Schist, County Tyrone

Sperrin Mountains: www.grangecourt.co.uk/things_to_do.htm

The Sperrin Mountains, in the west of N. Ireland, are made up largely of ancient metamorphic rocks, such as schist (inset) and quartzite. These rocks were originally deposited as muds and sands on the sea-floor, between 600 and 700 million years ago, but were squashed and metamorphosed during the Caledonian mountain-building that took place around 400 million years ago.

During the much more recent past (around 10,000 years ago), the Sperrins were covered by ice that carved out glacial valleys and, as the ice began to melt, left behind deposits of moraine and outwash gravels deposited by meltwater streams.

In some places, the remaining ice blocked valleys, forming lakes from which melt-water could only escape by carving out new valleys such as the one shown in the lower photograph.