Visible Bedrock of Oil Creek State Park
There are three major types of bedrock based on their origin: Igneous, Metamorphic, and Sedimentary. Igneous rocks from magma from inside the earth that may or may not come to the surface of the earth. The magma cools creating rock. Types of igneous rock are basalt and rhyolite. Sedimentary rocks form when sediments are carried by water and deposited into a body of water. Over eons of time, the sediments build up hundreds or thousands of feet thick eventually becoming buried deep in the earth. Pressure, heat and chemical reactions cause the individual grains to become cemented together. Common of examples of sedimentary rock are shale and sandstone. Metamorphic rocks are formed when existing rock is changed when exposed to great heat. Examples of this: Metamorphosed shales and siltstones become hornfels, sandstone becomes quartzite and lime stone becomes marble. Most bedrock under Pennsylvania is sedimentary but the other two types of rock are also found in PA.
This Graphic represents the layers of bedrock in the park. Note which sandstone is above the other.
There are two readily observable and identifiable bedrocks in the park. Shenango Sandstone formed over 325 millions years ago in the Mississippian period. It is fairly easy to identify because it has cross-bedding and pockets of more easily eroded sandstone which erode faster than the rest of the rock leaving “pock marks” in the face of the stone. Cross bedding is the criss-cross pattern in the rock layers and occurs in this sandstone due to large ripples making underwater sediments move and creating sand-dune like patterns in the sediments. On the whole, Shenango Sandstone is quite resistant and forms most of the scenic cliffs, overhangs and water falls in the park.
One way Cross Bedding Can Occur
One Example of Cross Bedded Rock
Corry Sandstone is another commonly seen bedrock in the park. It is also a sedimentary rock and is made of sand sediments. It is an older rock and therefore is located under the Shenango Sandstone in its natural position. This does not exhibit the cross-bedding or pock-marks common to Shenango Sandstone. A distinguishing feature of Corry Sandstone is some layers of it contain abundant fossils of lamp shells, clams, stem segments of sea lilies, and sponges.
Typical Non-Cross Bedded Sedimentary Rocks
To complete this earthcache you must visit the designated points, answer the questions and email them to us. Feel free to upload photos, but don’t upload photos of the stone blocks which are used for this earthcache exercise.
1) Go to Stage 1 (the coordinates at the top of the page). Look for the large bedrock sandstone block. Describe the bedrock- color, texture, does it have cross bedding? Does it have pock marks? Do you see any fossils? Take the elevation at the boulder. From this information, is this the Shenango or Corry Sandstone?
2) Next go to the stage 2. Look for the large bedrock sandstone block. Describe the bedrock- color, texture, does it have cross bedding? Does it have pock marks? Do you see any fossils? Take the elevation at the boulder. From this information, is this the Shenango or Corry Sandstone?
3) What is the difference in the two elevations? Does the relative elevation positions seem to support that the Shenango Sandstone is younger and therefore on top of the Corry Sandstone? Why or why not?
Email the answers to our email via our profile. You may log your find at the same time you send your answers. Any log not supported by answers emailed to us, will be deleted. Enjoy our Earthcache!
This Earthcache is placed with permission of DCNR- Oil Creek State Park. Please thank the Park staff for allowing us to enjoy Geocaching in Oil Creek State Park.
Source for this Earthcache are:
Harper, J.A., 1998, Pennsylvania Trail of Geology, Oil Creek State Park, Venango County, “Ice and Oil Shape the Land”, Park Guide 22, PA Geological Survey, 4th ser., 12 p.
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