Top, to mention several.  Diabase was formed by magma that never reached the surface, but instead cool and solidified into a rock at least one mile in depth.  The composition of diabase is similar to the better-known volcanic rock basalt.

View of the Stoney Brook diabase dike looking west.

So how did the diabase form?  Different from the ocean environment involved with the limestone here, the diabase is a relict of the supercontinent Pangaea.  Some 200-150 million years ago, Pangaea was splitting apart into the continents we know of today.  Although the main area of the rifting was occurring in northern York County (as told with previous blogs), some of the Earth’s crust was torn adjacent to the main rifting.
Here, there was a 20-foot wide rift that opened, allowing magma to fill in.  This crack or today known as a dike runs in a near northward direction from northern Maryland to near Elizabethtown, Lancaster County.  A small ridge has formed marking the location of the diabase, due to its resistance with weathering and erosion.  Today this exposure remains one of the best examples of a dike in Pennsylvania.

A group of geologic students listen to the leader who discusses the dike

Another location to see the diabase belonging to the Stoney Brook dike can be seen along trail #6 in the southern end of Rocky Ridge County Park.  Small rounded boulders with an orange “skin” can be seen in the trail.

Geologic map of the Rocky Ridge County Park area. The solid red line represnts the Stoney Brook dike passing through the Hellam Conglomerate.

The brownish rock you encountered on the walk between the limestone and diabase is known as hornfels.  Hornfel is a metamorphic rock that was formed by heat.  In this case, the heat from the magma (which is believed to be about 1100° C., baked the limestone, slightly changing the minerals and making the rock denser.  With that in mind, now walk east from the diabase dike and try to find the border between the hornfel and original limestone.  In larger diabase intrusions in southeastern Pennsylvania, valuable mineral resources have been formed in the hornfels.  Several examples include the Dillsburg magnetite mines and Cornwall Iron Mine in Lebanon County.

So as you leave this site, just realize that you have just witnessed about 280 million years of history.  Between the ocean environment that formed the limestone and the breakup of Pangaea, this area was land just like it is today.  Between that time, no sediment was laid down to create new rocks, but there was much shifting of the crust about 320 million years ago when Pangaea began to assemble through a series of collisions.

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