Burren Landscape Karst and Karren

The relatively pure calcium carbonate limestone of the Burren is very susceptible to the solutional effect of the high rainfall levels in the region (rain is a mild carbonic acid) - it has been estimated that 0.005mm of limestone is dissolved annually. This effect is accentuated in places through the humic acids produced by heathy vegetation pockets. Solution of the limestone has resulted in the formation abundant karren and karst features in the Burren.

Karst, a term originating from a limestone region in (the former) Yugoslavia, is derived from the Slovenian word kras, meaning a bleak, waterless place. It is used to describe a landscape containing erosional features such as bare pavements, subterranean water systems, dry valleys, dolines and poljes. The Burren is recognised as one of Europe's finest examples of a karst landscape, abounding in all of these features.

Some 100 closed depressions of various sizes occur in the Burren. Those of moderate dimension are called dolines, from the Serbo-Croat for valley or hollow, and a good example is found at Poulavallan near the Glen of Clab. Dolines are more or less circular, formed by solution, directly in some cases, or indirectly through the collapse of cavern roofs. Larger closed depressions known as poljes are more frequent in the Burren, and the best examples are found at Carran, Poulawillan, Caherconnell and Kilcorney.
 

Often providing a focus for agricultural settlement, these features were originally formed by ancient (Tertiary period) shale-based streams burning through the limestone beneath. The word polje derives from the Slav word for field, or an area that can be cultivated, and they are not unique to karst areas. Carran polje is c. 3.2 km long, 1.6 km wide, and over 60m deep.

Turlough is a term derived from the Irish words tuar and loch meaning ‘disappearing lake’. Turloughs are temporary (largely seasonal), shallow, groundwater-fed lakes surrounded by a rocky rim and lined with boulder clay, which are replenished and drained through discrete openings connected with the water table. They are strongly associated with zones of higher permeability in the aquifer in western Irish limestone lowlands. Some good examples in the Burren include sites at Carran, Aleenaun, and Turlough.

Probably the most characteristic karst feature of the Burren is the extensive pavement network, largely a product of glacial erosion and subsequent soil loss. There are two main pavement types, depending on the nature and structure of the limestone: smooth and shattered, with smooth pavements more common in the west Burren, shattered in the east.
 

These pavements are susceptible to solution, which can eventually expose vertical lines of weakness in the limestone to form characteristic clint-grike systems. The clints or blocks range from a few square centimetres to over 65m2, with most ranging between 0.4 and 2.8 m2. The grikes, or crevices that dissect them, are usually less than two meters deep and 3.2 to 6.5 centimetres wide, but extend down to six meters in places.  

Karren (or ‘lapies’) is a general term used to describe the total complex of superficial micro-solutional features of limestone pavement, widespread in areas such as the Burren. Karren forms include sharp-ridged grooves (rillenkarren) and their larger, elongated cousins (rinnenkarren), as well as rounded runnells formed beneath a soil cover (rundkarren). Other forms include the ubiquitous solutional hollows (kamenitzas), deep cleft-like ruts or grikes (kluftkarren), clints (flachkarren), and horseshoe shaped stepped structures (trittkarren) found on the Burren coast.

Karren forms vary depending on the extent and nature (where present) of the former soil cover. For instance, karren features that developed subaerially are fretted and rough, such as rillenkarren, while under an acid soil cover ‘cuspate’ and ‘arcuate’ forms such as rundkarren predominate. Furthermore, a bare karren surface of sub-soil origin is rounded and smooth, and not as sharp and angular as a similar surface that has never had a soil cover. Most karren features in the Burren are smooth rather than angular, suggesting that an extensive mineral soil cover must have existed in the area at one stage, a finding supported by pollen records from the area.