Geology
Both the Grwyne Fawr and the Grwyne Fechan drain the Old Red Sandstone hills of the Black Mountains. These are the Brownstones and Senni Formations which date from the Breconian stage of the Devonian period. The sandstones and mudstones of these formations can frequently be seen in the beds of the two rivers and sometimes exposed in the often steep valley sides. The lower reaches of the river run through an area blanketed with glacial till, a legacy of the last ice age.
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