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Tuesday, 22 April 2014

Quartz blocks at Rhosyfelin are not erratics


There are several big blocks of quartz low down in the sedimentary sequence at Rhosyfelin, which I have previously interpreted as erratics transported into the area along with other foreign stones and boulders.  Well, I have had a change of opinion on this.  Now, following the completion of the 2013 dig, one can see that the biggest block actually seems to be sitting on an outcrop of quartz bedrock.  So there must be a big mass of it incorporated into the rhyolites.  Perhaps Rob or Richard will tell us if this is typical --  maybe it is a secondary feature, formed some time after the rhyolites were laid down?

I must admit to having been a bit worried about the "fresh" appearance of the quartz blocks, with few signs of abrasion and many sharp edges.  This would be consistent with the blocks having travelled hardly at all from their place of origin.

A bit more digging in 2014 will reveal whether this is really the bedrock floor in the digging area, and how thick the sediments are beneath the lowest digging surface from 2013, as shown in my posts of last September.

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