Form and Relation to Present Sea Level of Pleistocene Marine Erosion Features

Abstract

Wind fetch and land slope are the two oceanographic factors most easily determined for ancient shores, and these factors can account for many of the variations in terrace form at different levels in one area and in different areas at the same level. On a vertical cliff face no erosion terrace is initiated, but on soluble rocks a sharp solution notch leaves a clear record of the sea level. Assuming maximum depth of erosion of rock by waves to be 30 ft., the geometry of terrace erosion is outlined, and the limiting states under conditions of varying land slope and subaerial weathering. Maximum terrace width is commonly about 3,000 ft. with a slope of about 1/100 since accumulation of a sand beach protects the cliff at this gradient. Small eustatic oscillations can cut two solution notches, two small terraces, one terrace, or one terrace and a notch, depending on the land slope and the amplitude and duration of the oscillation. Large eustatic changes tend to eliminate traces of small oscillations and to modify, but not necessarily eliminate, the terraces caused by long Stillstands. Thus terraces may survive with younger terraces both above and below, and altitude is not simply correlated with chronology. Earth movements during formation of a sequence of terraces cause predicTable alterations of spacing, and it is possible to analyze superimposed sequences of different ages on continuously tilting land.