The Pentland Hills are formed from rocks that were formed 430 million years ago under the ocean. They were made from mud, silt and sand which was washed into the ocean that separated two continents. These two continents came together, and the rocks that had formed under the ocean were folded and uplifted to create land. A walk up to the Howe (to the west of Loganlea Reservoir) reveals these rocks in vertical layers in the hillside to the north of the stream.
The following period of volcanic activity produced most of the rocks and hills in the Regional Park. Volcanoes situated somewhere near Swanston and Colinton produced ash and lava, which created rocks 5000ft thick. A visit to any of the high hills of the Pentlands will allow you to pick up a rock produced by one of those ancient volcanoes.
Rivers and lakes left deposits forming sandstone, which can best be seen on East and West Cairn Hills.
A huge crack in the earth's crust, the Pentland Fault, ran along the present route of the A702 enabling the hills to be lifted up higher than the surrounding land. The ice age created the final shape of the hills seen today: ice flowed over the top of the hills, rounding them off, then melt water eroded the glens and "cleughs".
For more information about regionally important geological Sites, visit the Edinburgh Geological Society website.