Margaret M. Loos
Traveling toward the harbor, we encounter the pleasant crescent of a small beach lying between the jutting rock formations of Lighthouse and Forbes Bluff. This beach is produced by the irregularities in the coastline and the availability of sand between the outcrops of bedrock. Waves bend to conform to the shape of the coast and their energy concentrates on the areas that project out the farthest to meet them. The amount of cutting the waves do on the shoreline depends on the makeup of the materials that they encounter and the energy of the waves. High energy waves occur in winter or storm conditions and low energy waves occur in summer or gentle waves. Sand is deposited in summer conditions and the beach is built up, and it is removed in winter conditions.
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A. Erosion is usually greatest on projecting land.
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B. However, the material of the projecting land is very resistant.
Evidence of the energy level of the waves that encounter the shoreline can be interpreted by the nature of the beach that is created. The high energy waves can remove the fine sediments and leave a steep slope, and the low energy waves generally leave a gentler slope and do not remove the fine sediment. Therefore, the gentler the waves the finer the beach sand. The amount of erosion caused by the waves at Lighthouse Point and Forbes Bluff is limited because of the resistant nature of the rock formations there, but ordinarily any projecting coastline would experience the greatest erosion. Beaches such as that at Morris Cove continue to recess but residents eager to retain their land build man-made barriers in an attempt to withstand the wave effects. Students should examine these barriers to see what happens to the sand deposits when an interference to waves is presented.
If we watch waves break on the beach we may guess that the energy of a wave is mostly expended there, but what can we discover about the effects of the waves on the material under them? It is important to point out that waves are actually movements of energy and not of the actual molecules or particles of water. The molecules themselves only travel in a circular path theoretically half as great in diameter as the wave’s length. If we can examine a flat area of coastline or obtain pictures of it at low tide we can gather evidence of the wave action there. If the path of the waves’ circular movement never touches bottom they leave no ripple marks. If the waves are not breaking, but moving in gentle movement (oscillating waves) they leave symmetrical ripple marks and if the crest of the waves override their low points or troughs, they leave asymmetrical ripple marks.
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No movement in bottom sediment
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Ripple marks symmetrical
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Ripple marks asymmetrical
7
If we place some floating objects on the water a short distance from shore at Morris Cove we may be surprised to see that the water does not carry them directly into the shore, but moves them parallel to the beach a distance. This movement is created by the interaction of the waves’ onshore and offshore movements and is termed a longshore current. It can be observed on a grander scale where surfers ride the waves. Also any student who has ever floated on the waves knows he will reach the beach a good distance down shore from the point offshore where he began to float.