22nd August 2007

HEADS OF AYR   

Access: Shore walk from Doonfoot.

Tour: self

Weather: A very fine, and long overdue, summer's day.

Summary:

A Lower Carboniferous vent at Heads of Ayr intrudes slightly older Carboniferous sediments.  Most of the material associated with the vent represents pyroclastic flow deposits, relatively soft bedded tuffs and agglomerates, which have been deeply eroded, so much so that much of the wavecut platform in front has been carved out of the same materials. The tuffs contain frequent fragments stripped from local rocks by the volcanic activity, including peridotites of deep-seated origin and the Carboniferous (cementstone) sediments.

A prominent northwest-southeast trending Tertiary dyke cuts sediments in the wavecut platform and tuffs of the eastern vent.

  The Heads of Ayr volcanic vent complex. The western vent at the right is separated by a central recess from the nearer eastern vent. Crag and tail feature suggest glacial ice movement from north to south (from right to left). Cementstone group sediments in the foreground predate the igneous activity and are gently dipping towards the viewer. They dip more steeply and become contorted near the vent margins.

  A typical tuff (volcanic ash) deposit from the eastern vent.  Fragments are of both igneous origin (carbonated peridotite at top), and of sedimentary origin (layered cementstone at bottom left).  Elsewhere, fossilized wood fragments can be found.

 

 
  This large composite dyke of Tertiary age dips steeply towards the north-east and cuts the cementstone strata in the foreground and tuffs in the cliff section. Most of the wavecut platform below the cliffs is built of deeply-eroded tuffs and agglomerates.
  The western margin of the Heads of Ayr vent complex.  A variety of structures can be found in the tuffs in this area, from cross-stratification to bomb sag.  Fragments in the tuffs include O.R.S. lava, cementstones, fossilised wood and carbonated peridotite.
  The promontory at Greenan Castle, approximately 2 km east of Heads of Ayr (visible in the distance) is built of well bedded grey-green tuff which is similar to that of the Heads of Ayr vent, which was the probable source.
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