C. Sedimentary Rocks
Examine how sedimentary structures, such as bedding planes and joints, influence the development of landforms.
All rock types produce their own unique landforms. Sedimentary structures are easily identifiable and distinguished from those of other rock types due to the quite fragmented structure of this rock type. Limestone is an example of a sedimentary rock which produces many landforms of a unique nature.
Limestone is a rock that is permeable, jointed, has bedding planes and rich in calcium carbonate. These attributes determine the rock to be particularly susceptible to erosion. A sinkhole is a sedimentary landform which highlights vividly the erosive tendencies of limestone.
Rainwater is a weak carbonic acid formed when rain water passes though carbon dioxide in the atmosphere and absorbs this greenhouse gas as a direct consequence. When rain falls on the surface of limestone a mild chemical reaction occurs. During this reaction the carbonic acid in the water separates the calcium carbonate into two distinct atoms; calcium and bicarbonate. Both these atoms are soluble in nature and are consequently washed away in solution.
This water can navigate about and through the limestone rock quite easily due to its jointed nature. These vertical joints allow water to trickle deep within the rock surface and continue to erode and fragment the rock further creating underground passages through areas of weakness.
Joints and bedding planes divide the limestone into blocks. Following the erosion of a limestone block by means of the underground water passage, the limestone block above becomes unstable and will eventually fall due to a lack of support from below. This results in an opening from the passage to the surface, through which the surface river plunges vertically downwards. This passage is known as a sinkhole. The former river bed laying the surface will, over time, run dry forming a dry valley.
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