The evolution of coastlines

Erosion and deposition, continued over long periods, gradually change the configuration of a coastline, tending eventually to straighten it by wearing away the headlands. Over many millions of years these processes would have reduced long stretches of shore to virtual uniformity were it not for the changes in relative levels of land and sea which have occurred from time to time throughout the earth's history (Steers, 1969). The causes of these changes are incompletely understood, but variation of world climate has certainly been one of the major factors during the last million years by altering the volume of water in the oceans. During this period there have been a series of 'ice ages' when the world climate has become colder than at present, polar ice caps have extended to much lower latitudes, and more snow has remained on the mountains instead of melting and flowing into the sea. Because a greater proportion of the earth's water has been locked up in frozen form, sea level has fallen. During the warmer interglacial periods, the melting of ice and snow has increased the volume of water in the oceans, and raised sea level.

Changes in ocean volume do not produce equal relative changes of land and sea level in all parts of the world. The enormous weight of an ice cap can depress the level of the underlying land. When the ice cap melts, although the sea becomes deeper, that part of the land which has been relieved from the huge load of ice may rise considerably more than the sea around it so that the sea level falls relative to the rising land. Depending upon the change of relative levels of land and sea, the changes of the coastline may be submergent or emergent.

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