Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- due for publication on June 1st 2018. After that, it will be available by post and through good bookshops everywhere. Bad bookshops might not have it....
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Monday, 25 May 2015

Rock mechanics at Rhosyfelin

Here again is Adam Stanford's fantastic Gigapan image of the Rhosyfelin rock face, scree bank and "proto-orthostat."  It helps to give the context for the previous post.

If we look at the big stone, the right (fresh) end was originally at the bottom of the standing pinnacle, resting on a jagged fracture or set of fractures,  and the  left (weathered) end was the one at the top, exposed to the atmosphere.  The bottom (buried) face of the stone would originally have been positioned approximately along the rock face as we see it today, facing NW.

By going to the source of the image you can zoom in very closely and examine things in detail.  Thoroughly recommended!

ADDED 25 May;  Here is that rather interesting diagram, courtesy Phil Morgan, showing the two mechanisms of rockfalling that we should be thinking about.  Although the rockfall debris has accumulated in an almighty jumle of blocks, slabs and fragments apparently at random, there are some signs of order.  One of the most interesting is that the weathered -- top -- ends of the pillars and pinnacles that have fallen tend to be furthest away from the rock face.  That means that the lateral fractures that run along the face, and which are at right angles to the foliation planes and the foliation surface fractures, have acted as ledges or hinges when the slabs have toppled over and outwards.

The top part of the rock face appears to have evolved as in Fig 4. and the lower part of the face might have evolved by sliding, as in Fig 5.  In reality the whole scenario has been far more messy, since accumulated banks of rockfall debris have provided a rough surface which, when snow-covered, could have acted to assist in the movement of blocks well away from the rock face.  Rolling and sideways sliding are also feasible.

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