MacGillycuddy's Reeks are composed of sandstone particles of various sizes which are collectively known as the Old Red Sandstone. The rocks date from the Upper Devonian period (310-450 million years ago) when Ireland was in a hot equatorial setting. During this 60 million year period, Ireland was the site of a major basin, known as the Munster basin, and the counties of Cork and Kerry were effectively a large alluvial floodplain. Chemical oxidation stained the material with a purple-reddish colour (and green in places from chlorination), still visible today. There are virtually no fossils in Old Red Sandstone. The composition of Old Red Sandstone is variable and contains quartz stones, mudstones, siltstones, and sandstone particles (boulders of conglomerate rock containing quartz pebbles are visible throughout the range). The Reeks were also subject to significant glaciation which led to fracturing of the rock, and resulted in deep corries (e.g. the Eagle's Nest), U-shaped valleys (e.g. Lough Coomloughra), and sharp arêtes and ridges (e.g. the Beenkeragh Ridge).