The Sperrin Mountains are made up of schists and quartzites - metamorphic rocks. The rocks had originally been deposited in pre-Cambrian times between 700 and 600 million years ago. The original rocks, mostly sandstones, mudstones and limestones, were changed by enormous heat and pressure caused by mountain building. These mountains were formed as the North American and Eurasian plates crashed against each other in Ordovician times, between 440 and 500 million years ago. The mountains themselves have eroded away and only their roots remain - the metamorphic rocks at the core of these once enormous mountains.
The last ice age has had a great impact on the Sperrins and the great ice sheets that covered the north of Ireland were probably at their thickest over the Sperrins. This area retained its ice sheet for a time after the retreat of the ice began - this has caused many of the present features of the Owenkillew Valley and the Glenelly Valley (see photograph). Numerous eskers, formed from sediments deposited by meltwater rivers below the ice, can be found. These are winding ridges, often 30 metres high, made up of sand and gravel. The bedding and sorting of the material indicates that it was laid down in water, rather than the unsorted materials such as boulder clay, laid down by the ice itself. So common are they in Ireland that the Irish word eiscir (anglicised as esker) has been adopted worldwide to describe these remnants of the Ice Age. One of the largest in the Sperrins is Eskermore (An Eiscear Mor - the big esker) at Beragh, Co. Tyrone. All of these eskers are of significant value and are exploited for materials for the construction industry.
As the ice retreated in the area, valleys were temporarily obstructed by ice sheets. This led to valleys filling with water which escaped into adjoining lowland. This often resulted in erosional features called glacial overflow channels. Found in many mountainous areas throughout Ireland, some of the best examples are in the Sperrins. Barnes and Gortin Glen are two such overflow channels. There are also the remains of deltas where rivers fed the glacial lakes. Called kames, a good example is Mossey's Hill above Gortin, County Tyrone.