Excepting the low-lying areas of the east coast, which mainly consist of Scarba conglomerate and Port Ellen phyllites, the isle of Jura is largely formed of Jura quartzite, a metamorphic rock which has its origins in the late Pre-Cambrian or Dalradian age. The predomination of quartzite is most apparent amid the Paps and the ridges in the island’s north-west.
A series of linear basalt dikes cut into the quartzite rock on a south-east–north-west orientation throughout the island, although they are more numerous in the south. They were formed during a period of intense volcanic activity in the Lower Tertiary period, some 56 million years ago, when upwelling magma filled cracks in the Earth’s crust radiating out from centres of eruption on Arran and Mull. Today, these dikes are most apparent on the west coast where erosion of the less-resistant rock into which they are intruded has left them exposed as natural walls.
Jura is internationally renowned for its raised shore platforms and immense raised beaches, which abound on the west coast along with numerous caves, rock stacks and rock arches. These geological phenomena reflect changes in relative sea level associated with the advance and retreat of ice sheets during the Quaternary Ice Age. During glacial periods, sea levels dropped as fresh waters were held captive by the advancing ice sheets and rose again when the glaciers melted. At the same time, the landmass initially sank under the weight of the ice cap then gradually rose again as the ice retreated in a process geologists describe as ‘glacio-isostatic uplift’. The land rebounded slower than the sea level rose, therefore the sea level was initially much higher locally than it is today and beaches and shore platforms were gradually lifted out of the sea as the land continued to rise. The relative rebound is greater in those areas where the ice sheet was thickest and this is why the uplift is more pronounced towards the north-west of Jura
The raised beaches along the west coast are impressively proportioned, occurring as terraces or ‘staircases’ of largely unvegetated shingle ridges, which evidence the gradual fall in relative sea level. The most impressive raised beaches lie between Shian Bay and Ruantallain, east of Ruantallain, west of Glenbatrick above Loch Tarbert, with perhaps the most remarkable formation being the massive pebble bank east of Glenbatrick, which forms a natural barrier between Loch Tarbert and the freshwater Lochan Maol an t-Sornaich. Raised beaches do occur on the sheltered east coast though these are most often covered in vegetation. The absence of vegetation on the west coast’s raised beaches makes these often huge tracts of large cobbles visually spectacular. The size of the cobbles and the depth of deposits combined with the scouring effects of wind and rain have rendered these beaches largely sterile.
Jura is also home to an excellently preserved medial moraine known as Sgriob na Caillich (the old woman’s slide), which extends for two miles westwards towards the coast from the flank of Beinn an Oir – highest of the Paps. The moraine takes the form of a series of parallel belts of boulders and is best viewed from across the Sound of Islay. It was formed by the retreat of the Late Devensian ice sheet and is regarded as the best example of its kind in Britain.
Wild Walks on the Isle of Jura 