OGG member, Graham Cunningham’s personal account of a visit he made to Crackington Haven, Cornwall, UK.
We took a one-hour break during a stroll on the coast of North Cornwall last May. Crackington Haven is a delightful spot between Boscastle and Bude where the tea-shop provides blankets as well as cakes – both are life-savers. Now, since I see geology as essentially rather like beach-combing, I decided to take a look…
As you can see in the picture facing out-to-sea, which shows the southern side of the beach, the rocks are bedded, and they dip quite steeply to the right, which is North. They’re Upper Carboniferous black shales and greywackes – no Coal Measures here. The greywackes resist erosion better then the shales, so they form most of what you see. But what’s this…?
(Limpets for scale). These are flute-casts, where small pebbles have scoured out channels (narrowing to the top left, so the current ran in that direction), which are then filled-in by the next sediment to be laid down. But these flute-casts are upside-down! The pool, top left, gives an indication of where up is. The scoured shale (scoured in the Carboniferous, that is) has been eroded away (Well, not completely. The lower right of the slab is made of surviving shale, so the flute-casts are not exposed there), uncovering the later bed from beneath it. All these rocks are upside-down! We need to look around.
We’ll turn 180 degrees to face more-or-less East. It’s an iffy day, so the exposures at the back of the beach (the shore end of the big headland on the right in the air photo) are not draped with towels/tourists, so let’s have a look…
Ok, these are the same black shale and pale quartz-rich greywacke we saw before, but the bedding is more or less horizontal, with little faults running diagonally lower-left to upper-right. The fault across the centre is normal (down-to-the-left). You can see the ‘dragging’ of the greywacke bed at the lower left, and the same bed re-appears centre right, assuming that’s the same thin greywacke bed just below it. Slip on the fault is about a metre. Now, the top of the bed I just mentioned interests me – it seems to be overlain by sizeable pebbles, at least the bit on the lower left does.
Here it is, a few metres to the right. Well, they’re not pebbles. You see the concentric growth on the two nice big ones on the right? They appear to be manganese nodules. We’re in very deep water here, with reducing conditions encouraging the nodules to grow, slow deposition of fine mud (the black shale) and occasional whooshes of clastic-sediment-transporting turbidity currents to form the sandy greywackes. So there’s a source of coarser sediment not too far away. See how the nodules sink slightly into the greywacke, distorting the bedding?
Above the nodules there’s the beginnings of another greywacke, but it quickly grades upwards from sandy white to black shale. Graded bedding is an excellent way-up indicator, with finer sediment always increasing upwards. And do you see the squiggly white structures dropping into the spaces between the nodules, like servings of Mr Whippy ice-cream? These are called flame structures, and they develop while the sediments are still soft and behave as fluids. A density inversion occurs when the upper, newly-arrived and denser, layer tries to sink into the lower one, in this case the mud between the nodules.
So three things – the sinking nodules, graded bedding and the flame structures – show us these beds are clearly the right way up.
Further right again, and we can see some folding. There are three little anticlines, best one in the middle, which are asymmetric with a steep right-hand limb and gentle left-hand. Two things suggest they are not formed by soft-sediment slumping. First, the nodules ‘track’ round the fold in the centre – one has been turned to the vertical above the right limb of this fold. Now nodules need time to develop, but slumping should have happened quite quickly after the greywacke was deposited – contradiction.
Also, I think I can see a weak fracture-cleavage dipping steeply left and maybe best-developed in the axis of this central fold, but visible elsewhere across the same bed. It may be a bit clearer if you look at the two limbs of the folds and try to imagine the angle-bisector between them. It won’t be vertical, because the folds are asymmetrical. The cleavage is an axial-plane cleavage, parallel to this direction. Ignore for the moment the quartz-veins.
Now, if these folds are tectonic, their asymmetry suggests they may be minor folds on the limb of a larger one. They can be said to ‘face’ right, because their steep limbs are to the right, and the axial-plane cleavage ‘leans’ that way. So there would be an anticline to the right.
Slightly fraudulent, that, since all it took was a step or two backwards. And here’s the larger fold. The right limb is overturned, so the teacup scale rests on the bottom of the bed, and the thin greywacke is no longer underneath, but on top of it. I reckon you can see at least one mirror-image of the minor folds just under the tea-cup, and the facing is also upside-down and towards the axis of the fold. Turn your computer over and see if you agree.
The shale inside the fold shows nice colour-change bedding layers, but also, perpendicular to bedding at the fold axis, a well-marked axial-plane cleavage. Shales do tend to develop a cleavage much more clearly than coarser-grained greywackes. By the way, some but not all quartz veins agree with the cleavage to some extent, again particularly close to the fold axis, slightly above and right of centre. Quartz veins can be useful sometimes, but they tend to do what’s easy, for example following a pre-existing cleavage, so they need to be treated with care.
Of course all this tectonic stuff happened later than those sedimentary structures, but you can see how useful the latter are, quite apart from telling us a lot about the sedimentary environment in which the beds were formed, in showing us which way is up! This coast is absolutely classic as a place to find these kind of relationships.
Another step back, and a sip of tea, and here’s a fault decapitating the fold, on the right. Matching across the fault is quite easy here, with the fold as a guide. Slip again about a metre.
At the bottom centre, underneath the next greywacke up the sequence, you can see directional bottom-structures a bit like the flute casts we saw earlier, and these are repeated across the fault by that little grey boulder on the right.
So the situation we saw at first on the beach is clear. The upside-down beds over there form the overturned limb of a fold like this, the horizontal limb having been eroded off. You can see that since we have only seen two bedding orientations, one horizontal and one steeply-overturned, the ‘style’ of this folding is zig-zag, with only the hinges at all rounded. This style can be found all along this coast at places like Millook, Bude and Boscastle.
So there we have it – Devonian and Carboniferous pulling-apart of continental crust to form a deep sedimentary basin (this one known as the Culm basin), filling (with interesting sedimentary structures) then closure ( marking the final assembly of the supercontinent Pangaea), resulting in folding, and finally faulting. The Variscan orogeny in a few metres of beach.