The Geological Detective - II

The Triton Stone Library

Having established that none of the samples in the Triton Stone Library could offer a good match to the stone used in the subject building, the next step was to obtain samples of stones that might be suitable to undertake the necessary repairs.

E-mails were sent off and phone calls made and I soon discovered that the quarry, which has been considered to be the primary source of this stone, has now been mothballed and that I would need to search elsewhere for a good match.

Stone matching in practice...

Time is of the essence, when planning and scheduling an extensive programme of works in the building restoration industry, and the first samples of stone were sent by a tried and tested contractor, who has a good working knowledge of the region.

The samples of Oat Hill stone from the recommended alternative marketed by various companies finally arrived, but a quick inspection with a hand lens revealed a medium grained, shelly oolite - with voids - which bears little resemblance to the subject stone.

The Art of Stone Matching is based on deep geological knowledge, and an understanding of chemistry - but it also relies on intuition, which microscopic analysis just can't reproduce. 

This particular compact, fine grained, moderately well sorted oolitic limestone is left with a very distinctive iron stain when subjected to a standard hydrochloric acid test.

The Geological Detective - I

Every now and then, I receive a few photographs of an old building – or fragments of stone in the post – along with a request to help with the identification of the stone used in its construction, ahead of a programme of restoration work.

Having devised Triton's stone library, with samples from all over the UK - for day to day stone matching purposes - I was very curious when I received photographs of various chimney stacks at a large red brick Victorian house.

In contrast to the Ham Hill stone, which is heavily weathered, the stone in question still retains sharp lines to the mouldings and appears remarkably clean - revealing its distinctive colour.

Having been categorically assured that it was a limestone, my first thought was that it could be a Jurassic oolite from the Cotswolds. 

Compared to the many geologically similar samples of “Bath stone” and “Lincolnshire limestone” that are now buried in a landfill site - but which once formed part of my own private collection - the large cube that I once obtained from this region stood out just for its colour. A quick look through a couple of reference books reinforced my initial thoughts that this might possibly be Guiting stone.

Although planned for inclusion in the Triton Stone Library, samples were never received and - except for the Permian dolomitic limestone that has replaced it - none of the samples are a good colour match for the stone that has been salvaged from one of the collapsed chimneys. 

The Triton Stone Library

Before and After

The Green Moor Quarry RIGS after cleaning and rock netting

Having completed the recording of the geological features seen at the Green Moor Quarry RIGS, I waited another month before I returned to photograph the site with its rock netting.

Views of the Green Moor Quarry RIGS

The UK Government’s objectives for planning are to conserve, enhance and restore the diversity of England’s geology and to contribute to a “better quality of life and to people’s well-being”.

Boston Park - a missed opportunity for Geoconservation?

I generally agree with these sentiments; however, in my experience, even when spectacular geological sites are owned and managed by the local authority, their conservation has been low priority.

Without financial resources and full support of the landowner, it is not always easy to put ideas into action.

In more than 20 years since I have been involved with Geoconservation, I have visited countless quarries - both active and redundant - throughout the British Islands and there are only a very few where I would feel comfortable, with a fully exposed rock face hanging over my back garden.

 

A few examples of rock outcrops in private gardens

From the outset, the developer was keen to enhance the old Green Moor Quarry site and, just by removing the old waste material that had been left behind when the quarry was abandoned 80 years ago, the appearance of the site has been improved considerably. 

Dry stone walling in perfect harmony with the Greenmoor Rock

Having discovered much better places where the Greenmoor Rock can be seen, as well as being accessible by the general public - as at Green Moor Delph and Hunshelf Bank - I thought that there was a very good case for deselecting the Green Moor Quarry RIGS and removing it entirely from planning control. As such, I think that the developer bent over backwards to incorporate the site into the housing estate, with the provision of a viewing point and access for management.

Stoneway Manor

I was paid well for my work to help rejuvenate the old Green Moor Quarry - and some people might say that I am a gamekeeper turned poacher - but I think that a good practical common sense approach was adopted for the development of this site and that the resulting housing estate constitutes a significant  environmental improvement.  

Views of the old Green Moor Quarry before redevelopment

Rock Netting

The Installation of Rock Netting

Following the cleaning of the Green Moor Quarry RIGS, all parties directly involved in this project agreed that the decision by the geotechnical engineer to apply netting to the rock face was good practical common sense. Although the conditions for the planning permission, to conserve the RIGS, were strictly adhered to by the developer, the elimination of potential Health and Safety hazards for the occupants of Stoneway Manor was considered to take priority.

