The smaller ones — known as granules — are gravel size, but still you could throw one. Sand ranges from 2 mm down to 0. Silt is essentially too small for individual grains to be visible, and while sand feels sandy to your fingers, silt feels smooth to your fingers but gritty in your mouth. Clay is so fine that it feels smooth even in your mouth. Are they sand, pebbles, or cobbles? If they are not too fine, you should be able to tell if they are well rounded or more angular.
The beach in the image is at Sechelt, B. This beach is subject to strong wave activity, especially when winds blow across the Strait of Georgia from the south. That explains why the clasts are relatively large and are well rounded. If you drop a granule into a glass of water, it will sink quickly to the bottom less than half a second. If you drop a grain of sand into the same glass, it will sink more slowly a second or two depending on the size.
A grain of silt will take several seconds to get to the bottom, and a particle of fine clay may never get there. The rate of settling is determined by the balance between gravity and friction, as shown in Figure 6. One of the key principles of sedimentary geology is that the ability of a moving medium air or water to move sedimentary particles, and keep them moving, is dependent on the velocity of flow. The faster the medium flows, the larger the particles it can move.
This is illustrated in Figure 6. Parts of the river are moving faster than other parts, especially where the slope is greatest and the channel is narrow. Not only does the velocity of a river change from place to place, but it changes from season to season.
During peak discharge [3] at this location, the water is high enough to flow over the embankment on the right, and it flows fast enough to move the boulders that cannot be moved during low flows. Clasts within streams are moved in several different ways, as illustrated in Figure 6. Large bedload clasts are pushed by traction or bounced along the bottom saltation , while smaller clasts are suspended in the water and kept there by the turbulence of the flow.
As the flow velocity changes, different-sized clasts may be either incorporated into the flow or deposited on the bottom. At various places along a river, there are always some clasts being deposited, some staying where they are, and some being eroded and transported. This changes over time as the discharge of the river changes in response to changing weather conditions.
The size of the clastic material determines the type of sedimentary rock that is formed. Organic sedimentary layers may not be caused by erosion at all.
Coal which is an example of a sedimentary material is formed when organic material like plants are buried between other sedimentary layers. The organic material is then changed into coal an organic sedimentary layer by heat and pressure.
Chemical layers like limestone form when the organic material of sea shells precipitates out of solution to the bottom of a body of water. The precipitates accumulate at the body of the body of water forming the thick layers of limestone found on the continents. Clastic sedimentary layers are formed by erosion and the accumulation of small broken bits of rock.
Chemical sedimentary layers and organic sedimentary layers are not the result of erosion. Chemical sedimentary layers are the accumulation of precipitates of organic material. Organic sediments are the remains of buried plants and other living things. How do clastic sedimentary rocks differ from organic and chemical sedimentary rocks? Boundaries of individual grains may be another grain, cement or empty pore space.
Overall rock is generally porous and not very dense. If the grains are visible, all of the above characteristics may be noted. Because the grains are invisible, examining the ease in which grains silt or clay may be knocked off is the best test to perform.
Unlike igneous crystalline textures, however, sedimentary crystalline textures are typically formed from one mineral throughout the entire rock.
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