Harrow Mountain Fault EarthCache
-
Difficulty:
-
-
Terrain:
-
Size: 
(not chosen)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
This earthcache brings you to Harrow Mountain where a fault has formed. This formation will show you not only what happens but you will be able to see how it happened.
During the Lower Devonian Period, approximately 400 million years ago, the accumulation of lava flows and beds of volcanic ash formed volcanic islands in an ancient sea. The flows and ash beds formed Harrow Mountain, which is made of Traveler rhyolite. The geologic evidence suggests that no important marine submergence have occurred after that period of time.
There is evidence of a fault at this location. A fault is a planar rock fracture, which shows evidence of relative movement. There are two parts to this simple thrust fault. Since faults do not usually consist of a single, clean fracture, the term fault zone is used when referring to the zone of complex deformation that is associated with the fault plane. The two sides of a non-vertical fault are called the hanging wall (east side) and footwall (west side.) By definition, the hanging wall occurs above the fault and the footwall occurs below the fault. This terminology comes from mining. When working a tabular ore body the miner stood with the footwall under his feet and with the hanging wall hanging above him.
The faults can be again classified into the types "reverse" and "normal". A normal fault occurs when the crust is extended. Alternatively such a fault can be called an extensional fault. The hanging wall moves downward, relative to the footwall. A downthrown block between two normal faults dipping towards each other is called a graben. An upthrown block between two normal faults dipping away from each other is called a horst. Low-angle normal faults with regional tectonic significance may be designated detachment faults. A reverse fault is the opposite of a normal fault — the hanging wall moves up relative to the footwall. Reverse faults are indicative of shortening of the crust. The dip of a reverse fault is relatively steep, greater than 45°. At the foot of the cliff you can see a pile of talus that was formed when the glacier moved over the faulted area destroying the edge. You will also see a great deal of very small angular rock as you approach the bottom of the opening. This is due to current damage being done to the zone by weathering.
This fault zone has had extensive modification due to weathering. There is a great deal of evidence for a process called frost shattering. This type of weathering is common in mountain areas where the temperature is around freezing point. Frost induced weathering, although often attributed to the expansion of freezing water captured in cracks, is generally independent of the water-to-ice expansion. It has long been known that moist soils expand or frost heave upon freezing as a result of water migrating along from unfrozen areas via thin films to collect at growing ice lenses. This same phenomena occurs within pore spaces of rocks. They grow larger as they attract liquid water from the surrounding pores. The ice crystal growth weakens the rocks, which in time, break up. The phenomenon is caused by the almost unique property of water in having its greatest density at 4 C, so ice is of greater volume than water at the same temperature. When water freezes, then it expands and puts its surroundings under intense stress.
Freeze induced weathering action occurs mainly in environments where there is a lot of moisture, and temperatures frequently fluctuate above and below freezing point. When water that has entered the joints freezes, the ice formed strains the walls of the joints and causes the joints to deepen and widen. This is because the volume of water expands by 9% when it freezes. When the ice thaws, water can flow further into the rock. When the temperature drops below freezing point and the water freezes again, the ice enlarges the joints further. Repeated freeze-thaw action weakens the rocks, which, over time, break up along the joints into angular pieces. The angular rock fragments gather at the foot of the slope to form a talus slope (or scree slope). The splitting of rocks along the joints into blocks is called block disintegration. The blocks of rocks that are detached are of various shapes depending on rock structure.
At the posted coordinate you will find yourself at the bottom of an opening that way formed as a fault. You will be climbing up a steep slope of small angular rocks that have been formed by weathering in the fault zone. To log this Earthcache: You must post a photo of you and your GPS with fault backgound in your log and then send an email to me through my profile with the following information: At the above coordinates you will see a fault. As you are looking directly at the fault from the bottom tell me your estimate of the distance that the rock traveled. Look for matching rocks on both sides. You may want to go to the top where the distance is easily seen. Please begin your email with the name of the earthcache and make sure your log includes the number of people in your group. This is a hard trip so in your log make sure to let people know of your adventure.
MAKE SURE TO MARK YOUR VEHICLE WITH YOUR GPS BEFORE YOU ENTER THE FOREST.
If you enjoy this earthcache you may want to check the Maine Geological Survey located at (visit link)
They have developed a number of information sheets or field localities giving a great deal of information about geologic features. They also have a number of books and maps about Maine’s natural history/ geology that you might find interesting.
Additional Hints
(No hints available.)