Basic Geology 2007

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Basic Geology Day 2007 took place on Sunday 18 March. The English weather was at its best for the occasion:

It was a great day out and thanks go to Cliff for getting through a difficult day.

Photos both by Don Cameron ©.

Richard Thomas

We met up on a cold and breezy morning in Millersdale for the Basic Geology field trip, planning to visit a series of cuttings, quarries and exposures in the c350 Ma old Carboniferous limestone along an abandoned railway line. After a short introduction from our leader for the day Cliff Bartlett, and a look at basic bedding structures in a quarried area near the cap park we set off along the trail in flurries of snow and hail (a taste of things to come).

We spent a while examining an abandoned lime kiln that had been built on a lava base, an example of how local industry made use of the local geology. On a track away from the railway line we found an assemblage of semi-intact crinoids on a step cut into the limestone Photograph 1.

Back in the railway cuttings we compared the bedding in different localities and searched for fossils; we found more crinoid fragments, numerous brachiopods (in beds and individually) a few in life position, also several solitary corals found. In one section there was a band of styolites, vertical sutures formed by pressure dissolution during deep burial.

Unfortunately the conditions started to deteriorate with the snow/hail becoming heavier, making it harder to spot the details in the rocks, Photograph 2 but we pressed on to a change in the lithology where a submarine lava flow was exposed showing pillow lava and a debris conglomerate that formed at toe end of the flow. It was pointed out that there were mineral veins formed during hydrothermal circulation driven by the heat from this and other volcanic eruptions and intrusions in the region, sadly they were not observed due to the conditions.

As the conditions began to resemble a blizzard it was decided it would be best to retreat to our cars for lunch, while we walked back the sun came out for a while giving views of the way changes in lithology effect the slope/shape of hills in the area.

After lunch Cliff concluded the day by discussing and demonstrating the use of the compass-clinometer on car park wall coping stones.

Thanks to Cliff for leading this trip and for the useful handouts provided, also to those who undertook the reconnaissance trip to ferret out the best locations to ensure that the day went well despite the weather.

Valerie Astill

A group of us met Cliff and his wife Brenda at 10.15am in the Millers Dale car park where he provided some excellent handouts before we started our walk at 10.30am. The idea was that we should make notes on the handouts during the walk but the weather was atrocious with freezing winds and sleet and most of us protected our precious handouts in our rucksacks.

Before we started off down the dale, Cliff asked us to observe the rocks from a distance. A long distance reading of the rocks above a quarry on the opposite side of the valley indicated that they were dipping at about 4˚ towards the west (although we didn’t know the direction at the time) while rocks on our side of the valley were dipping about 6˚ in the same general direction.  The deep rock beds were divided by horizontal cracks indicating unconformities. The cracks were not level, indicating that erosion had taken place between deposition periods. These limestones had different names, one of which was Millers Dale Limestone (but I didn’t make a note of the others).

At the start of our walk, a wall had been built beside the path consisting of blocks of a more yellow coloured rock.  Cliff scratched this with his penknife, showing that the rock was quite soft and after inspecting it through our hand lenses; we decided that it was probably constructed of sandstone.  On the side of one of the blocks, parallel lines were engraved, the upper ones dipping to the right, the lower ones dipping to the left.  It was explained that these were ripple marks from a tidal estuary, formed as the tide came in and out.

The undergrowth on either side of the path had been cut back so we were able to walk up to the rock face on either side.  Cliff used the acid test to show that the rock consisted of calcium carbonate or limestone. The rock was quite pale in colour showing little evidence of fossils and is apparently quite pure in this area.

Further down the path, we turned off and climbed some steps on the right which brought us to the remains of a lime kiln.  The kilns were once used to produce quicklime for agriculture but the limestone overlies a bed of lava which, over time, weathers to clay.  This lava bed became unstable and the rock face started slipping forward so the kilns were closed for safety reasons in 1930.

