This Earthcache will require you to visit two rock formations separated by approximately 9 miles: King Phillip’s Cave in Norton, MA and Anawan Rock in Rehoboth, MA. Both of these formations have great historical significance to the indigenous populations of Massachusetts, specifically but not limited to, the Wampanoag and its related tribes. When visiting each site, please be respectful of the properties.
Both of these rock formations were formed by the Wisconsin Glacier that passed here approximately 13,000 years ago and are examples of conglomerate rock. Conglomerate rocks are formed by the sedimentary rock process of erosion, transport, deposition, and cementation. (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. These 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’.
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.
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 (See gallery for photos of each):
- 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 rocks 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. Rock 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.
Visit each site and make note of the following:
- The composition of the clasts,
- The size of the clasts, and
- The amount and composition of the matrix.
To earn your smiley for this earthcache, please answer the following. Do not include the answers in your log; send them separately to me. Correct answers earn a smiley. After sending your answers to me, log your find. I will only contact you if I have questions about the responses. Rock on!
- Were the conglomerates at both sites of the same or different composition? If they were the same, which composition (monomictic or polymictic) was represented at the sites? If different, which composition was represented at King Phillip’s Cave and which at Anawan Rock?
- What were the sizes of the clasts at each site (cobble, pebble and/or granule)?
- Did the matrix appear of the same or different material between sites and were the conglomerates paraconglomerate or orthoconglomerate?
CONGRATS TO KARBA1 AND FRIENDSHIP7 FOR BEING FTCOMPLETE THE EARTHCACHE!