Dalradian Limestone

Schiehallion Limestone Pavement 

The Dalradian Limestone is 600 million years old, much older than the Carboniferous Limestone that makes up the Pennines and the Burren in Ireland. It is unusual in that it does not contain fossils and was produced by precipitation on the sea bed. It has passed through the Caledonian orogeny (mountain building process) and so has been hardened but is not quite a marble. There are bands of mica within it and when it weathers it crumbles into sugar-like grains.

In the Rannoch area it occurs as Limestone pavements and produces potholes, dolines, caves and swallow holes. In places drinking water is hard because of calcium in the water. Like other limestones it is dissolved slowly by acid water and it is this that has given the features listed above. The Schiehallion Limestone Pavement is a Site of Special Scientific Interest (SSSI) and is probably the best example of a Dalradian Limestone Pavement in Scotland. Many plants like lime conditions and flourish on the Dalradian Limestone. Examples include Rock Rose, Globe Flower, Thyme,  Purple Milk-Vetch (Astragalus danicus) Autumn Gentian (Gentianella amarella)  and Wild Strawberry.

Rock rose is found in profusion along the Schiehallion Road, at Dun Coillich and many other places in the Rannoch area where there is limestone.

Globe Flower may be found at Keltneyburn nature reserve, Dun Coillich and Trinafour.

The grooves that are dissolved in the Pavement are called grykes and may be very deep - perhaps six feet deep. The higher areas between grykes are called clints. The calcium carbonate which is the dominant mineral in the Limestone is in the form of calcite although when it gets incorporated into shells such as the snail Cepaea nemoralis it is converted into Aragonite. Snails are common where there is Limestone, as is a white lichen which grows on the rock.

The grykes provide protection from grazing for various woodland flowers such as Dog's Mercury, Herb Robert, Herb Bennet, Wood Anemones, Wood Sorrel and Ivy as well as small Birch and Rowan trees.

The Schiehallion Limestone Pavement showing the characteristic grykes and clints as well as white lichen.

Ivy growing in a gryke.

Ivy growing in a gryke.

Biotite Schist Erratic with honeycomb weathering. The honeycomb weathering is produced when the iron in the rock is differentially weathered more rapidly than other minerals leaving a pock-marked effect.

At Braes of Foss there are some small potholes and a swallow hole where the small burn disappears below ground to reappear on th other side of the road. In places the ground has collapsed (dolines) because the limestone beneath has been dissolved away.

On the south side of Schiehallion is the Giant's Cave which is actually a relatively small Limestone cave.

At Lassintulloch there are further potholes on the flanks of Schiehallion.

At Trinafour there is a small Limestone Pavement and another swallow hole.

The Giant's Cave is actually not a very large cave - it is found on the south side of Schiehallion. It results from the solution of limestone.

There are vaious limestone features near Braes of Foss - Potholes, a Swallow Hole, and Dolines. All result from limestone being dissolved by water. A Swallow Hole is where a burn disappears into the ground when an underground watercourse has been created as the rock has been dissolved away.  The same applies to the Potholes. Dolines are sunken areas where the surface has collapsed into the cavity left when limestone beneath has dissolved away.