To log this Earth Cache you have to answer the following questions by contacting me via my profile (erenei).
Questions:
1) What is the color, texture of the rock? Also estimate the width and height of "Castle Rock".
2) How is Sandstone rocks formed?
3) How do you think castle rock was formed? Refer to the type of erosion you think caused it and also to the nature and different sections of the rock.
4) Optional: Post a photo of you or your gps with Castle Rock visible.
Castle Rock is a sandstone formation that formed due to coastal erosion in Brenton on Sea. The rock is part of the Brenton Formation. The following has been documented on the Brenton Formation:
Fossil Heritage: Marine invertebrates (especially molluscs, including ammonites, also echinoderms, various groups of microfossils eg forams, ostracods), plesiosaur reptile teeth, plant fragments.
Rock Types and Age: Grey mudrocks, sandstones, subordinate conglomerates, coquinites. Late Jurassic (or Early Cretaceous) period.
It is the only extensive onshore marine succession of the Mesozoic age in Western Cape.
Castle Rock is mainly made of Sandstone (sometimes known as arenite) and is a clastic sedimentary rock composed mainly of sand-sized minerals or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow, red, grey, pink, white and black. Since sandstone beds often form highly visible cliffs and other topographic features, certain colors of sandstone have been strongly identified with certain regions.
Rock formations that are primarily composed of sandstone usually allow percolation of water and other fluids and are porous enough to store large quantities, making them valuable aquifers and petroleum reservoirs. Fine-grained aquifers, such as sandstones, are more apt to filter out pollutants from the surface than are rocks with cracks and crevices, such as limestone or other rocks fractured by seismic activity.
Sandstone is formed in two principle stages: First, a layer or layers of sand accumulates as the result of sedimentation, either from water (as in a stream, lake, or sea) or from air (as in a desert). Typically, sedimentation occurs by the sand settling out from suspension; i.e., ceasing to be rolled or bounced along the bottom of a body of water or ground surface (e.g., in a desert or erg). Finally, once the sand has accumulated, the sand becomes sandstone when it is compacted by pressure of overlying deposits and cemented by the precipitation of minerals within the pore spaces between sand grains.
Erosion
Coastal erosion is the wearing away of land and the removal of beach or dune sediments by wave action, tidal currents, wave currents, drainage or high winds (see also beach evolution). Waves, generated by storms, wind, or fast moving motor craft, cause coastal erosion, which may take the form of long-term losses of sediment and rocks, or merely the temporary redistribution of coastal sediments; erosion in one location may result in accretion nearby. The study of erosion and sediment redistribution is called 'coastal morphodynamics'. It may be caused by hydraulic action, abrasion, impact and corrosion.
On non-rocky coasts, coastal erosion results in dramatic (or non-dramatic) rock formations in areas where the coastline contains rock layers or fracture zones with varying resistance to erosion. Softer areas become eroded much faster than harder ones, which typically result in landforms such as tunnels, bridges, columns, and pillars. Also abrasion commonly happens in areas where there are strong winds, loose sand, and soft rocks. The blowing of millions of sharp sand grains creates a sandblasting effects. This effect helps to erode, smooth and polish rocks. The definition of abrasion is grinding and wearing away of rock surfaces through the mechanical action of other rock or sand particles.
Hydraulic action: Hydraulic action occurs when waves striking a cliff face compress air in cracks on the cliff face. This exerts pressure on the surrounding rock, and can progressively splinter and remove pieces. Over time, the cracks can grow, sometimes forming a cave. The splinters fall to the sea bed where they are subjected to further wave action.
Attrition: Attrition occurs when waves causes loose pieces of rock debris (scree) to collide with each other, grinding and chipping each other, progressively becoming smaller, smoother and rounder. Scree also collides with the base of the cliff face, chipping small pieces of rock from the cliff or have a corrasion (abrasion) effect, similar to sandpapering.
Solution: Acids contained in sea water will dissolve some types of rock such as chalk or limestone.
Corrasion: Corrasion or otherwise known as abrasion occurs when waves break on cliff faces and slowly erode it. As the sea pounds cliff faces it also uses the scree from other wave actions to batter and break off pieces of rock from higher up the cliff face which can be used for this same wave action and attrition.
Corrosion: Corrosion or solution/chemical weathering occurs when the sea's pH (anything below pH 7.0) corrodes rocks on a cliff face. Limestone cliff faces, which have a moderately high pH, are particularly affected in this way. Wave action also increases the rate of reaction by removing the reacted material.
Sources:
http://www.sahra.org.za/fossil-layers/brenton-formation
http://en.wikipedia.org/wiki/Sandstone
http://en.wikipedia.org/wiki/Coastal_erosion