Dry Sandford Pit Geology Walk

On Sunday 3 October, our group of eight was blessed with a warm, dry and sunny morning following the deluge of the previous day. The paths were, however, quite dry – perhaps there’s a clue in the name!  We were under the expert guidance of Lesley Dunlop, who introduced us to the wonders of the Corallian succession in this former sand quarry, which is a SSSI and BBOWT reserve.

The several exposures we examined in the quarry face were composed of interbedded limestones, sandstones and marls (a limey clay).  We learned that these were deposited in conditions similar to those on the Bahamas Banks today, ie. a high energy environment with shallow off-shore islands in a shallow warm sea.

Interpretive noticeboard.

An information board gave an idea of what lived in the sea at the time – ammonites, coral reefs, sea urchins, and a particularly strong shelled bi-valve, Trigonia, which was able to withstand the buffeting of the waves. Is consequently well preserved and gives its name to the Upper and Lower Trigonia Rock beds which are of such importance that the quarry has the status as a geological SSSI.

Examining the strata.

The limestones were laid down slowly in relatively undisturbed off-shore waters whereas the sandstones were deposited in shallower conditions and over a shorter period of time. We were shown some metre or so wide rounded iron hardened blocks of sandstone called Doggers which had been left by the quarrymen. Several layers were composed almost entirely of fossils which had been sifted together by currents after they had died. Such layers, not unsurprisingly, are called ‘death assemblages’. Jonathan was happy to climb up and demonstrate some of the most noteworthy details of this rock face.

Testing the pH.
Testing the pH of one of the ponds.

Intriguing pH tests were carried out on the water of two small ponds. Both results – 5.78 and 5.4 – showed more acidity than expected in a limestone rich area. This, we concluded, was because the water in these ponds had its origin in the sandstone layers.

All in all a fascinating morning’s excursion.

John Baker