by Ian Donaldson

Squinting through a hoarding around a building site on King's Square Avenue at Stokes Croft recently - I love watching other people at work - I noticed a pile of blue-grey stones along the edge.  Could this be the local blue Lias?  Just then a workman passed, so I asked him about the rocks. - No, he replied, they hadn't been uncovered at the site, in fact they had put down piles to a depth of seven metres and all they'd found was red sandstone. Nothing green, nothing blue or grey.

This bedrock therefore is presumably the "New Red Sandstone" of Triassic age, so called by early geologists to distinguish it from the Old Red Sandstone of Devonian age.

I was astonished to find this formation under my own house on the Redland-Bishopston border some years ago during excavations to shore up the bay windows. Imagine! Here in Bristol was a rock I'd first encountered as a student in Belfast, where it is part of the strata underlying the tertiary basalts of the Black Mountain. It was called Keuper Marl in those days, (perhaps it still is?) and with it were the tea-green marls and Bunter Beds. And here in Bristol was the same tea-green colouration in places, representing reduction of the iron in wetter conditions than the oxidising hot dry desert conditions that formed the Keuper.   And that meant that this bed extended all the way from Belfast, under the Irish sea to Bristol! I've since learnt that the same bed extends far into Europe. And in a desert you sometimes find salt lakes, which when they dry up form  evaporate deposits of salt, gypsum and alabaster. Hence the salt works in Carrickfergus, the large and as yet unexploited salt deposit under Brent Knoll, and the minerals which remain after the salt has been washed away, which can be seen so well at Aust Cliff. There, if you know just where to look, you can find beautiful salt (halite) pseudomorphs. Just ask, and I'll show you.