Please do not pull over at the above coordinates. It is a tight corner. Park at the coordinates on the waypoint section just up the road at Back Rd, and walk down to the above coordinates. There is a large pull off there that will be able to accommodate any size trailer or car. Additionally, this entire earth cache can be completed and should be completed while on the shoulder of the highway. Please do not walk on to private property or jump any fences as it's unnecessary.
The above coordinates are for a unique rock outcrop of the Conococheague Formation that I stoped at during my Sedimentation and Stratigraphy geology class while an undergrad at the University of Maryland in the Fall of 2019. It is probably the best example I have seen of stromatolites and a transgressive/regression surface in the field. I'll explain what these terms are and how they form later, but I was amazed at what I saw here, and I am sure you will be too.
Stromatolites are lithified rocks that formed by cyanobacteria, usually in shallow saline waters. As the cyanobacteria grows, it creates distinct wavy layering or banding depending on the rate of growth. The waviness in the bands is the dead giveaway that this rock was formed by this processes. Darker layers are periods of stagnate growth, while lighter colors indicate periods of expansive growth. These rocks take thousands of years to form and even more so to be preserved in the rock record. The Conococheague Formation is a shallow marine water deposit, typically from that of a oceanic shoreline. There are places in the world today where stromatolites are actively forming today, and they typically only form in warm tropical shallow waters.
The main rock type located here is called dolomite which forms under unique conditions. Usually, limestone (CaCo3) forms in these types of environments, however under certain conditions dolomite (mineral) can form if there is enough free magnesium in the water that can substitute for calcium as the limestone is forming. Along with this there must be also low salinity and low amount of sulfate ions (SO4) in the water if dolostone (the rock name of dolomite) is to form instead of limestone.
How do you tell if the rock is limestone or dolostone? Well, we can use a few drops of diluted sulfuric acid (H2SO4) and see if there is any fizzing (bubbling). Limestone will react with the sulfuric acid and fizz right away, and fizzes heavily, forming carbon dioxide (CO2) gas. Dolostone will fizz, but slightly on a a clean, unweathered surface. This will require the use of a rock hammer to cut a fresh piece off and use acid to observe if it fizzes.
A unique aspect about this outcrop is that you get to observe several regressions and transgressions occurring. A regression sequence is a period of time where the water is retreating from the land. In the rock record, you will observe deeper deposits below shallow deposits. For example, you may see sandstone below limestone/dolostone which is below stromatolites. On the other hand, a transgressive sequence is a period of time where the water moves towards the land. In the rock record, a geologist may see shallow marine deposits on top of deeper ones (reverse the order from the list above). One odd fact is that transgressive sequences do not preserve well in the rock record. This is because a transgressive sequence erodes surfaces much faster than sediments can be preserved in the rock record. That means, a regressive sequence is more likely to be seen in the rock record and there will be noticeable periods of time missing from an outcrop where the sediments appear to be deeper marine deposits such as shales and all of a sudden above that is stromatolites or beach deposits. This is what is more likely to be seen in the field and what is quit common.
After the rock is lithified and preserved, several diagenesis events can occur. Diagenesis in geology simply means any physical or chemical changes that occur to a rock while it buried and is becoming lithified. One interesting, and common, effect that can form are stylolite. A stylolite looks like a jagged crack in the rock that formed when increasing pressure partially dissolved the rock leaving a void. This likely occurred during the formation of the appalachian mountains as they were being tectonically uplifted. There are several noticeable stylolites at this outcrop. Below is a photo I took while visiting here. Keep in mind, some cracks are commonly mistaken as stylolites which are not the same thing.
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL THE QUESTIONS BELOW. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER TO SEND YOUR ANSWERS. ANY INCORRECT ANSWERS MAY RESULT IN A CLARIFICATION RESPONSE FROM ME.
1. "Conococheague Formation" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.
2. Take a close up photo of your GPS or finger pointing, if your GPS is the same as your camera, at your favorite stylolite that you can find. As these are quite small, but very noticeable features, you may want to make sure what you are finding is indeed a stylolite. Use the photo above to help guide you.
3. Look for stromatolites that are prevalent here, using first photo on the cache page to help you find them. What two dominant colors are they AND what does that tell you about how they originally formed?
4. Looking at the entire rock outcrop from bottom to top, count the number of transgressive and regressive sequences you see here. Which one is much more prevalent AND why?
5. Say you have some diluted sulfuric acid (H2SO4) and you wanted to see if this outcrop is made of limestone or dolostone. You decide to use your trusty rock hammer and obtain a fresh sample. After dripping a few drops on this clean, unweathered surface it doesn't fizz very much and it is hard to tell it is fizzing at all. What type of rock is this outcrop mostly made of?
Works Cited:
https://www.dmme.virginia.gov/webmaps/DGMR/
https://en.wikipedia.org/wiki/Stromatolite
https://en.wikipedia.org/wiki/Conococheague_Formation
http://www.geologyin.com/2015/09/how-to-identify-transgression-and.html
https://epod.usra.edu/blog/2017/05/stromatolites-in-glacier-national-park.html