Sandstone is a type of sedimentary rock. The sediment particles are clasts, or pieces, of minerals and fragments of rock. It is composed mostly of sand particles, which are of medium size; therefore, sandstone is a medium sized clastic sedimentary rock. More precisely, sand is between 1/16 millimeter and 2 mm in size (silt is finer and gravel is coarser). The sand grains that makeup sandstone are aptly referred to as framework grains.
Sandstone may include finer and coarser material and still be called sandstone, but if it includes more than 30 percent grains of gravel, cobble or boulder size it's classified instead as conglomerate or breccia (together these are called rudites).
Sandstone has two different kinds of material in it besides the sediment particles: matrix and cement. Matrix is the fine-grained stuff (silt and clay size) that was in the sediment along with the sand whereas cement is the mineral matter, introduced later, that binds the sediment into rock.
Sandstone with a lot of matrix is called poorly sorted. If matrix amounts to more than 10 percent of the rock, it is called a wacke ("wacky"). A well-sorted sandstone (little matrix) with little cement is called an arenite.
The cement in sandstone is usually one of three materials: silica (chemically the same as quartz), calcium carbonate or iron oxide. These may infiltrate the matrix and bind it together, or they may fill the spaces where there is no matrix.Sandstone forms where sand is laid down and buried. Usually, this happens offshore fromriver deltas, but desert dunes and beaches can leave sandstone beds in the geologic record too. The famous red rocks of the Grand Canyon, for instance, formed in a desert setting. Fossils can be found in sandstone, although the energetic environments where sand beds form don't always favor preservation.
Depending on the mix of matrix and cement, sandstone may have a wide range of color from nearly white to nearly black, with gray, brown, red, pink and buff in between. When sand is deeply buried, the pressure of burial and slightly higher temperatures allow minerals to dissolve or deform and become mobile. The grains become more tightly knit together, and the sediments are squeezed into a smaller volume. This is the time when cementing material moves into the sediment, carried there by fluids charged with dissolved minerals. Oxidizing conditions lead to red colors from iron oxides while reducing conditions lead to darker and grayer colors.
Along the Northumberland Strait, Prince Edward Island and the Gulf of St. Lawrence, there are any areas contaiinng red sandstone. gaining it's colour from oxidization and the presense of iron. Since the area of Londonderry, a few kilometers away is known for iron mining, it's safe to say that the formation near GZ obtained it's colour from oxidized iron. .
The sand grains in sandstone give information about the past:
- The presence of feldspar and lithic grains means that the sediment is close to the mountains where it arose.
- Detailed studies of sandstone give insight intothe kind of countryside that produced the sand.
- The degree to which the grains are rounded is a sign of how far they were transported.
- A frosted surface is generally a sign that sand was transported by wind—that, in turn, means a sandy desert setting.
Various features in sandstone are signs of the past environment:
- Ripples can indicate the local water currents or wind directions.
- Load structures, sole marks, rip-up clasts, and similar features are fossil footprints of ancient currents.
- Liesasang are signs of chemical action after burial of the sand.
The layers, or bedding, in sandstone are also signs of the past environment:
- Turbidite sequences point to a marine setting.
- Crossbedding (truncated, tilted sandstone layering) is a rich source of information on currents.
- Interbeds of shale or conglomerate may indicate episodes of different climate.
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1) Estimate the height of the red sandstone formation
2) Desribe the layers you see. At what angle are the layers, horizontal or verticle?
3) Do you think the opening in the formation is natural or man made? What features led to this conclusion?
4) Estmate the grain size in the sandstone directly above the opening.
5) What evidence of erosion do you see? Do you see any ripples? If so, please describe.
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