Sunday, 8 December 2013

Erosion at Sanna

With the wind firmly set in the southwest, bringing low clouds and a thin drizzle off the Atlantic, we decided to walk along the shore at Sanna this morning, starting at the southern end of the beach....

....which gives views across the bay to Portuairk.

Sanna is famous for its sands.  We had always assumed that the sand was the product of the erosion of the underlying rock, a dark igneous rock called gabbro, but it isn't.  The sand is almost entirely formed of smashed fragments of sea shells.

These shell fragments are washed up by the waves to form the beach, but the wind then takes over and blows them beyond the beach, to form some dune fields which, if they weren't covered in grass, would not look out of place in the Sahara.  Beyond them is an area of flat land, where the car park stands, which is also underlain by shell sand.  But there is every indication that these constructive processes are no longer active.

This area of calcerous but fertile soil, upon which the dominant vegetation is a coarse marram grass, is called machair in Gaelic.  This rare machair habitat is only found on the west coast of Scotland, most commonly on some of the outer isles, such as the Uists, Barra and Tiree.  There's more about the machair here.

Machair promises to become even rarer.  Each time we have a gale, the seas erode the dunes at the back of the beach, and there is no sign of the sand removed from here being deposited elsewhere, nor that the sand is being blown inland as it once must have been.

Perhaps it is the result of rising sea levels, though the land in this part of Scotland is also rising in isostatic response to the removal of the glaciers that once weighed it down.  Perhaps the dominant winds are changing, so there is less of the constant onshore westerlies that must have built the machair. Perhaps there are other reasons, but Sanna beach is changing.

No comments:

Post a Comment

Post a Comment