A Lesson Learned

Rock Art

 

When a shortlist of 4 sites was selected by the teachers leading the XP School “Rock On” project, I did have some reservations about going to Barnburgh Cliff, as I thought that the students would not have enough time to fully appreciate the points of interest here.

A view of the Dearne Valley in 2007

Although the geology is excellent, and the geomorphology of the Dearne Valley can be appreciated here, it is relatively remote and it takes a moderately long walk before the Carboniferous sandstone and the best exposures of the Permian limestone can be seen.

When I visited this site in February 2007, various geological features could easily be observed on a cloudy day.

An exposure of a Permian reef in 2007

Making up for time lost at Cedar Road Quarry, the students concentrated on the measuring and drawing tasks at Warmsworth Park and, by the time we arrived at Barnburgh Cliff, we only had 15 minutes to explore the site - before the group had to get back to school.

As the students jogged past the bryozoan reefs, I noticed that the undergrowth appeared to be much more hazardous and all of the rocks looked very dark and dank - beneath a dense canopy of leaves.

Stopping at a cave, I was asked to give a short explanation of the geology that the students could see behind me and then we had to leave, and I didn't take a single photograph.

On the way back to the minibus, a couple of the students – including one who was fascinated by the rocks at Cedar Road Quarry – told me that they had thought that rocks were all the same, and very boring, but that they were now much more interested in them. They had also noticed the Rock Art that can be seen here...

Dolomitic limestone and a sand filled rift  in 2007

Carboniferous Sandstone

An outcrop of Carboniferous sandstone

The survey of potential RIGS in Doncaster in 1997, like others in the rest of the county, was based on information gleaned from old maps, geological memoirs and local knowledge of the area. For some reason, Barnburgh Cliff was not identified during the desk top study and I first saw this site in 2007, when temporarily working with the British Geological Survey to produce the Doncaster Geodiversity Assessment.

Permian dolomitic limestone

Walking through a plantation along a straight bridleway, with a continuous exposure of well bedded dolomitic limestone up to 4 metres high - on one side - I thought that this place must have been quarried as a building stone at some time; it has been suggested that a Roman road passed closely by.

It is a very good place to examine the lowest parts of the Cadeby Formation - with its shelly ooid-limestones and reefs - which has a totally different character to the Sprotbrough Member seen at Warmsworth Park.

The yellow colour of the limestone often coincides with the appearance of large fissures, parallel to the rock face, filled with orange coloured sand of Quaternary age. Some of these have developed into small caves, which elsewhere in South Yorkshire have been investigated by speleologists, and which are the home for various species of bats. 

Although the Permian limestone contains some very interesting geological features, Barnburgh Crags is one of the very few places that I know of in South Yorkshire, where the underlying Carboniferous rocks can also be seen at the same locality. Unlike Ashfield Brick Pit, which as a SSSI was controversially infilled, the exact position of the unconformity that separates these geological periods can only be inferred from field evidence – but it is still an excellent field trip locality, at all levels of study.

Upper Carboniferous Sandstone

Taking Measurements

Weathered dolomitic limestone at Warmsworth Park

On both days of fieldwork for the XP School “Rock On!” project, when 3 sites would be visited, Warmsworth Park was chosen for its suitability as a place to take lunch, apart from the interesting geology that can be seen here.

Delays at Cedar Road Quarry on the first morning, due to the overgrown condition of the site, meant that we arrived in Warmsworth much later than planned. Without any leading, our group descended into the landscaped quarry and took up various positions on the walls of the small amphitheatre like area - to eat their lunch.

Dolomitic limestone at Warmsworth Park

Except for the ivy, which had further encroached on the rock faces, it was as I had remembered it on the previous occasions that I had visited.

Not long after we arrived, the other group turned up and it seemed that the trip to Barnburgh Cliff had not quite gone as planned either. Nonetheless, with the first stage of the project completed, both groups enthusiastically explored the various rock faces that can be seen in Warmsworth Park.

With large expansive outcrops and no nettles, brambles or obstructions, they soon completed their measuring task. Using a ranging rod and a tape measure, the students were encouraged to make actual and estimated measurements and include these in an annotated drawing of the rock face.

Before they commenced, I outlined the basic geology found here and pointed out some of the structural and physical features that they could see, such as bedding planes, joints and fractures. The students were also encouraged to closely study the rock face, looking closely at the colours and textures.

