What is sandstone?
Photo by Brent Eades.
Display rock 19 is sandstone that was fashioned as a building block.
Sandstone is a clastic (composed of fragments, or clasts) sedimentary rock, made up of sand-size grains of mineral, or rock, that are cemented together. The cements that bind the grains together are typically calcite or silica. Other minerals that act as sandstone cements include hematite, feldspars, and clay minerals.
The mineral grains of sandstones are usually quartz. Sometimes the quartz content can be very high – up to 90% or more. These are sands that have been worked and reworked by wind or water, and are said to be “mature”. Sands that contain significant amounts of feldspar and other minerals are said to be “immature.” Sandstone is one of the most common types of sedimentary rock and is found in sedimentary basins throughout the world. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow, red, grey, pink or white. Sandstone beds often form highly visible cliffs and other prominent topographic features.
Image courtesy Tulane University.
Sandstones are often porous, making them important as an aquifer for groundwater, or as a reservoir for oil and natural gas. Porosity is a measure of the “empty” spaces in a rock.
What to look for
Photo by Julie Lantos.
Note the parallel linear features that are easily seen on the top surface. Geologists call this parting lineation. This tells us that the sand was deposited in flowing water. We can see these lineations because the flowing water caused the sand grains to align in tiny ridges and valleys parallel to the downstream direction.
What does this tell us?
Parting lineation forms in flat, wet sediments, most commonly on beaches. It can also be created in tidal channels or other shallow marine environments. Our interpretation from reading this rock is that the sand was deposited in a shallow, near shore environment.
Look for other cut blocks of this unique sandstone behind the old town hall on Bridge Street, on the pathway of the Almonte Riverwalk. Some were rescued from the river bottom where they had been left as surplus building blocks during the early days of the town.
What do we know about this sandstone?
The sand forming this rock was deposited some 500 million years ago in the shallow waters of the Iapetus Ocean. The supercontinent Rodinia had been rifted apart by convection currents in the mantle, and a new ocean basin was created. The Iapetus formed between ancestral North America, Laurentia, and ancestral Europe, Baltica. Shallow seas lapped across our part of the world. Sediment was transported from the highlands of Laurentia into the shallow seas on the continental margins.
What is sand?
To a geologist, the word sand in sandstone refers to the particle size of the grains in the rock, not the material of which it is composed. Sand grains range in size from 0.0625 to 2 millimeters in diameter. Rocks with smaller grain sizes (not visible with the naked eye) are known as siltstones and shales. Rocks with grain sizes greater than 2 mm include conglomerates and breccias.
Public domain photo by Michael P. KLimetz via Wikimedia Commons.
Like sand, sandstone can be found in a variety of colours.
What is a sedimentary environment?
Any area where sand, or silt, or gravel is capable of accumulating is a sedimentary or depositional environment. Sediment can be wind, water and ice borne, and can accumulate both on land (think of deserts) and in water (think lakes, rivers, deltas, beaches, shallow and deep marine). Understanding modern environments of deposition allows geologists to understand the environments in which ancient sedimentary rocks were deposited.
Public domain image by Mikenorton via Wikimedia Commons.
Why is this important?
By recognizing depositional environments geologists are able to reconstruct the climate and geography of the past and track the enormous changes that have taken place over time.
Public domain image by Michael P. Klimetz via Wikimedia Commons.
Rocks are made of minerals.
How are sandstones formed?
Sandstones are formed by the weathering and erosion of other rocks. Pieces of rock are loosened then transported by water, wind, or ice to another area. Sandstones form where deposits of sand have accumulated on land, or in ocean environments. As more and more sand arrives, the layers of sediment are buried, compressed, and cemented over time to form sandstone.
During their journey, sand grains undergo further chemical and physical weathering occurs. The more time and distance separating the sand deposit from the source rock, the more its composition will change. If a granite is the source of the sand, the original material could include grains of hornblende, biotite, feldspar, and quartz. Hornblende and biotite are the most susceptible to destruction, and will be lost in the early stage of transport. Feldspar lasts longer, but quartz has the greatest chance of survival. It is chemically inert, harder, and not prone to cleavage. For this reason quartz is typically the most abundant type of sand grain present in sandstone.
What is weathering?
Weathering breaks down and loosens the surface minerals of rock so they can be transported away by agents of erosion such as water, wind and ice. There are two types of weathering: mechanical and chemical. Mechanical weathering is the physical disintegration of rock into smaller and smaller fragments. Chemical weathering transforms the original material into a substance with a different chemistry. Chemical weathering is caused by rain water reacting with the mineral grains in rocks to form new minerals and soluble salts.
Where are these rocks forming today?
Huge deposits of sand are forming all over the world today in marine environments such as river deltas – Mississippi (the other Mississippi in the U.S.A.), Nile, Mekong – and as beaches and tidal flats. The sands of the Sahara may one day be preserved as sandstone, as may other terrestrial sands that have been carried and deposited by rivers.
What is sandstone used for?
Sandstone has been used as a building material since ancient times. The old Almonte post office on Mill Street is a sandstone building. Some sandstone deposits are relatively soft, making them easier to carve and shape. Some hard, fine grained sandstones make excellent grindstones for grinding grain, and for sharpening blades and other implements.