Erratic mysteries

 Erratic boulder, Blido, Sweden (click to enlarge)

I'm often asked why, if the bluestones at Stonehenge are glacial erratics, they aren't covered with scratches (striations) and glacial facets which would demonstrate for all to see that they have been transported in or on glacier ice.  Partly, that's because dolerite does not striate very easily -- it's very coarse, and because the component crystals are quite large, when glacial abrasion occurs on a dolerite surface the rock crumbles rather than becoming smooth.  On the other hand rhyolite does smooth reasonably well, and I have seen striations on some ice-smoothed surfaces, on Carn Alw for example.   But there is not a lot of rhyolite exposed above the surface at Stonehenge.  Some sandstones can smooth and accept striations -- maybe the two sandstone stumps in the bluestone circle may give some clues as and when anybody gets to look at them.  Ashes are sometimes too soft -- but given the right circumstances maybe we could find glacial scratches on the smoothed faces of some Stonehenge monoliths.

To give us some idea how complex are the effects of glacial transport on contained erratics, have a look at the boulder above.  We dug it up last year from the thick till near our summer cottage in Sweden.  This boulder has at least nine facets on it -- the seven that I have labelled, and two that we can't see.  Number 1 probably shows the original shear face -- probably this boulder was sheared off the bedrock roughly along a weak contact between pink rock and bluish-grey rock.  (I hesitate to give them names -- the pink rock is thought to have been Pre-Cambrian granite originally, but there has been tremendous metamorphism since it was originally emplaced.  The bluish-grey rock is altered too.)  You can click to seee the other facets more clearly -- they are all sheared or fractured faces of different ages.  It's difficult to give them relative dates.  Note that some are VERY rough, while others have been subject to varying degrees of abrasion and smoothing.  The smoothest facet of all is Number 6 -- beautifully abraded, with very clear striations and also chatter-marks or pressure fractures created by tools being forced down onto the rock surface.  If a tool is very big and the pressure exerted is very large too, then the whole boulder will be split or fractured, and thence reduced in size.  And we will have another facet on the boulder.  If this process goes on for long enough, the boulder is reduced to a collection of smallish bits and pieces, gravel and sand.  If the boulder is protected, it can be carried for hundreds of kilometres without further modification. 

This is typical of what happens to a boulder being carried at the base of a glacier.  Boulders carried englacially or supraglacially are affected by a different set of processes.