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Monday, 2 November 2015

Waun Mawn -- the post-glacial sediment sequence

On the track from Tafarn y Bwlch to Gernos-fach there are a few nice exposures at the moment.  Here is a photo of one of them.

This is the sequence:

At the top, a very thin modern soil horizon -- c 10 cm thick, with streaks of humus and one thin band of dark organic matter.

Below that, c 30 cm of colluvial hillwash, buff-grey colour, sandy / silty texture, with patches of iron-staining in the top 20 cm.

Below that, c 40 cm of head or broken shale bedrock debris, with no signs of bedding but much churning -- maybe indicative of severe periglacial and even permafrost conditions.

Below that, iron-stained clay-rich till, probably of Late Devensian age, incorporating large boulders and other clasts which are sub-angular and sub-rounded.  The base of this layer is not seen.

In this open environment with a rather gentle slope, sediment accumulation has clearly been rather slow, with less than a metre of accumulation over a period of c 20,000 years.

The sequence matches well with Rhosyfelin and other exposures in the area, but what we see is great variation in the RATE of sediment accumulation, dependent upon the rate and type of sediment supply, the slope gradient, and the type of vegetation cover.  In some places, out on the moor, the till is exposed at the surface, without any overlying sediments at all.

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