Wednesday, 7 August 2013

In search of glacial sediment!

When we went and looked back through all the academic articles on Peak District Glaciation, we were amazed to find that diamicton (unsorted sediment with a wide range of clast sizes) had been widely observed by field scientists. Diamictons can be formed by a wide range of processes, including by ice sheets and glaciers, in which case they are referred to as 'tills'. Finding till in the Peak District would therefore be really good evidence of ice in the region, although we would still have to work out when it had been deposited. 

Striated rock shown to us by the caver!
First of all we visited a disused limestone quarry near to Eyam, where till had been previously reported. We were hoping to see a drape of till on top of the limestone. Unfortunately, despite scrabbling around for a good while we could not find any. Then Chris Clark suggested we visit an old caver he new in a nearby village, in an attempt to tap into some local knowledge. 

Chris' caver was extremely keen to help out. On walking into his house we were met by a veritable treasure trove of fossils and rocks, including one large rock covered in striations. Striations are small scratches, formed when rock fragments carried by the ice rub agains other rocks. This gave us hope that our mission would not be in vain; and indeed the gentleman pointed out several locations nearby where he had seen rocks like this - a sure indication of glacial activity!

In good spirits we headed into our second quarry, where immediately we discovered an absolutely massive section of diamicton, chock full of striated rocks and exotic rock-types (erratics), which must have been carried by the ice to this locality. We had found our till! 

Of course, now we are left with the age old question - when was the till deposited? On top of the till we identified a sequence of clays and silts, which are found at the bottom of lakes. Given that the lake sediments lie directly above the till, they were probably deposited in a lake dammed by the retreating ice. Crucially, we can date the lake sediments in a future visit to tell us when the ice had retreated. 

We therefore intend to return to the site later on in the summer to try and date the silts above the till, and also do further work on the till itself to try and decipher its depositional history (e.g. was it deposited under a glacier?).
Thick sequence of diamicton (note all the rocks jutting out), with silt and clay on top.

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