The Highlands and Islands lie to the north and west of the Highland Boundary Fault, which runs from Arran to Stonehaven. This part of Scotland largely comprises ancient rocks, from Cambrian and Precambrian times, that were uplifted to form a mountain chain during the later Caledonian orogeny. These foundations are interspersed with many igneous intrusions of more recent age, the remnants of which have formed mountain massifs such as the Cairngorms and Skye Cuillins. A significant exception to the above are the fossil-bearing beds of the Old Red Sandstone found principally along the Moray Firth coast and in the Orkney islands. These rocks are around 400 million years old, and were laid down in the Devonian period. The Highlands are generally mountainous and are bisected by the Great Glen Fault. The highest elevations in the British Isles are found here, including Ben Nevis, the highest peak at 1,344 metres (4,409 ft). Scotland has over 790 islands, divided into four main groups: Shetland, Orkney, and the Hebrides, further sub-divided into the Inner Hebrides and Outer Hebrides.