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Just over 4 Kms. west of Gander, on the North side of the Trans Canada Highway, you can easily see a great example of tilted sedimentary rocks. This cache is quite easy to get to - you can drive to within about 25 feet of it.
Sedimentary rocks are the most common rock type on the earth surface. If you've ever held a piece of sandstone, walked into a concrete building decorated with limestone blocks, or sprinkled salt on your food, you've encountered sedimentary rock.
Sedimentary rocks are the result of either deposition and subsequent cementation of material, or are formed by chemical processes. Pieces of rock as big as boulders, or as small as grains of dust can make up a sedimentary rock.
These pieces, called clasts, can be carried by wind, water, or glaciers. As they are deposited and accumulate into layers called strata, water creeps into the spaces between clasts. The water can carry dissolved chemicals that dry into cement, gluing the clasts together to form a rock.
Often, clasts of a certain size will become cemented together into a single layer of rock. The different sizes fall into specific categories, from which a general rock name is derived. Thus, mud, the smallest particle sizes, turns into mudstone and shale. The next largest, silt, forms siltstone. Sand makes up sandstone. The three largest clast types, pebbles, cobbles, and boulders, make up rocks called conglomerates (if the clasts are rounded) or breccia (if they are angular).
Certain sedimentary rocks don't come from clasts, but have chemical sources. Some of those have inorganic origins. Examples include evaporites, salts that remain behind when a body of water evaporates. Some of these salts are quite useful, such as halite, used as table salt, and gypsum, which is used in plaster. Other chemically deposited rocks come from organic sources. These include calcium carbonate structures such as shells and coral reefs that are consolidated into limestone and the calcium- or silicon-based "oozes," sediments made up of dead microorganisms that settle to the ocean floor.
As the sediments become buried under other sediment layers, pressures and temperatures increase. The sediment hardens into a sedimentary rock, or lithifies, after it has gone through the stages of compaction, dewatering, and cementation. During compaction, the grains of sediment are packed more tightly together. With increasing pressure some of the water between the sediment particles is squeezed out, dewatering the sediment. This process reduces the pore space, or open spaces between the grains. At this point, pressure and temperature conditions are such that certain minerals, usually calcite or quartz, fill some or all of the pore spaces and adhere to the sediment fragments, cementing them into a sedimentary rock.
The rocks that you see here on the side of the TCH consists of very fine-grained sandstone and shale that were deposited on the sea floor during the Ordovician, which began approximately 490 million years ago and lasted for about 50 million years.
If you look closely, you can see that compression of the rock has aligned very fine mica flakes across the bedding to form a cleavage. The rock not only breaks evenly across the bedding planes, but also splits along them.
As you’ve surely noticed, these rocks are tilted upward. Originally horizontal, these beds were tilted and folded during the Silurian period, which immediately followed the Ordovician. During the Silurian, glaciers melted and the seas began to rise. Not only did the seas rise, but so did the land. This was due to mountains being built as the continental plates collided.
Now that you have learned a little about the earth’s landscape, you can claim this cache by first including a photo of the rocks (with your GPS in the photo) with your log entry, then send me an E-mail with answers to the following questions. Do not put the answers in your on-line log entry.
1)There are many layers to the sedimentary rocks in front of you. What is the thickness of the thinnest layer that is visible: less than 0.5 mm, between 1.0 mm and 2.0 mm, or greater than 2.0 mm?
2)What are the three different types of sedimentary rocks,
3)What is the period of time that preceded the Ordovician, and the period that followed the Silurian.
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