37 Glacial and karst landscapes of the Gort lowlands and Burren. Michael J. Simms Introduction The Burren Hills The Burren is one of the most striking and distinctive landscapes in the whole of Ireland, a vast undulating and terraced upland of often bare or thinly vegetated Carboniferous limestone rising often steeply from the coast to an altitude of more than 300 metres in places. In an Ireland-wide context, however, the Burren represents an intriguing anomaly. Some 40% of Ireland is underlain by Carboniferous limestones often barely distinguishable from those of the Burren yet over almost their entire outcrop these mechanically tough limestones seldom rise significantly above 50 metres O.D. Indeed, the eastern edge of the Burren plateau ends abruptly at a steep scarp overlooking the Gort lowlands, again developed on virtually identical limestones, which here seldom rise more than 30 metres above sea level. The reason that the limestone outcrop across Ireland is predominantly lowland arises from its solubility compared with other rock types. Limestone is removed in solution, a relatively low energy process that is not dependent on water flow velocity. In contrast, other rock types with very much lower solubilities are removed largely by erosion, a relatively high energy process that occurs only when water flow is faster than about 0.2 m/s even for the smallest and most loosely packed sediment particles. Water percolating slowly through these relatively insoluble rocks will alter their composition and mechanical properties through weathering, but it cannot move the particles themselves until a threshold flow velocity is exceeded. Hence erosion is dependent on occasional high-energy flood events whereas dissolution of limestone is much more continuous. In the prevailing climate of Ireland, with endless ʻsoft daysʼ punctuated by only occasional downpours, dissolution of limestone and weathering of other rock types is favoured much more than erosion. As a result, over time scales of millions of years limestone outcrops across Ireland have been lowered at a faster rate than outcrops of other rock types, in a kind of denudational ʻTortoise and Hare raceʼ (Simms, 2004). The Burren limestone owes its survival as uplands to a protective cover of mudstones and sandstones which was stripped away only relatively recently. In contrast the shale cover over what is now the Gort lowlands was eroded away millions of years earlier, exposing the limestone beneath to prolonged dissolution. Through much of the Cenozoic Irelandʼs climate probably has been largely humid temperate, accounting for the overall topographic contrast between limestone and other rock types. Subsequent repeated glaciations through the Pleistocene had a profound influence on the topography of Ireland.