This earthcache-n-dash is just off of the Fifteen Mile Creek Exit (exit 62) on a road parallel to the Interstate (DO NOT attempt to access directly from the Interstate lanes!)
Logging Requirements: Send the answers to #1-#4 to me through my geocaching profile. DO NOT post the answers to any logging requirements on this site.
1. List the name “GC**** MD - Shale State on Town Hill I-68 EC-N-Dash Earthcache” in the first line of your email. THis is done automatically with Message Center. Group Messaging is fine.
2. Slickensides: Find a piece of rock at GZ that shows evidence of slickensides (see description below). Describe how the slickensides surface DIFFERS from the base rock.
3. Fissile Shale: give a description of the shape and nature of these sheets visible at your feet at GZ. See the description below to help you identify the fissile shale.
4. Small Fold: see if you can figure out A) which side (left, right, upper, lower) is "younger" (was the "top" layer of sediment before the fold. To do so you'll have to figure out from what direction the pressure that created the fold came from AND what direction it was pushing in (up down horizontally, at an angle, etc.) B) how steep the angle of the fold is on average (5 degrees, 10 degrees, 20 degrees, 30 degrees or greater)
5. (Required as of July 2022) Post a picture of yourself and/or your GPS with your log that shows you near GZ. DO NOT show any of the pertinent information panels in your picture or your log may be deleted.
I will only respond if you have incomplete logging requirements. Go ahead and log your cache
Geology:
The Brailler Formation (of which this road cut is a part) is part of the Acadian Mountain-building event in the early/mid Devonian Period.
So what happened? Here's the skinny.... The area west of what is now the Allegheny Mountains (the Acadian Mountain event actually built mountains much taller than the present Alleghenys), was a shallow sea. A variety of layers were created as sediment settled 1,000’s of feet deep. Compression and plate tectonics altered the layers of sandstone and siltstone created a variety of features visible at this location. According to accepted geological theory, in Ordovician time/Silurian time, the "Taconic Mountains" were created largely by the Continent of North America slamming into some "continent fragments" and islands in the area that is now western Maryland. Of course, tall exposed mountains begin to erode, which these did, creating a huge sediment wedge to the west of the mountain range.
This wedge of sediment is slammed up against the remnants of the Taconic Mountains and becomes part of a new mountain building event, the Acadian Mountains. on the western edge of this range, the sediments from the mountains' erosion collected, and compaction created shale, all of which was covered by a calm and shallow sea....
Why the compacting? The number one reason is the collision of the African Continent as Pangea was created. The force that created the Pangea-era Alleghenies was very great, so the sediment was compacted about half of its former depth...yep, that's metamorphism! On display in this easy-access roadcut, you see a significant exposure of the Brailler Formation that scientists believe was once part of a deep & quiet basin in the Acadian Mountains.
At this site you can see three things: 1. Slickensides 2. Fissile Shale 3. several small folds in the once pliable shale Specifically, the Brallier Formation was formed during the Devonian period, during the Acadian mountain building event. The low-oxygen water of the shallow sea (caused by convergent pressure) deposited 1,000’s of feet of sediment that turned into sandstone and siltstone, of which the Brallier Formation is a part. What is unique at this location is that the Brallier Formation has undergone some change, creating three geological “shale states” – Slickensides, Fissile Shale, and Folds.
Rocks:
1. Slickensides: Slickensides are polished, scratched surfaces resulting from friction between beds that occurred during folding of faulting. This is one of the few exposures in this region that actually demonstrate slickensides! For Requirement #2, find a piece of rock that demonstrates slickensides and describe how the polish & scratched surface DIFFERS from the base rock.
2. Fissile Shale is formed when clay hardens and is pressurized. In this case, over 2,000 vertical feet of shale has been created as part of the Brallier Formation in this area (about ____' of it is exposed here in this roadcut. This happens because clay molecules are flat like a sheet, so when they harden into shale (as they are compressed under hundreds of feet of sediment), the layers look like those in the rock debris pile in front of you. You will give a description of the shape and nature of these sheets in the Logging Requirements (#3)
3. Small Fold Folds are formed during the metamorphic process, First, sedimentation forms layers of varying colored sediment, sometimes centimeters, sometimes feet think. The colors and texture change based on the composition of the sediment, thus forming "bands" that are different colors and composed of different materials. Since Shale is formed from soils (clay, silt, mud), when it is still forming into rock, it is rather ductile (able to move and bend) and thus when tectonic forces or uneven pressures from a uni-lateral direction take place, a layer or layers of sediment "ripple" creating a fold like the one you see in front of you.
For logging requirement #4, see if you can figure out A) which side (left, right, upper, lower) is "younger" (was the "top" layer of sediment before the fold. To do so you'll have to figure out from what direction the pressure that created the fold came from AND what direction it was pushing in (up down horizontally, at an angle, etc.) B) how steep the angle of the fold is on average It is interesting to note that sediment formation / accumulation occurs at a rate of 2- centimeters per 500 year period (at current rates). Thus EITHER rates were more rapid in the past or this roadcut represents an VERY LONG period of time!
Congratulations to naomi48 for FTF on 5/3/2012.
Resources:
Means, John. Roadside Geology of Maryland, Delaware, and Washington, D.C.Missoula, Montana: Mountain, 2010. Print.