Crackington Haven sole structures 1

These elongated ridges are seen on the inverted bedding surface of a turbidite sandstone bed (Crackington Formation, upper Carboniferous) on the south side of Crackington Haven, north Cornwall. The scale can be deduced from the limpets - the largest is about 4 cm across.

The ridges are casts of sandstone infill into what was originally a hollow groove in the underlying sediment. The grooves were formed at the time of sediment deposition by fast-flowing water making small erosion scours around irregularities on the Carboniferous sea bed. Sometimes the current flow was sufficiently powerful to drag along pebbles and small boulders, which created long linear grooves.

Here, the rocks have been turned almost completely upside-down by intense folding at the end of the Carboniferous, so the grooves now appear as ridges. These sedimentary structures are generally known as 'sole structures' and are valuable as 'way-up' indicators - we can tell whether the rocks are the right way up or have been inverted, as is the case here.

Also in this example, the scours are asymmetrical - deeper at the upstream end (lower right) and shallowing off at the downstream end (upper left), so they also provide evidence for the water current direction at the time of deposition. So in this case, the water flowed from lower right to upper left. But the rocks have been inverted by folding, so.....

Here, the rocks are dipping (sloping) to the north (right) at about 30°, having been completely overturned by the folding. To restore this slab into its pre-folding position we need to imagine it 'flipped over', by rotating it through approx. 150° in a clockwise sense, like closing the left-hand side of an open book.

Once we do this, the sole structure ridges on the upper surface of the slab become protrusions on the underside of the slab, with the asymmetric deeper upstream side to the left (south) and the shallower downstream side to the right (north). Therefore, the original current direction which transported the sediment of the Crackington Formation was from south to north: the source of the sediment lay to the south, and was transported northwards by the water currents.

NB: The information board by the public toilets at Crackington Haven is excellent, except for where it states that the Crackington Formation sediment derivation was from the north. Sorry, but they've got that bit wrong! The evidence is there on the beach for all to see.