The Geology of the Valley (click on maps to bring up bigger/better versions).

Solid Geology

solid geology
Drift Geology

drift geology

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.)