You've come a long way, baby! EarthCache

When I first came across the information sign for this rock formation identifying it as a prehistoric rock shelter, I was pretty excited. Cool stuff to be sure. As I often discover during EarthCache research though, I end up thinking about, learning, and publishing something totally different than I started with. So, let's get the obvious out of the way: This is a big rock sticking out of the dirt that you can stand under if you're not a big fan of rain. That's true today, and it's been true for humans going back thousands of years. Heck, if it's raining now, feel free to stand underneath it.

This rock is called a “Schist”. A schist is a metamorphic rock, meaning it was changed from its original form by heat, pressure, strain, and/or fluids. This schist is likely part of the Prettyboy Schist Formation that extends from northern Baltimore County through Howard County and into Montgomery County. It is one of several schists found in the central part of Maryland. It predominately contains the platy mineral mica, but its greenish color indicates that the rock also contains the mineral chlorite.

This impressive formation had humble beginnings though. It started out as mud or clay sediment under an ocean. So, how did ocean sediment end up as schist in Wheaton, Maryland? Plate Tectonics and Continental Drift are the answer. I won't go into the details of Plate Tectonics, but the resulting movement and collision of land masses over millions of years ultimately transformed the ancient sediment into schist, all the while moving it over thousands of miles. You can view an excellent animation of Plate Tectonics and Continental Drift at the UCMP website (http://www.ucmp.berkeley.edu/geology/anim1.html). It might help you visualize the dynamics behind this EarthCache a little better as well.

The schists in central Maryland have been estimated to be between 500 and 700 million years in age (or between the late Pre-Cambrian/Ediacaron Period and the Cambrian Period of the Geologic Time Scale). At that time in our geologic past, the North American continent was a lot different than it is today. Back then, ancestral North America, or Laurentia as geologists refer to it, was actually below the Equator and turned clockwise approximately 90 degrees. Laurentia had just rifted, or detached, itself from the super-continent, Rodinia, which was centered at the South Pole, and was slowly drifting northward towards the Equator. The remaining part of Rodinia is referred to as Gondwana. The body of water that formed between Laurentia and rest of the Rodinia landmass was the Iapetus Ocean. If you are familiar with Greek mythology, Iapetus was the father of Atlantis, for whom the Atlantic Ocean is named. Much of Laurentia, including what is now Maryland, at that time was submerged under the Iapetus Ocean and the schists in central Maryland were once the ocean sediments deposited in the Iapetus basin!

Another super-continent formed between 300 million and 250 million years ago called Pangea. The formation of Pangaea affected Maryland considerably, in particular, the collision between ancestral North America (Laurentia) and ancestral Africa (part of Gondwana). The convergence of these two continents uplifted the sediment/rock formations that had accumulated in the shallow Iapetus Ocean, forming the Central Pangean Mountains. Geologists refer to this mountain building event as the Alleghenian Orogeny.

Approximately 200 million years ago, Pangaea began to rift apart and the individual continents began to drift to their present day positions. As North America pulled away from the other continents, a basin was created which began to fill with water. This was the beginning of the Atlantic Ocean. Material on the eastern side of the newly formed Appalachian Mountain range (there are similar mountains on the African side of the split) was being eroded and transported eastward by streams and rivers and began to accumulate in the ocean basin. Over the last 200 million years, this sediment has built up to over 8,000 feet in depth and today forms the Coastal Plain and Continental Shelf of Maryland.

To sum up, this rock began as sediment deposits on an ocean floor near the South Pole, was carried North and East until it was rammed into North Africa, then moved Westward to it's present position.

To log this Earthcache eMail me with the answers to the following:

1. How far do you estimate the tip of the largest rock is above the ground and how far out from it's base?

2. Approximately how old is this rock?

3. What are the main minerals that comprise this rock?

4. What gives the rock its green color? Find a location on the rock that does not have any organic growth (algae/lichens). Describe the natural color of the rock.

5. Does the rock appear to have a uniform content? In other words, do you see any other mineral streaks or inclusions, or does the content of the rock appear uniform and consistent?

6. What is the name of the mountain-building event that resulted from the collision between ancestral North America and ancestral Africa?

7. Can you make out any individual grains in the rock? If so, what size are they?

8. Optional: Post a picture of yourself or your GPSr at this location.

Logs containing spoiler pictures will be deleted without warning.
You may post your log at any time, but emailed answers must follow within 7 days or your log will be deleted.

There are several other caches located nearby, including Maryland's oldest MultiCache. Allow yourself a couple hours on your visit to enjoy this great area and all it has to offer.

Plate tectonic maps (C) C. R. Scotese, PALEOMAP Project (www.scotese.com). Used under non-commercial license
Special thanks to Dale Shelton of the Maryland Geological Survey (www.mgs.md.gov) for reasearch assistance and content.
Thanks to the Maryland Geological Society (www.ecphora.net/mgs) for pointing me in the right direction!