Cliff explained that we would see some pillow lava further down the path, showing that the lava had been produced under water.  He told us that this area was probably a small back basin where the sea floor had stretched and thinned and lava had erupted creating a small spreading zone. After the limestone had been deposited, the heat from the underlying lava had created water currents carrying dissolved minerals up through cracks in the limestone.

Further down the path, we saw the first evidence of shell fossils in the rocks.  We turned off to the right again at Millers Dale Quarry.  We climbed the steps and near the top, Cliff washed and brushed down one of the steps to show us a lovely exposure of crinoids. Photograph 1 These would have grown in warm, shallow waters containing plenty of oxygen, probably on a reef. 

Cliff had given us a map which showed we were in what had been a shallow lagoon surrounded by reefs.  Within this lagoon were small deeper basins.  The crinoid stems had not been broken up and it was surmised that there had been a storm which had washed the crinoids down into one of these deeper basins where they rapidly became covered in sediment, protecting them as they fossilised.  

We continued down the Dale, finding it increasingly easy to spot the edges of the shells of bivalve brachiopods, which appeared as curves standing slightly proud of the surrounding limestone which dissolves more quickly in the rain.  These shells mostly curved upwards in the same orientation although we later saw some on their sides or curving downwards where they had probably been washed down into deeper water and stuck at an angle in the mud.  Cliff also pointed out some pieces of crinoid and a single coral. We were later told that each time we go down the Dale looking for fossils, we will find many more, as we come to know what to look for.

In this area, beds were dipping in the direction we were walking, approximately towards the east, and appeared to dip a little more steeply further down the dale.  Cliff later explained that we were on a dome of limestone surrounded by younger rock and the direction (and angle) of dip changed as the direction of strike changed around the edge of the dome.

In the rocks we could occasionally see smooth-sided semi-circular vertical tubes about two feet long and we were told that these were drill holes where explosives had been inserted to blow the rock apart. Some vertical lines, very close together in the lower rocks, were pointed out to us as styolites. These are pressure marks from the weight of the bedding above, formed while the lower rock was still wet and not completely lithified.  Nearby some of the upper beds of limestone dipped down into ‘V’ shapes in the lower beds, these showed that the beds were the right way up.

Cliff showed us that at least one of the horizontal gaps between the rocks was filled with clay or mud but he loosened a piece of infill from one of the vertical cracks and tested it with acid.  It fizzed, showing that it was a carbonate.

We came to the pillow lava, large, almost circular lumps of a dark rock among a loose, crumbly foliated rock – which we were told was clay, formed when the basalt weathered.   Further along we came to nodules of flint in the rock and narrow strips of chert bedding.  Cliff explained that the chert and flint were made of silica, a form of quartz, from the skeletons of siliceous marine organisms.

By this time, we could hardly see the rocks through the swirling fine hailstones Photograph 2 which were lying about an inch deep on the ground so we cut the walk short and made our way back to the car park for late lunch in the shelter of our cars.  Afterwards, we met by Cliff’s car where he had a large collection of rocks for us to examine through our hand lenses.  We were shown a complete fossilised bivalve shell (about six inches across) so that we could see what the curved ridges in the chalk actually belonged to.

Cliff then demonstrated how to use the compass and clinometer to measure direction of strike and angle of dip and explained the geology of the area from the Ten Mile Map and a local geological map. 

By this time, we were all shivering with the cold and although the information we were being given was all extremely helpful and interesting, our brains were beginning to seize up.  Don Cameron thanked Cliff on behalf of the group and Brenda, as Secretary, presented him with a bottle of wine in appreciation of all his hard work. 

Despite the weather, it was a very enjoyable day and it made all the difference seeing the rocks in situ with experts to explain exactly what we were looking at.  I am looking forward to more field trips in the future.

Editor

I would also like to thank Muriel for all the work she put in for this trip; she initially identified the locality as being suitable and was instrumental in the successful running of the day. Thanks Muriel.

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