Swallow holes Rivers also erode limestone by the process of solution, hydraulic action and abrasion. A river, which may begin in a non-limestone area, disappears underground when it reaches a limestone surface. The place where a river disappears underground is called a swallow hole. Swallow holes may be tiny or large depending on the size of
the stream.
Pollnagollum swallow hole in the Burren is over 6 m wide and 16 m deep.
Fig. 8 Map showing a disappearing stream in the Burren. (See the blue line suddenly disappear between Croagh Bridge and the R480.)
Harmless colour dyes are poured into swallow holes to see where the water will reappear. This helps geologists to map underground water reserves.
Fig. 9 A doline in the Burren, County Clare Dolines or sinkholes
Another characteristic feature of karst areas is that its surface has many enclosed hollows called dolines. They are also known as sinkholes, e.g. Dunmore Cave, County Kilkenny. Dolines are conical-shaped depressions and it is thought that these hollows form as a result of underground caves collapsing. They may be anything from a few metres up to a kilometre wide and a few metres to 150 m deep.
Slight hollow over cave Land surface 30 m
CAVE
1. Cave exists close to surface. As the roof weakens, a depression may appear on the surface.
Blocks of collapsed cave roof
2. Roof collapses leaving a sinkhole or doline. Fig. 10 Exam Diagram: The formation of dolines or sinkholes 96