Allsbury Formation EarthCache
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This rock cut is an excellent example of the dip of the rock. What is most interesting about this site is that it clearly shows cleavage of the rock, nearly vertically dipping beds, a fault and the possibility of seeing graptolites.
The Allsbury Formation is a deep-water formation that underlies much of the region where the Central Maine and Aroostook-Matapedia basins merge. It was formed during the Ordovician/Silurian period some 430 million years ago. The rocks are light brown in color, which is caused by the rare mineral siderite, a carbonate of iron. What is most interesting about this site is that it clearly shows cleavage of the rock, nearly vertically dipping beds, a fault and has the possibility of seeing graptolites.
Cleavage is when the rock always breaks in certain manner. In some rocks minerals, bonds between layers of atoms aligned in certain directions are weaker than bonds between different layers. In these cases, breakage occurs along smooth, flat surfaces parallel to those zones of weakness. In some minerals, a single direction of weakness exists, but in others, two, three, four, or as many as six may be present. Here the rock breaks along two very different planes. If you look at the rocks along the bottom of the rock face you will see the planes. This tells scientists something about the atomic structure of the rock.
The dip gives the angle below the horizontal of a tilted stratum or feature. Typically the angle of dip is included on a geologic map. This tells scientist what has happened to the rock in the area. Once, long ago this rock was on the bottom of the ocean, forces caused the land to be pushed up so that what was once flat is now vertical. Dip, along with strike are a method of describing the orientation of a plane in three-dimensional space. It is usually applied to the orientation of tilted layers of rock. Remember, dip is the angle of tilt, measured from the horizontal. Think of the direction of dip as the direction that a ball would roll if placed on the surface. The angle of dip is measured in degrees.
There is evidence of a small 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. The hanging wall occurs above the fault and the footwall occurs below the fault. This tells scientist how forces were applied to the rocks in the past.
Graptolites, the fossil colonial animal, commonly appear as feathery black tracing on the bedding surfaces. The name graptolite comes from the Greek graptos, meaning "written", and lithos, meaning "rock", as many graptolite fossils resemble hieroglyphics written on the rock. They were originally regarded them as 'pictures resembling fossils rather than true fossils'. Graptolites are common fossils and have a worldwide distribution. Scientists use these important index fossils for dating rocks as they evolved rapidly with time and formed many different species.
To log this Earthcache: You must send an email to me through my profile with the following information: At the above coordinates you will see a thrust fault. As you are looking directly at the fault from the roadside tell me if the direction that the hanging wall moved was to the right or left? Make sure to check out for bedding dip or perhaps fossils while you are at the location. You should be able to view this earthcache from your car on a rainy day. Please begin your email with the name of the earthcache and make sure your log includes the number of people in your group. It also would be nice if you would post a photo so others would know what they have to look forward to at this earth cache.
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.
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