4th July 2006
GIRVAN
Tour:  self            
Weather: fine and sunny.

Summary of Visit: 

A visit was made to examine the Ordovician and Silurian rocks of the foreshore just south of Girvan.  A range of interesting sedimentary structures was found in the Ardwell Flags, at Port Cardloch and Whitehouse shore, as shown below.  All the beds were folded into a near vertical position during the Caledonian Orogeny.

( Toe of boot, where present, indicates scale).

 
A typical series of alternating dark grey, hard sandy bands and lighter coloured soft muddy bands in the Ardwell Flags. The beds are close to vertical. Match up of beds across the fault is not obvious, but turn of the beds close to the fault plane indicates movement to be dextral (to the right as viewed from opposite side).
  A fine example of beds of alternating hardness - two prominent hard bands at top and bottom are separated by a soft muddy layer. A vein of white calcite or quartz  intruded the beds before lithification - it passes straight through the competent sandy hard band, but was squeezed out during lithification and compression of the incompetent muddy layer.
  Another fine example of vein behaviour in incompetent muddy layers - the white vein has been stretched into a series of thin zigzags.
  This is believed to be an example of flute marks.  The lower muddy layer was laid down first, and was followed by the upper sandy layer before the mud had hardened. This caused eddies to form on the sea bed, which become shallower and wider down-current.  The current movement was therefore from right to left as seen here.
  Graded bedding in near vertical sandstone units.  The unit at centre right can be seen to grade from coarse to fine from right to left, so the succession youngs in that direction, and is therefore slightly overturned.
  Detail of the graded bed described above, shown in the same orientation.  When a pulse of material of various grades falls to the sea floor, the coarsest material will settle first followed progressively by finer material, so the bed youngs upwards (from right to left as seen here).
  A series of turbidite units, each having a sharp base (at left) and fining progressively upwards (towards the right).  Recent erosion carves out the soft, bluish muddy layers more easily than the coarser, sandy layers.
  When successive beds have a high competence contrast, as between these dark sandy layers and the intervening muddy layers, deformation during lithification may cause the the competent layers to stretch out, or separate into lozenge shaped pieces known as boudins    (french = sausage).
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