21st July 2007

SICCAR POINT   

Tour: Glasgow University Geological Society. 

Participants: about 25

Leader: Con Gillen - the author of an excellent guide to Scottish geology "Geology and Landscapes of Scotland" (ISBN: 1-903544-09-2). This was our first geology trip together since Finland in 1970!

Weather: Light rain at first, becoming very heavy. Very cool and breezy.

Summary:

Siccar Point, on the Berwickshire coast, is one of the most important geological sites in the World.

It was here, in 1788, that James Hutton (1726-1797) saw gently dipping red sandstones lying unconformably on top of tightly folded, almost vertical older strata and recognised the sequence of events that had taken place. He recognised the immense periods of time involved..............

"......No vestige of a beginning. No prospect of an end".

A year earlier, he had discovered his first unconformity, at Newton Point on the Isle of Arran, and this is described in one of my field reports.

Today, we have a fuller understanding than was possible in Hutton's time of the sequence of events represented at Siccar Point:

Deposition of sediments on the sea bed during Silurian times, about 400 million years ago. Occasional graptolite fossils prove the marine origin.
These beds were compacted, folded and lightly metamorphosed during the Caledonian Orogeny, as the continent of Avalonia to the south collided with the continent of Laurentia.
The folded, near-vertical strata were eroded down to a nearly flat, horizontal surface on which was deposited red sandstones in a river or lake environment during Devonian times, about 345 million years ago. The unconformity therefore represents a period of some 55 million years. The sandstones contain many poorly rounded pebbles of greywacke, eroded from the Silurian rocks beneath.
The sandstones were, in turn, consolidated with the passage of time and gently tilted - perhaps during the Variscan Orogeny or as a result of the upheavals in the west of the country in Tertiary times - and recent erosion has revealed the unconformity as we see it today.

 
 

 

The Siccar Point unconformity. Gently dipping Devonian red sandstones lying unconformably on steeply dipping Silurian greywackes.

 

 

Another view of the unconformity.

 

 
  A close up view of the unconformity.
  TOP NEXT FIELD TRIP >