This earthcache was set up for SEMAG Meet & Greet #100 - The Centennial Event (GCA56E4). It is located in the Bioreserve off a trial I have nicknamed the "Rocky" trail after a multi that begins near the trailhead. Special thanks to groopsoop for the help with the earthcache description!
Rock formations known as ‘erratics’ were typically formed by the Wisconsin Glacier that passed through here approximately 13,000 years ago. (A glacial erratic is a piece of rock that differs in size and type from rock that is native to an area. The erratic rock was carried by a glacier over distances of many hundreds of miles, and can range in size from pebbles to large boulders.
Erratics take their name from the Latin word ‘errare’ which translates ‘to wander’). All glacially- transported erratics tend to show evidence of that glacial transport, with scratches (striations), rounded edges and polished faces. Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. Erratics are significant because they are one of a series of indicators which mark the path of prehistoric glacial movement.
The specific rock introduced in this Earthcache is a ‘conglomerate’ rock. Conglomerate rocks are formed by the process of erosion, transport, deposition, and cementation. [The word ‘conglomerate’ comes from the Latin ‘conglomeratus’ (to roll together) and ‘glomerare’ (to gather
into a ball). Conglomerates are also known as ‘Puddingstone’ because the rounded pebbles whose color contrasts with the matrix, or cement, resemble the appearance of a raisin or Christmas pudding.] To form erractics, rocks, gravel, cobblestone and even boulders are
exposed to an erosion process by being transported from their original location via wave action from rivers, beaches or, in the case of these two examples, glaciers. The sedimentary particles are known as ‘clasts’ and measure 0.079 inches or more. They are held together or cemented by a mixture of calcium carbonate, iron oxide, silica and/or hardened clay referred to by geologists as ‘matrix’.
Conglomerate rock is relatively uncommon, estimated by geologists to represent only one percent of all sedimentary rock on the earth.
The following characteristics are used to classify and categorize conglomerate rock:
- The composition of the clasts. If all the clasts are the same type of rock or mineral), the rock is categorized as ‘monomictic conglomerate’. If the clasts are made up of two or more types of rock or minerals, the rock is a ‘polymictic conglomerate’.
- The size of the clasts. Rock comprised of large clasts is ‘cobble conglomerate’. If the clasts are pebble-sized, the rock is called ‘pebble conglomerate’. If the clasts are small granules, the rock is called ‘granule conglomerate’.
- The amount and chemical composition of the matrix. If the clasts do not touch each other (lots of matrix), the rock is ‘paraconglomerate’. Rocks in which the clasts touch each other is called ‘orthoconglomerate’.
- The environment that deposited the material. Conglomerates may form from glacial, alluvial, fluvial, deep-water marine, or shallow marine environments.
Orthoconglomerate Rock
Paraconglomerate Rock
Visit the site and make note of the following:
A. The composition of the clasts,
B. The size of the clasts, and
C. The amount and composition of the matrix.
Logging Requirements
1. Which best describes the composition of the conglomerate: monomictic or polymictic?
2. Which best describes the size of the clasts: cobble, pebble and/or granule?
3. Which best describes the matrix: paraconglomerate or orthoconglomerate?
4. (Optional) Please upload a photo of you or a geocaching item at or near GZ as a proof of visit.