The Green Moor Quarry RIGS after cleaning

After the scaffold had been dismantled and all remaining loose material had been cleared from the base of the quarry face, I arrived very early on site to take photographs of the cleaned RIGS - before the rock netting contractor commenced work.

Tools and materials

Even though I wasn’t paid for my time to take these photographs, having been involved with the Green Moor Quarry RIGS for 17 years, I was not only curious to see how this was done, but I also thought that it was needed to record this project - from start to finish.

In the time that I was on site, I was only able to observe the setting out of safety lines and drilling and netting to the south of the RIGS - the highest point on the old quarry face - but it was enough time to provide me with a good introduction to the techniques, tools and materials used to undertake this kind of work.

Looking at the materials – the 80 mm mesh rock netting, rock anchors, cables, various other nuts, bolts and epoxy resin – these were obviously of a grade that would be used when unstable geological conditions make the rock susceptible to rock falls and landslides. 

A detail of rock netting

As a geologist, with an appreciation of structural defects and structural loading - acquired in the general construction and specialist restoration industries - I agreed with the opinion of the engineer.

Conservation projects like this are so rare but, ideally, I think that there might be scope for the rock netting industry to make a “conservation grade” product which allows unwanted plants to be removed and still let the geological features be seen.

Drilling

That said, with Stoneway Manor essentially complete, except to finish off some road surfaces and gardens, I was very impressed by the way that the whole team worked in the tight spaces available; I was also quite surprised by the speed at which they progressed.

I particularly liked the way that the mobile access platform was used. The operative skillfully manoeuvred it into position - lifting rock netting to the top of the rock face, where other site operatives had previously sunk ribbed stainless steel dowels, ready to receive it.

When the cleaning work took place at the beginning of May, after a long winter that had extended well into April, the vegetation along the quarry face had largely died back but, 6 weeks later, the root balls that had been left on the RIGS had sprouted into plants. To the south of the RIGS, where vegetation covering the overburden had been left untouched, large ferns formed obstacles to the placing of the rock netting and, in some instances, they were very difficult to remove.

The removal of a deeply entrenched fern

A Site Report

Ironstone nodules found at the Green Moor Quarry RIGS

A condition for the planning permission to develop Stoneway Manor was that, once the Green Moor Quarry RIGS  was cleaned, a report on the geology would be made, along with a systematic photographic record of the exposed rock face.

In the old quarries that I had seen around Green Moor, there were exposures of relatively thick beds of sandstone, once capable of supplying very large,slabs of paving stone and I envisaged that the cleaned quarry face would reveal rocks with the same character.

Once the cleaning started, it was soon obvious that the Greenmoor Rock here was full of oblique fractures and sections of rock parallel to the quarry face, 50 – 300 mm wide, were becoming detached. 

Most of the rock that had fallen down from the quarry face was, at best, only suitable for dry stone walling, except for one large piece of rock, which one of the general workers on site moved by himself from the quarry face to the back of a small van – to everyone's amusement.

A section through the Greenmoor Rock

After dead vegetation from the top lift had been removed, only the uppermost section of relatively solid rock, with flaggy beds, could be seen, with the overlying material being made up of soil, waste rock and man made items.

Here, natural weathering processes have opened up beds, joints and fractures in the rock and these have been deeply penetrated and further disrupted by the roots of brambles, ferns and other invasive species.

Lower down, the weathered sandstone passes into more massive blocks and, except for fine partings of silt that are weathered to yellow clay, with occasional ferruginous beds and associated nodules, most of the exposed rock has a similar character. 

Details from the cleaned Green Moor Quarry RIGS

The spur of rock, the most prominent feature of the RIGS was found to be unstable and large blocks of stone were taken down during cleaning work. At the back of the spur, a wide joint has been infilled with loose breccia and finer material - also weathered to clay.

Especially where soil and weathered rock had accumulated on ledges, the rock was extremely loose and very easily disturbed when cleaned with a trowel, and it was usually pervaded by fine rootlets.

Although every attempt was made to leave the quarry face in a safe condition, the oblique fractures, which are an inherent feature of the Greenmoor Rock here, constitute a structural weakness and a decision was made by the geotechnical engineer to apply rock netting to the entire length of the old Green Moor Quarry face. 

The Green Moor Quarry RIGS - after cleaning