On the way back to the minibus

Using a piece of stone that had long since fallen from the quarry face, the students were shown how to take a small rock sample using a geology hammer, taking care to wear goggles.

They then used a hand lens and grain size scale to closely examine the limestone and to note the differences between this rock and the sandstone that they had seen earlier – and many made comments on the mineral pyrolusite, which they had noticed...

A geology hammer

Permian Dolomitic Limestone

Warmsworth Park in 2007

Magnesian limestone comprises approximately 25% of the solid rocks that underlie Doncaster and, when I first saw them, I didn't think that they were very interesting - as I sped past along the M18. Although there is a distinctive escarpment, as seen when driving north from Hellaby, I could only see layer upon layer of pale coloured, gently tilted rocks - for a few miles.

Variation in colour and texture

When I examined the Magnesian limestone closely - in Rotherham and Doncaster - whilst identifying the RIGS, my opinion of these rocks changed. 

The dolomitic limestone has been extensively quarried for lime, stone and aggregates and for use in the steel making and glass industries. Many road and railway cuttings, as well as natural exposures, provide opportunities to study the geology of Doncaster in much greater detail. 

At Warmsworth Park, a former building stone quarry, there are extensive exposures of the Sprotbrough Member of the Cadeby Formation, which is typically a massive, wedge bedded, fine grained and compact limestone, with virtually no fossils. As an established public park, it is still regularly maintained and is an ideal field trip site for all levels of students.

Several safe and accessible quarry faces demonstrate the vertical and lateral changes in thickness of beds, and there are many examples of joints and fracturing, often at oblique angles. Stylolites are common and, on a fresh surface, the stone exhibits a black speckled appearance which, viewed through a hand lens, is seen to be the manganese bearing mineral pyrolusite, with its distinctive dendritic pattern of crystal growth.

Glacial Till

Although vegetation inevitably obscures much of the rock, a closer investigation also reveals sandy glacial till of Quaternary age, full of angular fragments and sub-rounded pebbles, which provides evidence of the great changes that have taken place on Earth during the last 300 million years. 

In places, the uppermost beds of the limestone are flaggy and highlighted by weathering and the formation of soil horizons – during very recent geological times. 

Also, a particularly interesting feature is a joint in a quarry face that shows widening and solution of the limestone by percolating groundwater and is associated with the formation of Karst topography on a once exposed and weathered surface.

A solution feature

Lithological Analysis

Cedar Road Quarry in 2014

In my last post, I briefly described my surprise at seeing the overgrown condition of this site and my first job at Cedar Road Quarry was to try and find a place where the students could make drawings and take measurements of the rocks – but this wasn’t so easy. Whereas before, there was an expanse of red rock, there were now insurmountable brambles, as well as nettles, and no obvious access to any rock face.

We soon decided that the students’ time, at this site, would be best spent examining the rocks at close hand and, once I had demonstrated that the brambles and nettles could be trampled and a path cleared, albeit with a small scratch or two, the students threw themselves into the task. 

Sandstone and mudstone

Although a geological hammer was at hand, taken from the toolbox by a boy who wanted to use it, I showed them that a small sample of this sandstone could be broken off by hand and easily crumbled with the tips of the fingers. 

They learned how to distinguish the sandstone from the thin discontinuous layers of mudstone that occur within it and to note the differing weathering characteristics of these rocks, as well as seeing the variety of large and small sedimentary structures that can be found within them.

Each took a small piece of sandstone and mudstone to a grassy area, where they used their hand lens to examine and analyse them and make notes in their science books. Although each had laminated grain size charts, I showed them my “old fashioned” Wentworth Grain Size Chart, which has actual grains of silt and sand glued to stiff cardboard. With their eyes closed, the students were encouraged to rub the card and to appreciate the value of using more than vision to investigate the physical properties of rocks.

Grain size charts

Also, to their great surprise - and to the initial disgust of some - I then proceeded to lick the small fresh piece of sandstone that I had taken from the rock face. I had first learned this when making a map of the Borrowdale Volcanics, to highlight the texture and minerals contained in the rock, but have subsequently discovered that the taste of a fresh piece of rock can vary considerably, as well as the smell that is generated when a stone is cut with a diamond saw or ground with abrasives.

Having emphasised that it is only safe to do this with fresh rock, and that this should only be done once you have gained sufficient knowledge of geology and mineralogy, I was later surprised to discover that one of the boys actually tried this.

An introduction to the science of geology