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Thursday, 2 October 2014

Is Flat Holm a roche moutonnee?

Flat Holm as seen from Steep Holm -- looking north.  In this view the asymmetrical profile of the island is striking.  The Anglian Irish Sea Glacier travelled from left to right.

An oblique aerial photo of the island, looking from north towards south.  Here again the cross profile is obvious.  The steep cliffs are to the left on the down-glacier side, and the gently rising land surface is on the right, on the up-glacier side.

In this vertical image, the south is at the bottom and the north is at the top.  Once again we see the steep cliffs to the right, between Castle Rock and Lighthouse Point.  The west coast is very different, with a large area of reefs exposed at low tide and a very low cliff line.

This all looks pretty convincing.  However, we must be aware that there is some structural control here, because the most dominant structural feature on the island is a pitching anticline in the Carboniferous Limestone bedrock, which runs roughly NE-SW across the island.

7 comments:

Myris of Alexandria said...

Not such a silly idea.
M
The first photo especially.

Alex Gee said...

Brian
May I, as a fellow swivel eyed loon, remind you about my opinions about the Rouch Moutonnee form of the hills on the Somerset levels;on the south flank of the mendips?

I've now finished my survey of the csa of cave passages on the mendip hills and their relationship to catchment area.

Would you like me to email you the data?

Funny how the karst conduits to the west with much smaller catchment areas are far larger than those to the east with much larger catchment areas?.

I wonder where all that extra water required to erode the passages to
much larger dimensions than their catchment area would allow, came from?

The location of these hydrological anomalies coincides with that of the deeply incised tunnel valleys I proposed in a previous post!

BRIAN JOHN said...

Thanks Alex -- yes, I remember your photos. I was not convinced at the time! But one is always open to new possibilities...... by all means send the info through. I'll be interested to see it. And if you want to put more info on this blog, feel free...

Alex Gee said...

Brian:

A Query? And some hope!

I know that opponents of the glacial transport theory have stated that is impossible because no erratics have been found in their studies of river gravels in river beds draining Salisbury plain.

But I've recently read that most academics believe that river gravels exposed to study, were most probably deposited during/since the last glaciation. Gravels deposited during the Anglian are most probably quite rare and only still extant in very few locations!

Perhaps you could tell them this!


Alex Gee said...

I know you visit relatives in Nunney on occasion! Might I suggest that next time you visit, drive south on the M5 to Bridgwater, thence on the main road to Glastonbury,turning south east at Street(heading to yeovil) for 10 miles. Be very interested
in your opinion of the hills and landscape!

Phil Morgan said...

Hello Brian,
A little late getting to this topic but never mind.
A thought ---- If Flat Holm is a roche moutonnee and Steep Holm is not, then would this give some indication as to the limits of the ice sheet, for Steep Holm is only 3.81km (2.37 miles) south of Flat Holm?

BRIAN JOHN said...

Interesting thought, Phil. But Steep Holm also has an asymmetric cross profile, although it is a much steeper island. There's structural control here too, with the dipping limestone strata, and the island is more elongated than some of the photos indicate. Looks a bit streamlined to me -- and worth examining! Maybe we should think about a Steep Holm trip as well........