Geology

Geology

In order to understand this area a basic knowledge of its geology is needed.

The study area is a limestone corridor which runs between the higher slopes of Cuilcagh Mountain and the lower glaciated Lough Macnean basin. From the block diagram (below), and map (below), the ground geology can be seen to be comprised of three different levels. From the summit of Cuilcagh the upper sandstone/shale layer overlays the Dartry Limestone layer below it and only on the lower slopes does this limestone layer become the surface layer. The effect of this can be best seen when you examine the hydrology of the area. When the upland rivers meet the limestone they go underground to form cave river passages, as the water dissolves its way through the limestone rather than flowing over it on the surface. It is only where these underground streams encounter the lower, less soluble, shaly limestone of the Glencar limestones do they resurface.

Limestone ‘corridor’

It is in this ‘corridor’ of limestone that the project study has been undertaken. The base rock of this corridor is limestone with a thin covering of soil, much of the area having exposed limestone pavement and ubiquitous glacial erratics. Interspersed throughout this karst terrain of hills and dry valleys are areas of glacial drift and bog.

Along this ‘corridor’ there is an absence of soil deposition and bog development resulting in the exposure of most of the prehistoric archaeology and, incidentally, it is also likely where most of the prehistoric farming and settlement would be located originally. The soils in this limestone area have better drainage than the thicker glacial soils on the sandstone. Even prior to bog development the soils on the upper sandstone region would also have been more acidic. The lowlands, where the underground rivers re-emerge, have substantial subsequent soil deposition covering any possible earlier settlement features.

Glacial erratics

Another aspect of the geology is the presence of glacial erratics. These sandstone glacial erratics have been deposited throughout the karst area in very large numbers, the Burren forest having one of the densest concentrations.

They were transported from the mountainous region surrounding Cuilcagh Mountain during the last Ice Age. These ‘erratics’ form the raw materials for the many stone features that are of interest in the area. As almost all the boulders are Glenade Sandstones there is practically no erosion on them, apart from weathering. As a result the many forms of rock carvings, and rock art, are well preserved.

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Pedestal boulders

As the sandstone glacial erratics are insoluble (Glenade Sandstone) and as limestone is soluble, even in slightly acidic water, many of the erratics have developed pedestals underneath them over the millennia as they have protected the bedrock immediately underneath them from erosion. Many of these ‘pedestal boulders’ appear monumental in their own right. An important aspect of the study was to distinguish between natural erosion/weathering and possible anthropogenic work. This resulted in a number of study techniques being developed by GB. The Glenade sandstone of the Cuilcagh area is an orthoquartzite sandstone (98% quartz grains and siliceous cement) and would not have been affected by chemical water based erosion. This is important when examining channels, hollows and other natural looking features which may appear similar to those on calcareous rocks where they would have been caused by water dissolution activity.

 

Pedestal Boulders

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PB47 G + JBB.jpg

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CB29 PB .jpg