Geology
The Rhinogydd are formed of hard sedimentary rocks of Cambrian age which occur as a major anticlinal structure known to geologists as the Harlech Dome. This structure which originated during the Caledonian Orogeny (mountain-building period) extends from Cadair Idris in the south to Blaenau Ffestiniog in the north. Its erosion by successive ice ages has left the valleys and peaks of the Rhinogydd visible today. The core of the area is formed from the hard-wearing greywackes of the Rhinog Formation. The formation also contains and is overlain by some siltstones and mudstones which form a broken zone of softer scenery around the periphery of the area's rugged core. In places, swarms of dolerite dykes cut through the country rocks in a generally northwest-southeast direction. Much of the lower ground is mantled by glacial till, a legacy of the last ice age.
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