The Geology of the Valley (click on maps to bring up bigger/better versions).
|Now I'm straying into an area that's certainly not my
field of expertise - the geology of the region - but I'll have a go at
describing it, and await a response from those who are better informed. The
majority of the solid geology of the Dyfi catchment area is Silurian
mudstone/siltstone, but with outcrops of Ordovician mudstone/siltstone
appearing mainly to the SW of Machynlleth, and Silurian greywacke on the far
eastern edge of the catchment area. The drift geology of the area comprises
glacial deposits (diamicton) in various locations, and aluvial deposits of
clay/silt/sand along the Dyfi Valley itself. Sand dunes appear near the
estuary, and you can see from the map that there are local concentrations of
peat in various places. I needed to find out a bit more about many of these
terms and, if you feel the same need, take a look in Wikipedia, where there's
enough to keep you engrossed for hours, if that's your idea of a fun evening.
Fundamentally though, greywacke, mudstone and siltstone are all sedimentary
rocks, the main difference between them being the size of the grains from which
they are composed.
(Maps are Crown Copyright, used with permission of Environment Agency Wales.)