This is an EarthCache, so there is no cache container hidden at these coordinates. An EarthCache is meant to provide an earth science lesson by having you make observations and reporting these to the cache owner. For more information about EarthCaches, visit EarthCache.org
Back in your junior high Earth Science class, you should have learned about the three types of rocks: igneous, sedimentary, and metamorphic. Perhaps your teacher also explained that all the exposed rocks in western Pennsylvania were sedimentary rocks (like sandstone, shale, and limestone), and that there were no rocks younger than the early Permian times, about 280 million years ago. Well, your teacher wasn't exactly 100% correct.
During the Permian period, almost all the earth's land masses had joined together to form the supercontinent Pangea. This supercontinent lasted for about 100 million years, but began to break up during the Jurassic period to form the continents as we know them today. One of the first phases of this breakup was the creation of a rift between what is now North America and Europe, resulting in the formation of the North Atlantic Ocean. Try to imagine the tremendous forces involved in splitting a continent and moving the pieces apart. Much of this driving force was provided by an increase in volcanic activity. But not all volcanic activity occurs at the surface, and it doesn't have to look like a Hollywood special-effects extravaganza.
As you go deeper into the Earth's crust, the temperature steadily increases until, many miles deep, it is hot enough to melt the rocks. This molten rock, or magma, is under tremendous pressure. This pressure can cause magma to seep upwards through any cracks in the existing rock layers overhead in a process called volcanic intrusion. If the magma reaches the surface, it forms one of the different types of volcanos, but not all the intrusions reach the surface. If the intrusion cools and solidifies before reaching the surface, it leaves behind structures known as dikes and sills. A dike is a vertical intrusion and a sill is a horizontal intrusion. Both types of intrusions can be exposed later through erosion of the overlying rock layers. Usually, the intrusion is a harder rock, and it resists erosion, making it stand out against the surrounding rock.
Shiprock, New Mexico
So, where am I going with all this information? It turns out that there is an exposure of igneous rock in our area, and it formed during the Jurassic period about 170 million years ago. It isn't as dramatic in appearance as the dike in the photo of Shiprock, but it is the only known surface exposure of volcanic rock in western Pennsylvania. (One other dike is known, but it was found deep underground in a coal mine in Indiana County.)
This remnant of a volcanic intrusion can be found on State Game Lands 238 in Fayette County. The dike extends for 2.7 miles, but is not exposed over the whole length. The igneous rock has been identified as a type of kimberlite. (Kimberlite is just one of many types of volcanic rock, but it has one special feature - many of the world's diamond mines are in kimberlite. This dike has been examined closely for diamonds, but, alas, there is very little chance of diamonds here.) It is very dark gray in color, but the surface weathers to a greenish-gray over time. Fresh exposures will have the dark gray color. In some places, water has seeped into cracks in the kimberlite and left white calcite deposits. The volcanic intrusion has sedimentary intrusions!
EARTHCACHE Bring a tape measure and a compass. From the parking coordinates, follow the mowed areas to the "trailhead". (It's really just the easiest place to get through the brush.) Go east to the waypoint, which is where the ravine is shallow and easier to enter. Follow the ravine downhill to the cache coordinates. The outcrop will appear on the right hand (west) side of the ravine. The photo is of the part of the dike sitting at the bottom of the ravine, but if you look up the side of the ravine you can see other outcrops of the dike, including a block in the exposed roots of a tree at the top of the ravine. Further down the ravine are blocks of the kimberlite that have fallen into the streambed. To log this Earthcache you must:
1. Measure the width of the block in the streambed.
2. With your compass, determine the direction the dike is going into the hillside.
E-mail your answers to the two questions, and feel free to post pictures of this unique geological feature.
Since you are on State Game Lands, you must follow their regulations, especially during hunting seasons. Rules are posted at the parking area or can be found at SGL Regulations
Especially pertinent are the following:
-It is illegal to remove mineral specimens
-It is illegal to be in groups of more than 10 without special permission
-It is illegal to be on Game Lands between November 15 and December 15 (except Sundays) without wearing 250 square inches of orange, visible for 360 degrees.
References The diagram of volcanic action and the photo of Shiprock are from IU Online Geology