Desert Turns the Land Red
From 415 to about 360 million years ago, Scotland’s climate was hot and semi-arid. Vegetation was sparse. The landscape was a desert like the Sahara of today, cut through by rivers that rose and fell rapidly. These rivers had wide floodplains and associated ephemeral lake basins. Sand dunes were kept on the move by fierce desert winds. The distinctive red and yellow coloration of these rocks resulted from the harsh desert conditions.
The scale of ancient rivers that traversed the desert plains is indicated by the former river channel seen in the A9 Crossgates road cutting west of Perth. Here, the internal bedding structure of the sandstones shows that the river channel must have been in excess of 15m deep which would compare well with that of the modern Mississippi. The Crossgates River would have dwarfed the present day Tay and its catastrophic floods of 1990 and 1993. However, the braided island sections of the Tay above Perth at Scone would, vegetation aside, be a reasonable model for the ancient regime, albeit on a smaller scale.
Some of the oldest of these rocks originated as river shingle and related deposits. The conglomerates are particularly resistant to weathering and erosion, and form hills such as at Knock of Crieff, Craigowl north of Dundee and Hill of Finavon by Forfar. The Sphinx at Arbroath is composed of gravels and sands laid down in these ancient environments. The Bannet Stane, northeast of West Lomond Hill, and John Knox’s Pulpit are composed of sand, blown by wind into ancient desert dunes. The sculpting of the Bannet Stane is partly the result of modern sand blasting by winds.