The Formation of the Old Redstone in the Island of Kerrera (Essay Sample)

THE OLD RED SANDSTONE FORMATION IN ISLAND OF KERRERA
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Introduction
The island of Kerrera is in the Firth of Lorne between the areas of Mull and the Scottish mainland and is opposite the town of Oban. The Island is approximately 6.5 miles long and two miles wide.
The Old Red Sandstone in Kerrera is a combination of red and brown sandstones, conglomerates, and shales deposited on the island during the Devonian age of geologic time. The old red sandstone, which contrasts with the other typical formations of the Devonian age, is largely a continental formation, laid down in both freshwaters and on land resulting from erosion from the highlands. (Infoplease.com, 2015).
The Devonian period refers to a period in geological times during the Paleozoic era. The Devonian period came after the Silurian Period and preceded the Carboniferous period. It occurred 419.2 million to 358.9 million years ago. Some geologists refer to the Devonian period as the age of fishes. This phrase came into use because of the different, several and shocking types of creatures that lived and swam in the Devonian seas. For instance, in the early Devonian there were forests and coiled shell bearing marines organisms known as ammonites emerged. In the late years of the Devonian period, there appeared the first four-legged amphibians that led to the vertebrates colonizing the land.
Formation of the Old Red Sandstone
Sandstone is a type of sedimentary rocks. Sedimentary rocks are types of rocks made when the other rocks break down into small fragments, and later the pieces join to form a new stone. Sandstones are also forms of rocks that form over centuries. It occurs when sand accumulates in rivers, lakes, oceans, valleys, in addition, the floors of other water bodies. The accumulated sand later blends with quarts or calcites and with time undergoes compression to form substances that are like rocks. The combination of sand and quartz undergoes pressure and these two or more elements after combining form the sandstones. Geologists have estimated the sand-sized grains to have a measurement of about one over sixteenth to two millimeters.
Several theories explain the formation of the old red sandstone in Kerrera. The old red sandstone was a representation of the period when the ocean closure and collisions between continents resulted in a geography of the world that was largely different much to those of the Palaeozonic times. (Barclay et al., 2005).
One of the theories explains and proposes that the old red sandstone is a form of a thick sequence of Devonian rocks, which formed from 416 million to 359.2 million years ago. The theory also explains that the sandstone is continental in origin but not marine (Encyclopedia Britannica, 2016). According to this theory, the rocks were deposited in the structural basins found between the mountains. These mountains came into existence during the Devonian period when a section of Europe and a large land mass collided. After the collision thick deposits of sand and mud, some as thick as, thirty-six thousand feet deep, slowly collected and accumulated at the sinking basins. These deposits of sand contained oxidized red minerals.
The second theory states that the old red sandstone formed from rivers systems that were meandering and flowing across the arid global environment. According to the proposers of this theory, the rocks comprise of red sandstones that are red in color and interbedded with fine-grained and rich in Mich. The rocks have sandstones and sandy green, mudstones, and shale that have a combination of red and purple colors. During the formation of the old red sandstones, the thick mudstones beds formed at the top while the pebbly sandstones and conglomerates collected and formed layers at the bottom.
Composition of sandstone
Petrographically (mineralogy and texture) or chemically are the two primary ways of ascertaining the composition of sandstones. (Eynatten, Barcelo ́ -Vidal And Pawlowsky-Glahn, 2003). Obtaining the petrographic composition of sandstone requires analyzing their framework components using light mineral analysis.
Sandstone mostly contains quartz, feldspar, and lithic minerals. However, sandstones may also contain other minerals determined by the mineralogical maturity of the sandstone. (Nelson, 2013). Sandstones also contain smaller clay or silt-sized crusts held together by cement that is usually made up of either carbonate or silica.(Sandatlas.org, 2013).The composition also contains little amounts of oxide, which are residues of iron-bearing minerals.
