The southern end of the Loch is crossed by the Highland Boundary Fault - a fracture caused by movement in the earth's crust hundreds of millions of year ago. The Fault marks the geological division between Highland and Lowland Scotland. The rocks around the Fault reveal a fascinating story. 600 million years ago (Ma), the pieces of the earths crust which eventually merged to become Scotland were apart. Three separate pieces (terrains)- each with a different history - have been identified close to the end of the Fault.
The line of the Highland Boundary Fault can be seen clearly from Conic Hill on the south-east shore, looking west across the islands of Inchcailloch, Torrinch, Creinch and Inchmurrin.
The Dalradian, as the rocks to the north of the Fault are called, began life as sand and mud on a sea floor, over 600 Ma. As the continents which bounded the sea moved closer together, the sands were buried tens of kilometers below the earths surface. Extreme pressure and temperature 'cooked' the sediments into the hard schists and slates we see today, (metamorphic rocks). they also folded and squeezed upwards, resulting in a mountain range that was once as high as the Himalayas.
The Midland Valley
South of the Fault lies a different terrain. It is composed of sedimentary rocks -sand and coal measures - formed around an ancient river system some 300-400 Ma. The Midland Valley rocks form a belt across central Scotland, interspersed with volcanic intrusions (igneous rocks), like the Kilpatrick Hills and the Campsie Fells to the south and east of the Loch.
The Highland Border Complex
Between the Dalradian and the Midland Valley is evidence of a third terrain - the Highland Border Complex. This is a mixture of rock types formed in a marine environment after 540 Ma. Since the Dalradian rocks were being eroded at that time. and the Border Complex contains no Dalradian sediments, geologists deduce that the two terrains had not yet come together.