Geomorphic Features

Historically, geomorphic features have provided the clearest evidence of a former change in sea level. Erosion and inundation during transgressions, or prolonged periods of rising sea level, often erase or submerge former sea level positions. Until the advent of offshore seismic stratigraphy and bottom profiling, the features most often available as possible indicators of former changes in sea level were likely to have been supertidal in elevation (e.g., raised beaches) and landward of existing shoreline position, and thus, representative of a time when sea levels were higher than present. Such evidence was fundamentally at best a very incomplete record of the sea level history of most areas. The actual possibility of higher sea level than at present during the late Holocene is a separate issue that will be discussed later.

In more recent decades, the incompleteness of the record has been seen as less of a problem than whether such evidence documents a change in sea level at all. That raised beaches, wave-cut scarps, and strandlines demonstrate that water levels were at one time higher has not been the issue; rather, were the episodes of former high water levels related to sea level changes, and not storms (Fig. 2.1, see color plate) (see Stapor, 1973)? It has been shown that storm surges can cause either deposition or erosion considerably landward of modern shorelines and, in fact, raised beach ridges have been use to reconstruct the tempo of storm periodicity since the early Holocene (cf. Fairbridge, 1987). Tsunamis are an additional complication. Although passive continental margins like the U.S. Atlantic Coast are hardly areas where such events can be expected, no coast is probably immune from tsunamis. About 7000 years ago, a submarine slide off Norway (part of the Amero-type trailing continental margin of Northern Europe) generated a tsunami which deposited a sand layer in eastern Scotland almost three-quarters of a meter thick at some sites (Long et al., 1989).

Still, though some remain suspicious about the connection between sea level and old supertidal "shoreline" features, the wide geographic extent of some scarps, like the 1.5-m above-mean-sea-level (MSL) scarp ringing much of the Florida Panhandle, has been argued as something only a transgression could create (Donoghue and Tanner, 1992). Even widespread scarps generated by storms, such as those formed by the 1991-1992 northeasters along the U.S. middle and north Atlantic Coasts, would be unlikely to be continuous and at relatively the same elevation, since local coastal configuration and regional variations in the storms would combine to produce irregularities in surge height and penetration. Perhaps the skepticism that greets reports of raised beaches and former sea level highstands more than anything crystallizes the debate alluded to by Flint in the quote that begins this chapter; highstands are undeniably fluctuations.

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