Derby

Reports of Geology Field Trips

5 July 2016

Group visit to Bradgate Park

11 members met at the Hunt's Hill car park on a bright sunny morning.

We began with an introduction about the geology of the area. The majority of the rocks are Pre-Cambrian in age, formed about 600 to 540 million years ago. These are known as the Charnian Supergroup. Next in the geological sequence in the park are the Triassic rocks, formed about 240 million years ago and the area was also later overlain by glacial deposits. Erosion has exposed the rocks as we see them today.

Our first stop was the unlikely location of the public toilet block! Here the Charnian rocks form the walls. There are some particularly interesting black slabs of Peldar Porphyritic Dacite with large feldspar and quartz crystals. The roof is tiled with Swithland Slates from the Brand Group, the youngest of the Charnian Supergroup.

Once inside the park we climbed up to the Old John Tower. This stone tower was erected on the site of a windmill shortly after the demolition of the latter in 1784. In 1786 the Earl of Stamford held a celebration here to mark the coming of age of his son, George Harry. The huge bonfire caused a large pole to burn through and collapse on some of the guests, killing an old retainer of the estate called John. After the accident the Earl decreed that the tower be renamed 'Old John Tower', and in 1792 the famous archway or 'handle' was added to the tower, resembling a beer mug, perhaps knowing the old man's liking for ale! The tower was used as a prospect tower to enable the Earl to watch his racehorses gallop round the base of the hill. The remains of the stable block can still be seen below the tower.

As we climbed the hill we noticed the hummocky ground formed by the boulder clay or till deposited during the Ice Age. The outcrops around the tower consist of mudstone, siltstone and sandstone, made up of volcanic deposits, known as volcaniclastic rocks or tuffs. The volcanoes thought to be located to the north-west ejected ash and other particles into the air which rained down into the sea and settled out into the water. The bedding planes between the layers can be clearly seen, dipping steeply towards the tower at an angle of about 60°, and cut through by cracks called cleavage planes created by pressure during periods of uplift. The rocks here are part of the Maplewell Group which is the oldest of the Charnian rocks exposed at Bradgate.

We then walked through the Old John Spinney, one of the spinneys planted in the park as game coverts and a source of timber, walled in order to keep deer out. Reaching the War Memorial dedicated to the Leicestershire Yeomanry, we again studied the rock outcrops, particularly noting the graded bedding with the characteristic pattern of coarse-grained particles at the base grading to fine-grained at the top.

Important fossils have been found here in the Maplewell Group which have the appearance of small fronds and disc-like impressions. The are known as Ediacaran fauna, the oldest multi-celled animals in the world, and are the oldest fossils in England.

Making our way to the next locality at Sliding Stone Crags we had excellent views of fine Red Deer stags in the bracken. The outcrops of the Maplewell Group rocks here show particularly good examples of chaotic ripped up blocks and layers of deformed sediment. It is thought that large masses of unstable, waterlogged sediment were rapidly deposited on the sea bed and then began to slump down the submarine slope, perhaps triggered by earthquakes or erupting volcanoes.

From here we descended past Dale Spinney to the main tarmac carriageway along the eastern perimeter of the park. At the next locality below Coppice Wood there was another set of craggy outcrops, where the bedding is down-folded into a gentle syncline.

Reaching the nearby cafe we had a welcome lunch break and a brief visit to geological section of the visitor centre. On approach to the ruins of Bradgate Hall we passed a big herd of Fallow Deer with colourful Yellowhammers singing on the fence. The hall was constructed c1499-1520, and is famous as the birthplace of Lady Jane Grey in 1537. Following the death of her cousin, Edward VI, she was proclaimed Queen of England, only to lose the crown nine days later. She was executed for treason in 1554 by Mary Tudor. (Interestingly, many of the oaks near the ruins were also 'beheaded' by the woodsmen as a mark of respect to her.) The house ceased to be lived in by 1720 and was in ruins by 1790.

The University of Leicester were carrying out archaeological digs near the house, and happily gave us an account of their excavations. The digs near the house were exploring the midden (medieval refuse tip) in order to find evidence of the diet of people living in the hall during the 16th century, and the other larger dig nearby had excavated an extensive moated building, pre-dating the hall, possibly a hunting lodge.

Stable Pit Quarry nearby exposed the youngest group of the Charnian Supergroup, known as the Brand Formation. These rocks were originally thought to have been Pre-Cambrian in age but have since been re-classified into the Lower Cambrian. The rocks are quartz-rich sandstones known as arenites, in some cases coated with the iron mineral haematite. Tension gashes can also be seen filled with white quartz. The main feature of interest here was the vertical igneous intrusion known as a dyke cutting through the arenite at the back of the quarry. The intrusion is composed of diorite, a medium-grained igneous rock, with crystals of green chlorite and pink feldspar.

