The geodiversity of the Isle of Harris : statement of significance and identification of opportunities

This report has been prepared for Harris Development Ltd. in order to provide a summary of the geological history of the Isle of Harris, to identify the key geodiversity features, and to suggest opportunities for geological interpretation. The entirety of the island of Harris is underlain by rocks of the Lewisian Gneiss Complex. The oldest rocks on Harris were formed over 3000 million years ago – they are two-thirds as old as the Earth itself, and they are some of the oldest rocks in Europe. For 1500 million years after their formation, these Lewisian gneisses were affected by a multitude of geological processes including stretching of the crust associated with extensive volcanism, and collision of ancient continental masses which led to mountain building. All these geological events are recorded in the rocks we see today on the Isle of Harris, and they are the focus of continuing research. After about 1600 million years ago, the Lewisian gneisses of Harris became part of a stable continental mass – and there they stayed, largely unaffected by geological events going on around them, almost until the present day. Finally, within the last 2.6 million years, the gneisses that were exposed at the surface have been eroded and polished by the action of glaciers and oceans, producing the landscape that forms the Isle of Harris today. The first section of this report describes this geological history in more detail, and emphasises some of the important events. The second section discusses the relationship between geology and the unique landscape of the island, whilst the third section focuses on some localities that are of particular importance in terms of their geological interest. Possible interpretation opportunities and suggestions for geotourism facilities are given in the fourth section. It is worth noting that, where possible, Gaelic spellings have been used for place names on the Isle of Harris (Hearadh). However, where anglicised spellings have previously been used in a geological name or in the name of a notified site, those spellings have been retained in this report. For clarity, the anglicised terms ‘South Harris’ and ‘North Harris’ have also been retained.