Recording Past Sea Levels in the Somerset Levels, UK

Figure 1.  From Bowen et al (2002), the limits of glaciation in Great Britain and Ireland.

Figure 1. From Bowen et al (2002), the limits of glaciation in Great Britain and Ireland.

The Holocene spans the last 11,700 years, and is marked by a return to warm interglacial conditions through much of the northern hemisphere, and the spread of humanity into areas of land that were previously under ice. As glaciers melted, two things happened to sea level relative to the land. Firstly, there was a global rise in sea level due to water being released from the ice (Quaternary scientists call this eustatic sea level change). Secondly, areas of land that were previously under ice rose relative to sea level as a result of the decreased load on the earth’s crust (isostatic sea level change). Whereas much of Britain was under ice during the Last Glacial Maximum, Somerset which is in south west Britain was not, and periglacial conditions prevailed. The lack of ice over Somerset, and the rapid loss of glacial ice over Wales means that the Somerset coast has been sinking relative to sea level (there has been no isostatic lift). This becomes apparent when we look at the River Severn.  Over the timescale of human occupation of Somerset, the River Severn has changed from a river cut through a rocky gorge flowing to a sea much farther west than the present day coast, to an estuary with one of the largest tidal ranges on earth, flanked by low-lying land prone to flooding.

Machines stripping the excavation area, Walpole Landfill Site, Somerset

Machines stripping the excavation area, Walpole Landfill Site, Somerset

Since 2000, C&N Hollinrake Ltd. have been carrying out archaeological work at the Walpole Landfill Site, close to Bridgwater on the southern edge of the Severn Estuary (map) ahead of landfill cell construction. This has allowed a view of the stratigraphic sequence over an area almost 1 km in length north to south, and some 500 metres east to west. At the most basic level, changes in relative sea level can be detected in the stratigraphic sequence. Clays, deposited during times when the site was flooded by the tide, are interspersed with peats, when freshwater wetland conditions predominated.

Haynesina germanica, a benthic foram

Haynesina germanica, a benthic foram

A more detailed picture emerges from the remains of animals preserved in the sediments. Different species of benthic foraminifera, tiny marine protists which live in the estuarine mud, have been found to live in distinct zones relative to how much of the day the mud spends covered by the tide (see Haslett et al. 2001 for more details), while examination of bryozoans (a colony-forming invertebrate) can serve as an indication of the salinity of the water (allowing brackish lagoons to be differentiated from the open coast) (see Law 2013). Work has only just begun on the analysis of biological remains from this site, however, by examining how the landscape responded to fluctuations in sea level in the past in such fine detail, we can begin to think more confidently about what might happen as global sea levels begin to rise in the future.

References

Bowen, D. Q., Phillips, F. M., McCabe, A. M., Knutz, P. C., & Sykes, G. A. (2002). New data for the last glacial maximum in Great Britain and Ireland.Quaternary Science Reviews, 21:89-101.

Haslett, S.K., Strawbridge, F., Martin, N.A. and Davies, C.F.C., 2001. Vertical saltmarsh accretion and its relationship to sea-level in the Severn Estuary, U.K.: an investigation using foraminifera as tidal indicators. Estuarine, Coastal and Shelf Science 52:143-53.

Law, M., 2013. Bryozoans in archaeology. Internet Archaeology, 35.

Advertisements

About matthewlaw

Archaeologist
This entry was posted in Uncategorized. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s