At the entrance to the quarry the till or boulder clay is well exposed with angular, poorly sorted stony deposits.

Continuing along the carriageway we reached our last locality which was to view the Triassic desert sediments exposed in a nearby cliff. These form part of the sediments which gradually buried and blanketed the Pre-Cambrian hills between 248 and 205 million years ago. The sediments are a deep red colour, mottled with occasional pale green more blocky layers.

We then made our way back to the car park after an interesting and varied day with not only the geological content, but also archaeological, historical and natural history interest.

Tuesday 30th June 2015

Group Visit to Stone Centre Wirksworth

15 of the group gathered at the Stone Centre at 2pm on one of the hottest days of the year.

Geoff our guide is a volunteer and first showed us the deep shaft of a lead mine very close to the visitor centre. These shafts were usually worked by individual families.
The shaft looked like a deep well with an iron cover over the hole for safety.
There has always been lead mining in Wirksworth and the surrounding areas. The fissures of limestone contained rich deposits of minerals, especially Galena – lead ore. The Romans were there and mining in the area carried on right through to the early 20th century.
Geoff showed us a piece of galena.

Our guide then took us to see a disused lime kiln with an explanation of how they worked. In a draw kiln kindling was laid on a grill at the base of the cylindrical, or oval, vertical bowl with alternate layers of fuel and limestone being laid on top. Most kilns had a small quarry immediately behind and a ramp so that stone and fuel could easily be tipped into the bowl from the top. Once a kiln had been filled to capacity, the rate of burn was regulated to allow actual burning in the central section of the bowl; hot air from here dried out and heated up fresh stone in the upper third; while that in the lower third slowly cooled down ready to be drawn through the draw hole - or eye - at the base. It was then packed in sacks or barrels or loaded on carts to be spread across nearby fields or transported to where it was needed.

Geoff then guided us along the Geo-Trail
The introduction states that “The limestone rocks you are about to see are about 330 million years old and were formed during the Carboniferous Period. At that time Britain enjoyed a tropical climate, lying just north of the equator.”
1.Viewpoint
From here we had to imagine we were on a tropical island,standing on what would have been a dry strip of land forming a reef ,enclosing a warm shallow water lagoon behind us and to the left and the deep sea before us.

2. Rock shelves
The rocks are made of limestone – calcium carbonate.
Looking down at our feet Geoff poured water over the lumps in the rock and revealed a brachiopod quite clearly ,further along the circular top a coral was revealed. From this it can be deduced that this rock was formed in a warm shallow setting as corals thrive in a warm climate and depend on sunlight for survival.

3. Lagoon
Looking at the quarry face,the limestone beds were deposited as flat layers of lime debris at the bottom of a broad and shallow lagoon. This lagoon actually makes up most of the limestone area of the White Peak. It stretched from Ashbourne in the south to Castleton in the North. The beds are gently tilted towards the east- this was caused by later earth movements, cracking the layers forming vertical fractures or joints in the beds.

4. Mineral vein and crinoid beds
Here we were shown the sidewall of a fracture in the limestone that were later filled with mineral rich, hot fluids to deposit typical Peak District Minerals of galena, barite, fluorspar and calcite. Small deposits of galena and barite could be seen.
We spent some time inspecting the crinoids bed which were on a sloping mass of rock which was the back slope of the reef sheltering the lagoon from the open ocean. Crinoids are related to starfish. They were filter feeders, sieving clear seawater with their frond like “arms” to catch microscopic plankton, whilst attached to the seabed by a along stem. The stems are what we could see as they are preserved more easily than the fragile fronds.

5. Top of the reef mound
This was the reef that formed a barrier between the deep ocean in front and the lagoon we had just passed.

Millenium wall
This was very interesting. The walls were constructed over the May Bank Holiday 2000 the styles were from different parts of the UK using different stones and techniques. Geoff had been there at the time and explained some of the techniques.

6. Inside the reef
The quarry is the reef core that has been quarried out. The limestone is not arranged in neat beds as in the lagoon, but instead forms a large mass of grey limestone which is lime mud held together by microscopic algae.

7. Looking into the abyss
This is the edge of the reef looking out towards the deep ocean. Looking down into the quarry the bulk of the rocks are grey unbedded reef limestones.This is the very end of the limestone massif forming the White Peak beyond this point there is only mud and sand.

This was the end of the trail and we were very ready for a drink or ice cream back at the visitor centre.

The Stone Centre is looking for more volunteer guides as they have a lot of school parties asking to visit.

I found an interesting booklet on the net about lead mining in Wirksworth,'Lead miners Heyday: the great days of mining in Wirksworth' and the 'Low Peak of Derbyshire' by Ron Slack