A Field Trip in Knaresborough - Part 1

 

Knaresborough railway viaduct

As a very active member of the Sheffield U3A Geology Group, I am always on the lookout for potential field trips and, having seen the Denys Smith Memorial Trip: The Geology of Knaresborough Gorge by Another Cooper, who was the editor of the Doncaster Geodiversity Assessment, I immediately thought that a shortened walk based on this would be ideal for us.

 

The original field trip route by Anthony Cooper

 

Back in 2017, during a weekend visiting an old friend from Nottingham University, I had a good walk along the Nidd Gorge and was very impressed by the geology and the surrounding landscape and, although one or our members took it upon himself to undertake a recce while in the area with his wife, I undertook my own research so that I could provide geological input on the day. 

 

The British Geological Survey bedrock map of Knaresborough

 

Less than a week after my exploration of Blacka Moor Nature Reserve in Sheffield, the group met at the Conyngham Hall car park at the west side of High Bridge and, walking from here along Waterside to the Grade II* Listed railway viaduct (1851), I noted several examples of dolomitic limestone from the upper Sprotbrough Member of the Permian Cadeby Formation. 

 

An exposure of dolomitic limestone on Waterside

 

These oolitic limestones, which very often contain a high content of sand derived from the underlying Upper Carboniferous Addlethorpe Grit, show massive subaqueous dune bedding that was formed in a Bahamas Banks environment. The ridge on which Knaresborough is set appears to be a primary depositional feature, as the limestone thins away from it in all directions. 

 

Inspecting the gritstone at Knaresborough railway viaduct

 

We stopped to take a quick look at the buff/light brown pebbly gritstone used to build the viaduct, built by the Scottish engineer Thomas Grainger for the Leeds and Thirsk Railway. The Leeds Geological Association (LGA) guide, adapted from the original by Bill Fraser, which our leader Dave had chosen for the route, suggests that this may have been quarried from the Plompton Grit a few kilometres upstream at Scotton. 

 

The shelter on the cliff path to Knaresborough Castle

Making our way along the winding path up the cliff to Knaresborough Castle, we had our first opportunity to look at the limestone close up at a shelter half way up, where the cross-bedding dips to the north, before continuing up to the ruins of the castle. 

 

Views of Knaresborough Castle

 

With our leader just reading Bill Fraser’s notes, which I had already read and digested, I made my own observations of the various rocks that I was looking at and, when arriving at the castle grounds, I had a quick wander around to take a few photographs of the remains of the keep, the Courthouse Museum and Knaresborough war memorial, before rejoining the group and returning back to the gorge. 

 

Knaresborough war memorial

 

The next stop was to look at a vertical cliff section on Waterside, which exposes the unconformity at the base of the Cadeby Formation. Addlethorpe Grit forms the lowest part of the section, with the upper parts stained red by post Carboniferous weathering. Immediately above it are 6-7 m thick basal zone of reddish-buff coloured, sandy dolomite, containing blocks and fragments of eroded Addlethorpe Grit held together by a dolomite cement, with 17 m of dolomite above it. 

 

Views of the cliff face on Waterside

 

At Knaresborough, the uplift and erosion during the Permian period resulted in the removal of all the Coal Measures strata and the upper parts of the Millstone Grit Group. The landscape had undulating hills and ridges where the resistant sandstones and gritstones occurred with shallow valleys and hollows in the soft mudstones. 

 

The Carboniferous-Permian unconformity

 

The plane of the unconformity dips at an angle of approximately 15 degrees to the south and, just a little further along Waterside where the red sandy dolomite is not covered with rock netting, its very irregular nature is clearly visible. 

 

The remains of a fossilised tree

Below the unconformity, standing slightly proud from the surrounding Addlethorpe Grit, is the very weathered remains of a fossilised tree. Unlike others that our group had encountered in the Millstone Grit – at Whirlow, Otley Chevin and Ilkley Moor – its surface is very weathered and none of the patterns that are typical of Carboniferous tree fossils can be seen. 

 

Listed buildings on Waterside

 

Continuing along Waterside,  where I took a few photos of the listed buildings at Castle Mill and March House for the British Listed Buildings Photo Challenge - all of which are built in sandstone or brick and not Permian dolomitic limestone - and another quick snap of Low Bridge, which the Conservation Area Appraisal says was rebuilt with Magnesian Limestone in 1779.

 

Low Bridge