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

Triassic Sandstone & Mudstone

A general view of Cedar Road Quarry in 1997

The first time that I saw this site, way back in 1997, a very brief survey clearly demonstrated that the extensive and easily accessible rock outcrops makes this an ideal site for educational purposes, at all levels. At the time of my visit, a vehicle and a couple of workmen were being gainfully employed in cutting grass, pruning a variety of shrubs and other invasive vegetation - and taking away the large amount of rubbish that inevitably accumulates in very many forgotten geological sites like this.

Sandstone and Mudstone - 2007

On my next visit, 10 years later, whilst producing the Doncaster Geodiversity Assessment, it seemed obvious to me that this former Adventure Playground was no longer actively managed by the local authority - with beer cans and medical syringes just being a few pieces of social debris that could be found there. 

Apart from a few brambles, which stopped me from getting as close enough to a rock outcrop as I would have liked to have done, my former observations concerning the educational value still held true - as there was plenty of rock to be seen.

At this moment - during winter - I had both the time and the technological benefit of using a Global Positioning Device to pinpoint the various places of geological interest that I had seen before, and could still easily see. I also discovered a few Quaternary rocks that I hadn't noticed before.

Glacial Till and Glaciofluvial Deposits

Although the Sherwood Sandstone Group, of Triassic age, forms the bedrock beneath 70% of the borough of Doncaster and it does form gentle escarpments and other topographical features in places, most of it is buried by unconsolidated Quaternary sediments. In the remote eastern part of the borough, this very friable sandstone has been exploited, along with the overlying sand and gravel, for aggregates and it is well exposed in several active and redundant quarries. However, except where seen in railway cuttings, exposures of rock in the public domain are extremely rare.  

 

A general view of Cedar Road Quarry in 2007

When returning again, this month, with students from XP School, I could hardly believe my eyes. Taking into account a very warm, wet year that has disrupted so many of the annual cycles of plants and birds, amongst other things, I just couldn't see a single place where they could readily undertake any of their allotted tasks - without cutting through the overgrowth.

Conservation Meets Education

An old fashioned measuring tool

Out of the blue, I was recently contacted by XP School in Doncaster, to ask if I would be prepared to act as a “geology expert” and accompany a group of first year students on a field trip to investigate the geology around the Don Gorge.

Martin Said, a teacher at the school, was impressed by the Doncaster Geodiversity Assessment that I co-produced for Doncaster MBC in 2007, whilst working for the British Geological Survey. Having previously undertaken a survey of the 26 geological sites that formed the basis of this report back in 1997, with the South Yorkshire RIGS (Regionally Important Geological Sites) Group, I was considered to be the ideal person to undertake this job.

A 20x magnification hand lens

Although a great effort was made by the South Yorkshire RIGS Group to highlight the very best sites in the county for their educational value, none of the sites identified in South Yorkshire had been used by any school for their field trips - to the best of my knowledge - and so I was very pleased to help.

25 students, split into two groups, were taken to 3 sites that were considered to provide the best introduction to the geology of Doncaster.

With each group travelling in an opposite direction, and meeting up for lunch in the middle, a total of 6 hours was spent out in the field to undertake the following tasks:

1)   Measure and draw the rock outcrop.

2)   Annotate the drawings.

3)  Closely investigate the rock outcrop and analyse the lithologies seen.

Grain Size and Roundness Charts