17th December 2006, Sunday
RIVER  AYR,  FAILFORD
Tour:   Self            
Weather:  Sunny and mild
Summary of Geology:

  

In Permian times, around 270 million years ago, all the major continents of the World became united as the single supercontinent of Pangaea.  Britain lay at the heart of this continent, at about 15 degrees north latitude, the same latitude as many of the deserts of the World today.  In this hot, arid environment, sands piled high into dunes, and were consolidated with time into sandstones.  These have now been largely removed by erosion, but are preserved in south-west Scotland on Arran, near Sanquhar and at Mauchline (cf. Bongate Scar and Hoff Quarry, studied during Durham field course).

A fine section through the dune-bedded red sandstones of the Mauchline Basin can be found in the River Ayr Gorge near Failford.  The characteristic cross-stratification may be due to changes in wind direction or overlapping of barchan (crescent-shaped) dunes and are particularly well exposed at Peden's Cove.  However formed, the observed result is frequent erosive truncation of lower bed units and deposition of further units on top.  This proves that the succession is the right way up; any apparent truncation of upper units by lower ones could only mean that the whole succession had been inverted - but that is unlikely in these relatively undeformed New Red Sandstones.

Close examination of the sandstones reveals expected similarities with desert sands being formed today - well sorted, well rounded grains, with pitted surfaces caused by multi-collisions in an air-borne environment.  Water-borne sands, in contrast, such as those typical of the Old Red Sandstone, tend to be less well rounded, but are smooth surfaced, due to the cushioning effect of the water.

The typical red colour of all terrestrial sandstones is due to a thin surface layer of haematite iron oxide (Fe2O3), which forms around the grains in the highly oxidising environment. The iron is derived ultimately from the chemical breakdown of even older rocks, such as basalts.   Despite their strong colour, the iron content of red sandstones is rarely more than about 1%.
The Permian rocks of the Mauchline Basin comprise lavas followed upwards by red sandstones, well displayed at Failford and Ballochmyle.
  Permian desert sands in the Ayr Gorge near Failford.  The brick-red sandstones readily take on a lime green algal surface cover, but large scale dune bedding can be detected.
  Cross-stratified sandstones at Peden's Cove.  (The steps just visible at left are thought to have been carved in the 17th century). 
  Brick-red cross-stratified sandstone, beautifully exposed at Peden's Cove.  
  Sand dunes build in the wind direction by movement of grains over the top of the dune and onto the top of the leeside, where they avalanche down and cross beds are formed in order 1 to 5.  Together, these form the lowest master bed.
     
  In the idealized situation, and if sediment supply is plentiful, a series of master beds may build up, one on top of another.  The orientation of cross beds within higher master beds may change due to a change of wind direction.  Cross beds always thin and tend to the horizontal at the bottom, i.e. they are asymptotic with the surface below,and are truncated by the next master bed above.
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