Huntley Quarry and Lydney

Introductory Day March 08 2008

Meeting at Huntley Garden Centre at 10:00 a.m. on a beautiful but slightly overcast March morning, we all gathered for a briefing of the day ahead. Dave Green, our leader for the day, explained that we were standing right in front of the Blaisdon fault. To our west, the Triassic Mercia mudstone and Sherwood sandstone plains stretch out across Gloucester and beyond; to our east (hidden by dense forest), the Blaisdon fault and a succession of dips, folds and faults.

We set off at a hearty pace, only to stop after a very short walk at location 1, where we examined the Bromsgrove Sandstone formation (fig.1). It is red in colour - a good indicator of dry conditions. There may have been dunes present at some stage.

Location 2 was a good walk through the forest and this is where we examined pebbly sandstone beds from the early Silurian Huntley Hill formation. These beds are sandstone with quartz and feldspar. The quartz grains are quite well-rounded but the feldspar gave us a first look at angular grains. Dave explained that these beds had been laid down by water (I must admit that Dave did say why, but I was enjoying myself so much with my hand lens that I forgot to listen). Again, these beds are red but now contain unoxidized grey patches. Interestingly, marine fauna (brachiopods) have been found in them.

At the quarry location we had the opportunity to see geology at its best. This is a site of many geological events. In fig.2 our trusty leader is pointing out the plunging syncline with his hammer (not that you can see the hammer!) The Blaisdon fault was first a reverse fault, then a normal fault, creating a beautiful syncline and bed folding.

The bedding planes in fig. 3 clearly show the extent of dip to the west. Also note the beds (towards the top right) that have completely folded. To the right of the plunging syncline limb is the actual site of the Blaisdon fault. Sadly, this was somewhat covered with loose sediment, but clear enough to make out. The rocks in this area range from 430 Ma (Huntley Hill beds) to 200 Ma (the Mercia mudstone beds). This is truly a terrific site to see such folding and faulting.

This small area of such outstanding geological importance and beauty (approximately 15 x 25 metres) has seen some fantastically violent times and yet still remains to show what our beautiful planet has made for us. At this point, the heavens opened and the hailstones pounded us; but true to form, Dave continued the story through time. My notebook became saturated, so any notes thereafter were soggy and illegible!

After a leisurely lunch (some at the café and some at the pub) we met at Lydney Harbour. "Not much to look at," I thought, but how wrong I was! Our first after-lunch location was Red sandstone bedding (fig 4). This bed was composed of very fine grains and clay materials. We did an acid test to discover whether there is calcite present. Results varied, so I must record an open verdict. The bed was mainly red (iron) but had many chloride lines and reduction spots cutting through. Dave demonstrated how to determine how soft (or hard) a compound is, which would reveal what mineral(s) make up that bed. The beds at this location were laid in a calm and gentle environment some 400 Ma (Upper Silurian). Again the red colouring of the beds suggests a desert environment.

The next location was very exciting. We were shown almost vertical beds of Bishops Frome Limestone (fig. 5), previously known as Psammosteus Limestone. This is a very important bedding plane and runs from Pembrokeshire, through Gloucestershire to Shropshire. The white markings are calcite burrows and if you look very closely you can see the pseudo-synclines and pseudo-anticlines!

Moving along to our last location, we noted more folding and faulting with a plunging anticline on the water’s edge. Dave then showed us another exciting bedding plane, that of convolute bedding. This is a quicksand bed. If you look really closely (Photo on page 1) you will see where the water has tried to escape as the bed was being compressed. This was a fantastically educational day and such good fun. Dave’s relentless enthusiasm made ‘learning’ easy. A huge thank you to Dave, for his knowledge, and to Jan for organizing such a great day out.

Tracy Fitzgerald