Cracks! In! Rocks! EarthCache

This Earthcache focuses on joints in a rock outcropping which is an exposed intrusive dike of Jurassic Diabase. Over 100 million years ago, as Africa was separating from North America after the Appalachian Orogeny, a rift basin formed allowing magma to rise towards the surface through cracks and fractures. These dikes and sills cooled and solidified under the surface. Millions of years later, the previous surface has been eroded away, leaving what you see here today.

The posted coordinates should take you to the top of this Diabase outcropping, along the Orange trail in Conway Robinson State forest.

Required Activities and Questions

1. Systematic Joint Array
   
   • When studying an area, geologists seek systematic joint arrays (joints occurring in patterns or sets). These usually tell us more than non-systematic joints (which have no obvious spatial relationship to others). The easiest to identify systematic joint array at this location is a series of parallel joints spaced fairly closely together. They are best observed in the rock faces which are (or were) perpendicular to the ground. From the posted coordinates, you can see good examples looking down upon some of the material which has peeled and fallen away from the main outcrop. You can also find good examples by following the path around to the base of the formation. These joints are especially evident in areas that have been exposed and weathered (and, conversely, are not as evident and covered by lichens in some areas that are more protected). Find this systematic joint array.
   • Questions: What is the distance (approximate) between the joints in the array? What is the (approximate) angle (ie, 33°, 45°, 66°, 90°, 180°) to the ground which these joints form (or originally formed, keep in mind that some material has fallen away from the main outcrop)? Observing multiple faces (or a corner) is required to answer the previous question, why is this so?
2. Joint Set and Joint System
   
   • A more complex joint set (series of interlocking joint arrays) can also be found, which together with the joint array earlier identified form the joint system (series of interlocking joint sets) of this outcropping. The results and overall pattern of this joint system can be seen in the shapes of the pieces of rock as they fall away from the main face. This joint set can be a little tricky to identify, but give it a try. It is best observed along rock faces that are (or were) parallel to the ground. At the posted coordinates, look down at the rocks at your feet (the first rocks you come to along the trail at the top of the outcrop). Also look at the faults (cracks with measurable and observable distance between the rock faces) which have formed as rock pulls and falls away, many of which are along what were originally joints (cracks without measurable and observable distance between the rock faces) in this set. And again, moving to the bottom of the outcropping and looking at what used to be the "top" of same of the pieces that have fallen away may be beneficial. I also noticed in this joint set an interesting pattern where more major, easier to observe joints had formed a further distance apart, but upon closer examination one can see harder to observe joints much closer together.
   • Questions: What is the distance (approximate) between the joints in this more complex joint set? Did you notice the same pattern effect I did where more major, easier to observe joints had formed a further distance apart, but upon closer examination one can see harder to observe joints much closer together? If so, what were the distances involved between these joints? What shapes seem to be formed by the intersections of these joints, and what are the (approximate) angles formed (ie, 30°, 45°, 60°, 90°, 120°, 180°)? What is the (approximate) angle (ie, 33°, 45°, 66°, 90°, 180°) to the ground which these joints form (or originally formed, keep in mind that some material has fallen away from the main outcrop)? Observing multiple faces (or a corner) is required to answer the previous question, why is this so?

Optional Activities and Questions

1. Optional Photograph
   
   • While certainly not required, a photo taken while visiting Conway Robinson State Forest is hugely appreciated. Thanks!
2. Formation of these joints?
   
   • Question: How do you think these joints formed? What order do you think these joints formed in? Feel free to use information presented on this cache page, or elsewhere, in addition to observations. I have to admit, I'm not knowledgeable enough about geology to really have a full understanding of the complex formation of joints in this outcropping. But, I'll respond with my thoughts, as well as aggregate any other interesting responses. Hopefully someone who knows more can shed some light!
3. Diabase
   
   • Question: This outcropping was formed as a Diabase intrussive dyke. Did you see other examples of Diabase today?

Simplified Geologic Cross Section of the Culpeper Basin. Map developed by C.M. Bailey, James Madison University

Geological Setting

This outcropping is an example of an intrusive diabase dyke which formed over 100 million years ago as Africa was moving away from North America after the Appalachian Oregony. The Culpeper Rift Basin formed as enormous cracks and faults developed between the two continents moving apart. Another rift basin formed a little further to the East, under what is now the Atlantic Ocean. Magma seeped towards the surface, forming a complex system of dykes and sills beneath the surface. What you see here is the result of magma which cooled beneath the surface in a column, which has now been exposed as the softer layers of rock above and around it erode away.

The diabase that can be found here is very common in the area. In fact, it's so common you might see it all the time and barely notice it. Diabase in the area is quarried for trap-rock or rip-rap. It's used along road sides and train tracks, in ditches and gullies to control erosion, and as that "salt and pepper" / "black and white" rock which you often see on roads, driveways, and in ornamental uses. There is a quarry a little to the East which produces diabase trap-rock (you pass it if driving along Rt. 29). There is also a large quarry to the North which produces diabase trap-rock (and is the subject of a fun Earthcache).

For another excellent educational cache in Conway Robinson State Forest, please visit Name! That! Tree! by rob3k.

Nominee: Best Historical/Educational Cache