Introduction

Variations in the extent and location of saltmarshes around the coast of the British Isles have occurred throughout recent geological time. During the post-glacial marine transgression in the Flandrian Period, saltmarshes were transient features formed on the (now flooded) continental shelves. As sea level rise slowed down, around 4000 years ago, the coastline took on the general character of its present form. The Holocene sea level history of Britain varies in detail from area to area, with some regions, notably southeast and southern England, having experienced slow net sea level rise (albeit with some small positive fluctuations) during that period, while other regions, including much of the north and west, have experienced a net fall in relative sea level (Shennan, 1989; Shennan et al., 2000). These changes and their timing have had a profound effect on the historical development of saltmarsh around the coast.

In some areas, such as on the North Norfolk coast and the Thames Estuary, marsh sediment sequences have accumulated vertically, albeit episodically, over several thousands of years as sea level rose during the Flandrian Period (Devoy 1979; Andrews et al., 2000). Periods of slight sea level regression are recorded in the sedimentary sequences as soil layers, peats or over-consolidated horizons. In these areas there was an only limited seaward progradation of new marshes during the later Holocene, and deposits of younger marsh sediments largely sit on top of older sediments. Elsewhere, as in West Lancashire and The Wash, early to mid Holocene marsh sediments now lie well inland of younger saltmarshes formed in the last few centuries (Tooley, 1992; Pye, 1995; Brew et al., 2000).

Typically, the older marshes around the British coast have been extensively reclaimed in a series of progressive stages, with younger marshes forming (or attempting to form) to seaward after each phase of reclamation.

The information available within this section summarises two of the key human influences on saltmarsh extent historically (and more recently); namely land claim and coastal squeeze.

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