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Unless otherwise stated, this image is the copyright of the submitter. Contact them for permission to reproduce it. |  | | Description | Grid ref: SH6524 3267
Can anyone provide explanation for impression in rock seen during walk in Northern Rhinogs? There is no sign of quarrying activity in immediate vicinity. |
| Posted Comments: Andy B
(2011-08-11) | Can anyone help with this question? | TimPrevett
(2011-08-11) | Is this volcanic or metamorphic rock? Curious if it's a pocket of air during solidification. | pab
(2011-08-13) | I see that it is only 2 miles from some markings I had come across in Aug 2008, which were acknowledged to be (probably) ancient and not naturally caused. Perhaps an email to the contributor at that time might be useful:
http://www.megalithic.co.uk/article.php?sid=20201
| Talsarnau Times
(2011-12-05) | I have received the following explanation for the markings, for which I am most grateful.
"The Rhinogs are the home of the "Rhinog Grits", Lower Cambrian,
coarse-grained sandstones which are well shown in the wide-
angle photo.
Sandstones are prone to developing "concretions", patches which are heavily cemented together by (normally) calcite or dolomite.
Because the process is controlled by diffusion through the rock, the
patches are usually spherical to ovate, having started at some nucleus. Typically that sort of thing happens fairly early in the rock's history, during burial and compaction.
Bearing in mind that these are 600-500 million years old, burial cycles have probably been repeated, and they have certainly taken part in the
Caledonian Orogeny, a mountain building process that caused rocks to be
forced downwards as much as upwards during continental collision, and
probably a previous event which is recorded as pre-Caledonian
collision on Anglesey.
My guess is that under extreme conditions, the carbonates cementing the rock have fled and been replaced by a pervasive silica cement
developed from within the sandstones - BUT the textures have endured,
including the concretion and the very obvious bedding of the sandy layers,
so well displayed in the picture.
No, not man-made in my opinion, and a most interesting curiosity.
Plus an evocative location for geology in the Harlech Dome."
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