Geology
Frosterley marble forms a relatively thin layer - up to 1 m - within a much more substantial non-fossiliferous limestone layer dating from the Carboniferous period (345 - 290 million years ago). This layer formed when this area was a shallow tropical sea-bed - a lot changes in ~ 300 million years! Many years of slow accumulation of organic and mineral detritus ("sedimentation") formed muds that eventually turned into rock in a process called lithification. The marble layer represents a period where the accumuating detritus was particularly dark, and when a large number of relatively intact solitary corals (
Dibunophyllum bipartitum) were incorporated into the forming mud. The tough calcite skeletons of the corals were preserved in the rock to give the incredibly detailed, light-coloured fossils.
Harehope Quarry was mainly worked for limestone that was used in iron smelting and road building, but also for the decorative Frosterley Marble. The quarry sides therefore expose a full record of the deep limestone layer.
The Bollihope Burn runs through the quarry and has over a long period of time dissolved some of the limestone to produce caves and potholes - a characteristic of limestone areas and known as karst scenery. In places the river can partially or completely disappear underground through these channels, then re-emerge further downstream, where it is known as a resurgent river. Of particular interest are regions where the riverbed is formed from Frosterley Marble since the wet, naturally polished surface shows the natural marble at its best. One such location is at the viewpoint at the main EarthCache coordinates.
The earthcache site
Logging the EarthCache
To log this EarthCache you need to visit two locations at this site - see map below for an overview.
- Geology garden [STAGE1]. From the parking coordinates walk down into the site, following the path to the left across the bridge, then left again to stage 1. Here you should find a stone display showing the layers of rock at the quarry.
What are the two layers immediately older than the (marble-containing) limestone layer?
- Fossil viewpoint [Main coords]. Continue along the path back to the lane you arrived on. Double back on yourself on a footpath to your right - the Weardale Way. Continue until a permissive path branches off to the right, down some steps. This leads to the viewpoint at the bridge, where as well as being able to see marble in the river, a block of polished marble has been placed to allow closer examination of the fossils.
The fossils are present as circles (fossil corals that have been cut across - transverse sections) as rods (corals that have been cut along - longitudinal sections) or as intermediate shapes (oblique sections).
Measure the diameter of 5 coral cross-sections either on-site or by photographing the corals and including a reference of known size such as a coin, pen, etc. What is their average diameter? What is the longest longitudinal section you can find?
This should give you an idea of the size of the corals.
- Finally, do some research, either on-site or at home, to find one other fact about Frosterley Marble that is neither in this description or in the previous logs. Add this to your log and I will periodically update the cache description with these facts. This EarthCache will get harder over time! Example facts include sites where polished marble can be found or the marble was quarried, or more information about the fossils or the rock itself.
Post the photo (if taken) and new fact with your log and
email the answers to the other questions to me through my
contact page.
You may log a find without waiting for a reply from me, but logs where I'm not satisfied that you have visited the site and learnt about the geology will be deleted.
Additional Information
More information is available in the
Frosterley Marble brochure written by the
North Pennines AONB Partnership. A piece of Frosterley Marble with a
scultpured re-creation of the corals makes up part of the Laing Touch Trail at the
Laing Art Gallery, Newcastle (near this cache).
Facts Collected by Cachers
Updated November 2011
These tiny beasts appear in the form of sea shells and for this reason Frosterley marble is known to local quarrymen as `Cockle'.
"A document from 1183 mentions ‘Lambert the marble cutter’ of Stanhope – almost certainly a reference to the working of Frosterley Marble."
"Imagine a shallow tropical sea, alive with bright corals and shellfish. This isn't the Bahamas but the North Pennines - 325 million years ago! Frosterley Marble formed in this sea, and its beautiful fossils give us a glimpse of the creatures that lived here in the distant past."
"The marble is actually crinoidal limestone, and as attractive as it may be, a great deal of it was placed on the early railways set up in the area and shipped out to the iron foundaries in places such as Teeside, as it is essential in the making of high quality pig iron. It took about half a ton of Weardale limestone, acting as a flux, to make a ton of pig iron."
"[This area] would have been like the great Barrier Reef in North Queensland, but they got the chills in the Mid-Triassic and died out unlike the crinoids which still exist today - they're now called sea-lilies."
" Frosterley is not a true marble but a black limestone that is speckled with the remains of prehistoric plants and marine creatures"
"The columns found in Durham Cathedral would have been roughly cut, possibly from the Bollihope Burn riverbed, then transported to the cathedral building site. Here, industrious Norman monks would have begun the laborious process of polishing, using blocks of sandstone lubricated with water and leather cloths impregnated with fine sand and silt, to bring the stone to a smooth decorative finish."
"The features of Bollohope Burn where the marble can be seen in the bed, including the gorge, potholes, caves, and resurgent streams are known as karst scenery resulting from the chemical weathering of limestone"
"All corals, like the fossils here are actually animals and have a life cycle that alternates between the 'attached' form and a 'swimming' form. The attached forms are called polyps and can be single such as a sea anemones, or colonial, living together, like corals where each little 'cell' ( you can see these if you look closely at the fossils) will house a miniature polyp. The free living stage is called a medusa, but most of us just call them jellyfish! Their job is to trawl the ocean for food and then reproduce to form more polyps."
"The lithology and mode of occurrence of the corals in the rock suggests that the original deposition of the sediments took place under the influence of stronge waves or currents."
Further places to see Frosterley Marble:
A sculpture at
Frosterley railway station.
Map
The font in
St Michael and All Angels' Church, Frosterley - It was rescued from a churchyard in Lincolnshire,restored and installed in the church in 1989.
Map