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Friday, 3 December 2010

Isostasy revisited

Wikipedia map showing gradual ongoing uplift in the north and gradual sinking in the south

This is a revised version of my post of 6th June -- to be read in conjunction with the posts relating to Holocene shorelines etc:

The map above shows that there is still slight uplift going on in the northern parts of the British Isles, but that there is sinking in the south. This sinking might be partly a compensatory isostatic effect, partly tectonic, and partly because of the massive sediment load carried by rivers into the southern North Sea and English Channel. The rate of uplift in the Highlands of Scotland is only about one third of that of the Gulf of Bothnia, since the British-Irish ice mass was that much smaller and since isostatic recovery is now more or less complete.

Another complicating factor is that when a large area like the Bristol Channel or the English Channel becomes submerged as a result of eustatiic rise,  the weight of water that was not there previuously has an isostatic effect -- it causes the crust to sink.

On the measurement front, there are often discrepancies when altitude or sealevel measurements are made in areas like the Severn Esuuary or the Somerset Levels, because sediments are compressed (ie they sink) over time; the degree of sinking or compaction can also be related to water content......

The highest post-glacial or Holocene shorelines in Scotland are nothing like as high as the shorelines I have studied in the Arctic and Antarctic. There are some at c 45m in south-west Scotland and Northern Ireland, although most of those identified have been beneath 30m. In West Wales there is no evidence of post-glacial shorelines above present sea-level; this means that the amount of isostatic recovery following the removal of the ice load has been less than the 120m or so of eustatic sea-level rise.

If we assume that isostatic uplift in South-west Britain is more or less complete, we would expect to find some evidence of prehistoric shoreline occupation BELOW present sea-level. This is just what we do find. During the Palaeolithic sea-level was more than 20m lower than it is today. During the Mesolithic c 7,000 years ago, sea-level was about 10m lower than today, and we find evidence of the Mesolithic sea-level rise in the submerged forests around the coasts of Pembrokeshire. In the Neolithic, sea-level was at c -6m, and in the Bronze Age it was at -4m. Around the time of Christ, it was about 1.5m lower than at present.

This all means that in many locations (for example, on low coastal forelands or in estuarine environments) HWMST would have been located out beyond the position of the present coastline. This is something that does need to be borne in mind by those who argue for the human transport of bluestones by land and sea. To my mind, the degree of difficulty would have been greatly increased -- with extensive boggy and heavily-wooded tracts to be negotiated in places where there is now sea.

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