Quartz makes more than two-thirds of all the sandstone minerals. The reason for this is that in crystalline rocks like granitoid, schist and gneisses, quartz is one of the most abundant minerals. Quartz has high hardness and does not have cleavage making it more durable, quartz is stable under the present conditions of the earth’s service, and it is not highly soluble in water. Quartz as a component of sandstone occurs as monocrystalline and polycrystalline grains and shows undulatory extinction, which occurs because of the deformation of the preexisting rock from which the pebbles came from and this also, leads to the sandstone also deforming. (Nelson, 2013).
Sometimes quartz may have a milky color, and known as white quartz. The white quartz forms because of bubbles filled with fluids. The white quartz is not common in sandstones, but their presence is an indicator that the quartz is either pegmatite or vein quartz. Sandstone with monocrystalline quartz does not undergo undulatory extinction because the monocrystalline quartz is more stable than the polycrystalline quartz grains in a sedimentary environment.
Feldspar is less stable than quartz at the earth’s surface conditions. Because of its less stable form, feldspar only forms only between ten to fifteen percent of sandstones. Feldspar in sandstones consists of plagioclase, alkali feldspar. Plagioclase source is both from the metarphohic sources and from igneous sources. However, plagioclase can also come from volcanic sources, and plagioclase from volcanic source shows zoning. Alkali feldspar comes from igneous, volcanic, and metamorphic sources. The types of feldspar from igneous and metamorphic sources are orthoclase and microcline. The kind of feldspar from volcanic sources is sanidine. Another source of feldspar is the plutonic igneous source that forms a type of feldspar known as microperthile.
The third type of sandstone composition is lithic fragments. These fragments are very unstable in sedimentary environments. Sandstones can contain any rock fragments, but some kinds of rocks are common in sandstones than others. The reason that some pieces are highly available includes the area of the source drainage basin, location of the drainage basin, the stability of the fragments in any sedimentary environment, and the size of the fragments crystals.
The strata and structure of the old red sandstone
Stratification refers to the process through which the sediment layers stack over each other occurring on the scale of hundreds of meters and scale down to smaller scales of millimeters. (Indiana.edu, n.d.). The old red sandstone of Kerrera structure is visible to the naked eye. Just like sedimentary rocks, sandstones also have layers. The sandstone layers are visible because of the variation in the size of the grains, and the layers are transparent because they have different colors.
There are some forms of structures and stratification of the red sandstones. One of the common structures is the graded bedding in which the grains gradually becomes smaller, and it’s more coarse at the bottom and becomes finer as it approaches the top.
Another form of stratification is cross bedding also known as cross stratification. In this kind of structure, the rock fragments become deposited at an angle in the main sequence. During the formation of cross bedding, the sand waves transport the sand grains during which the sediments erode along the slopes and then redeposit in the current down slope. Cross-bedded sandstone layers form when several of these sand waves carry sand into an area, and more sediment deposited than eroded. The inclination of the cross- beds determines the transport direction of the sediments and the flow of current. Another method for the formation of cross bedding sandstone structures is after the wind blows over a land surface covered with sand and creates sand dunes.
Classes of red sandstone in Kerrera
Kerrera as an island has water all around it. The combination of water and land surface brings several types of sandstones and the classification based on the mineralogy of the grains and the matrix type that is present between the grains. (Wiki.aapg.org, 2015). The first type is the shoreline sandstones. Shoreline sandstones are convex shaped, face upwards and mostly associated with marine and lagoonal shales and both are the sources of the sediments forming these sandstones. They occur in the areas with slopes that are gently sloping. There are two types of shoreline sandstones, which are beach and barrier sandstone. When shoreline sandstones deposit at the beaches, they become narrow, linear, and parallel to the shore while the shore barrier sandstones form finer grains and are more lateral and discontinuous. However, both beach and barrier shoreline sandstones have a similar characteristic, which is that they are unpolluted and well sorted with high porosity and permeability.
The next class is the Aeolian sandstones that are windblown by the sand waves found in the coastal environments. The sand dunes heap up ridges of sand forming several crests that are either parallel to the longitudinal dunes or perpendicular to the transverse dunes. Sometimes the peaks are crescent shape...