Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click

Tuesday 21 June 2022

Stockholm Archipelago glacial clasts


My art installation consisting of 11 clasts of diminishing size.  These were picked up at random from washed till on the foreshore of Blido.  They are all made of PreCambrian basement rock -- granite, gneiss, basalt, volcanic ash etc.  Only two or three of these rocks are bullet shaped.

You can pluck glacial erratic clasts out of till exposures and washed till all over the place in the islands of the Stockholm Archipelago -- they are generally clean and easy to find becuase the finer materials have been removed by "washing" as the land has risen from the sea during the process of isostatic recovery.  A. couple of things are noteworthy:

1.  The clasts do NOT carry traces of ancient weathered crusts or surfaces; all the facets and edges are equally unweathered, although some parts of the clasts have been stained or modified / abraded / fractured more recently than others.

2.  Rough blocks and slabs predominate, with multiple facets.  Maybe about ten percent of clasts have a bullet shape.

Some clasts have nine or ten facets -- some abraded and others clearly originating as fracture scars.  Some scars are old and some are young; some of the older ones are modified by later abrasion and by smaller and later fractures.  Almost all combinastions are possible.

Variations on a theme.  Three small clasts collected from a till exposure on Blido.  From the left: white granite, greenish volcanic ash and pink granite.  All have at least six facets.  All three have quite fresh fracturs scars on their lee (blunt) ends, and all have heavily abraded edges.  Interestingly, all three have lost their bullet tips (stoss ends) through fracturing.  A point is always vulnerable.

The greenish ash clast, showing a big fracture scar on the lee end and another smaller pressure fracture scar.  Subsequent abrasion has modified the sharpness of these features.

Striations on the same clast -- a deep one sub-parallel to the long axis but others running across it -- showing that at some stage the long axis has been at 90 degrees to the direction of ice flow and clast transport.

No comments: