Ancient Rivers on Rum
The reddish-brown Torridonian sandstones are best seen in the part of Rum north of Kinloch, although they also occur along the coast in the southeast of the island. They are sedimentary rocks - rocks that formed by the accumulation of large amounts of sediments such as sand. Northern Rum has a gently 'stepped' landscape, with obvious layers in the rock that are gently inclined down towards the northwest. When the sediments were originally laid down, nearly 1000 million years ago, they would have formed flat layers of sand and pebbles. Each layer represented an episode where a river flooded and deposited sediment across a floodplain. Later earth movements have led to these layers becoming tilted.
Because the sandstones were originally deposited in seasonal rivers, rather than deep in lakes or oceans, they were exposed to the air. This meant that iron particles in the sandstones became oxidised, in a process similar to rusting, producing a characteristic red-brown colour. In the south and southeast of Rum, the sandstones are interlayered with mudstones that were deposited in lakes and these rocks are typically grey rather than red-brown in colour.
The sedimentary rocks are softer than the rocks that make up the rest of Rum, and so they have been easily eroded, forming less mountainous areas. Most of the younger sedimentary rocks that once lay on top of the Torridonian have now been eroded away completely, but a small area of sandstones of Triassic age can still be found on Monadh Dubh in northwestern Rum, to the north of Glen Shellesder. These sandstones contain the poorly preserved remains of 250 million year-old plants.