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Sunday, 20 February 2011

The Teifi Varve dilemma

The top photo shows the Swedish aristocrat and geologist De Geer, who pioneered the science of interpreting varve sequences, examining exposures in a clay pit.  The map below shows the "Swedish varve chronology" -- successive ice edge positions during the wastage of the Scandinavian ice sheet.  Much of this chronology (established eventually by many geologists over many years) was based upon the early observations of De Geer, and his recognition that characteristic sequences could be traced across country, from one exposure to another,  until a more or less complete story could be recreated -- from places that were inundated by fresh of brackish water, in vast lakes, as the ice melted.

In the top photo we can see that the varves vary quite considerably in thickness -- but the basic principle is that the light coloured layers are summer layers, when the sediment load in lakes was high, with sandy and silty layers settling on the lake bed relatively quickly.  The dark layers (generally much thinner) are the winter layers, comprised of much finer silt and clay particles, that settled out during the winter, very slowly and probably under a seasonal ice layer.  Many of the lakes concerned were more than 100m deep.

Back to Lake Teifi and the other lakes.  If anybody out there knows how many annual varve layers have been counted thus far in the cores and exposures examined in the course of research, I'd be pleased to hear about it.  How much time elapsed between the initial ice advance across the coastline from Cardigan Bay and the eventual collapse of the ice sheet edge?  Shall we assume for the moment that we are talking about a period of c 1,000 years?

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