Bullet-shaped erratics

 

A roughly bullet-shaped erratic boulder on Alexander Island, Antarctica.  Photo by Andy Emery, published on the AntarcticGlaciers.org web site

The Newall Boulder, now in Salisbury Museum. (Acknowledgement:  BGS photo.)

This is quite fun -- a large erratic boulder in Antarctica with a shape that is very similar indeed to that of the famous "Newall erratic" much discussed on this blog and elsewhere.  There is a large specialist literature on the mechanisms that come into play in the shaping of subglacial and englacial erratics. I have done many posts on this.  Te see them, use the search box.

 I don't want to pretend that most glacial erratics are shaped like rough bullets, because erratics come in all shapes and sizes. Lithology initially (following entrainment) plays a large part in determining their shapes, as does distance of travel.  But what the literature suggests is that erratics slowly evolve towards an "ideal" shape in which we can recognize a blunt bullet-shaped and heavily abraded snub nose or upglacier end, a broader rough (and less heavily abraded) down-glacier end, and longer faceted flanks affected by streaming ice.  Here there may be striations or even pressure fractures on the flanks, although the number of facets may vary according to the rock's history,  texture and internal weaknesses.

See also:

https://brian-mountainman.blogspot.com/2022/06/sub-glacial-clast-modification.html

https://brian-mountainman.blogspot.com/2022/06/glacial-clast-morphology.html