Getting to know sedimentary rocks

Sedimentary rocks are formed from the mechanical breakdown (erosion) of igneous rocks, metamorphic rocks and other sedimentary rocks. Any hill made of any kind of rock is continuously subject to erosion. High hills weather faster than low hills because the effects of frost and gravity are more severe, and because high land streams are faster are more destructive than the slow meandering streams of the low lands.

Most commonly, sediments form in seas and lakes where streams and rivers discharge their load of sediment. Seasonal fluctuation in the flow rate of rivers leads to alternating layers of fine grained sediment (eg mud) and coarse grained sediment (eg sand, pebbles, even boulders) to give individual beds of distinctive appearance. There is also a lateral variation in grain size - fine-grained mud can travel far out to sea before it settles, whereas sands are more common near the shore.

Grand Canyon

Sample 7A (Plate 7, sample A) is a laminated sedimentary rock in which thin sandstone layers (composed of pure white quartz sand) alternate with dark-coloured layers containing clay minerals. Layers only a few millimetres thick are best termed ‘laminations'. Where the layers are a bit thicker than this they are usually called ‘beds' or ‘bedding'.

Plate 7

A. Laminated rock with alternating quartz-rich and clay-rich layers B. Shale, formed from mud or silt
C. Sandstone, formed from sand D. Concrete, resembling a pebbly grit

Sample 7B is a fine grained sedimentary rock which has formed from fine-grained or muddy sediment. This rock is called shale.

Sample 7C is sandstone consisting of naturally-cemented sand grains and it has a ‘sandy' feel. This sample is red coloured due to a significant iron content. Sandstone is a common building stone as it is attractive and easily cut into regular shapes. In Strachur it is commonly used as cornerstones and for the borders of windows, and for arches (The Square on Strachur Estate, and the Parish Church, for example).

Bridge Strachur Estate

Ornamental bridge made of sandstone, Strachur Estate

Sandstone rocks do not occur in situ in the Strachur area, but they do occur at Helensburgh, for example, which lies on the south side of the Highland Boundary Fault. It is common in nature for beds of sandstone to alternate with beds of shale or mudstone, and when these have been metamorphosed they become beds of quartzite alternating with beds of schist.

Sample D is concrete, but it mimics very closely what is found in nature. The little pebbles are basalt and the matrix is cement, which is manufactured from limestone. It is a familiar example of a soggy mixture consolidating to a hard, rock-like material, although for concrete it takes only a day or so compared to millions of years in nature. This sample of concrete is similar in appearance to the Crinan Grit, a coarse sedimentary rock which is found on Bute and in the Tayvallich area.

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