However, in contrast to the quantity of archaeological evidence found, the population of The Burren likely declined noticeably between the late Neolithic and the end of the Bronze Age. This was likely largely due to a shift in the climate, possibly related to the volcanic eruption known as "Hekla 4". Around the end of the 3rd millennium BC temperatures dropped and the climate became wetter. Heavy rainfall combined with previous clearing activity to cause serious soil erosion in the uplands. Evidence suggests that much of the soil cover over the limestone karst was in fact removed during the Middle and Late Bronze Age. This reduction in the area of fertile land put pressure on population numbers and probably increased conflicts over scarce resources. Building of new stone walls largely came to an end – there is a gap in documented building between the late Neolithic/early Bronze Age and early Christian periods. By the end of the Bronze Age, the uplands were largely depopulated.