The Brian John Boulder again

Photo: Dr Brian John

 We must thank Brian John for bringing this clast from Craig Rhos-y-felin to our attention:

In the words of 11 glacial geomorphologist:

1. If I had been doing a Reichelt shape classification I would have classed this as sub angular and typical of igneous clasts that have been dragged along the glacier bed.

2. I would say it has been glacially transported. That could account for the relatively smooth (abraded) surface and the other sub-angular surface and edges. There could even be some grooves on the abraded surface (upper image).

3. I would not be surprised if it is glacially transported. It looks to be the result of physical processes rather than chemkcal weathering. The light parts look hard and fresh and one side is flat. Corners and edges are neither sharp nor rounded. I could have picked it from a till in Bergen.

4. I am a bit less convinced. Looking at the right side of the first image the facets might be where cooling joints meet. That end is almost hexagonal. It is quite bullet-shaped though.

5. I would agree with my esteemed colleagues and say that there is certainly evidence for a possible faceted surface but difficult to say much more based on the photos.

6. In addition to the facets and chip marks that jumped out at first glance, the lower image to me has a slight bullet shape to it. Nothing definite from photos alone, and perhaps especially not from these two angles, but my very careful guess would agree with a subglacial transport path. Striae rarely preserve well (and on many lithologies don’t even form). Having said that, the clasts seem pretty weathered and battered.

7. Although quite hard to get a complete picture from just these two images, I’d say they seem to show a subrounded cobble/small boulder that is faceted, and has a shape that some people might say approaches a bullet-shape. I can also see some – what look like - chipmarks on some of its edges, the arrangement of which could indicate a responsible force from a single direction. From behind my wall of disclaimers and from within my cloud of speculation, I would probably guess that this boulder was subglacially transported. Striations on the faces would perhaps clinch it for me, but I could not see those in the images.

8. It's not possible to be definitive on the basis of these pictures alone. However, the presence of planar facets is consistent with subglacial transport. It would help if there were additional characters that might corroborate this, such as a stops-lee or double stoss-lee form. I guess there are no striations, or you would have mentioned them. Also rhyolite doesn't tend to striate.

9. My guess would be glacial. Not overly far travelled I’d say, but there does appear to be edge rounding and also chipping, with potential flat-iron faces. Looks like a lot of igneous clasts in tills in the north of Ireland.

10. I agree that this could be interpreted as subglacially transported boulder. Some rounding of the corners, but the facetted surface is not the best I have seen…if it has striations I would of course be 100% convinced.

11. Looks like a fluvioglacial clast. Definitely been in a fluvial system but only for short time as the degree of rounding is limited. The pic maybe misleading but I can see parallel lines -? Striations.

12. It looks partially faceted, edge rounded and abraded. The surfaces even appear to have some crude chattermarks/flip-outs. I cannot see any definitive striations but the lower image has an interesting set of linear marks that warrant a better image, though they may well be structural. I would say definitely glacially transported.

Sorry, the picture is of a clast at Craig Rhos-y-felin which hasn't been transported anywhere, the description is of the Newall Boulder. This is the "Brian John Boulder " described here:

Comparative Analysis of the "Brian John Boulder" at Craig Rhos-y-felin and the "Newall Boulder" from Stonehenge: Implications for the Origins and Transport of the Bluestones- a paper - DOI: 10.13140/RG.2.2.28445.01769

Here it is compared to the Newall Boulder, found at Stonehenge, also from Craig Rhos-y-felin having been manuported there by the neolithic builders  of the monument.

 The shaping of the Newall boulder has nothing to do with an imaginary ice flow.