Chevin Forest Park GeoTrail #12:
Root Fossils and Coal Seam
The 12th cache in the series, and the 5th of 8 EarthCaches, is located just north of the Yorkgate car park. If continuing from #11 or starting from the car park, proceed to the GeoTrail carved marker stone #6 at N 53 53.568 W 1 41.924 which depicts roots fossils. Then walk a short distance across the grass to GZ.
Please refer to GCB7RA7 for the background information on the geology of The Chevin, which provides explanations of how and when the various rock formations were formed.
At GZ there is a thick exposure of slump bedding Doubler Stones Sandstone dipping to the south. The surface of this (bedding plane) is dimpled with the fossils of tree roots. This sandstone is made of fine to medium size quartz grains with a few pink grains of feldspar and was formed in a river environment.
Due to advanced weathering, these very faint impressions are difficult to spot and may only be visible under certain light conditions, where shadows may help to pick them out. You may be able to find a loose piece of the yellow sandstone where the carbonised tree roots can be clearly seen with the naked eye.
So, the main feature of interest here is instead on the far (right) side of the outcrop where the sandstone disappears below the surface. Here in the low bank, the outcrop is overlain by a thin, black layer (seam) of coal. This is the Morton Banks Coal which, although very thin here is up to 1.5m thick some 12km east at Morton Banks (nr Riddlesden) where it was mined until c1856 when the coal was of such poor quality that ‘the cinder and ash which remains after combustion are almost if not quite equal to the unburnt coal in bulk’.
How was the coal formed? In the Carboniferous period, this part of the continent was close to the equator with a warm and wet climate, so tropical rainforests flourished. Dead plant material became trapped in stagnant swamps between river sands when the rivers in the delta changed position during floods. Gradually, as more and more layers of sediment accumulated above with each succeeding flood event, water and oxygen and hydrogen were driven out of the plant remains, leaving only the carbon in coal seams.
There are around 30cm of sedimentary mudstone - grey fireclay seatearth* (aka underclay) above and below the coal - the remains of the soil layers in which grew the trees and ground vegetation that formed the coal.
*Seatearth refers to the layer of sedimentary rock, particularly a clay-rich fossil soil, that directly underlies a coal seam (here it actually encases the seam). It represents the soil in which the vegetation that ultimately formed the coal was growing.
This layer is considered a paleosol - a fossil soil formed during a prior time period. It is essentially a preserved soil profile from the time before the coal seam formed and found directly beneath the coal deposit, acting as the foundation for the peat accumulation that eventually became coal. The vegetation that grew in the seatearth (a swamp or bog) accumulated organic matter (peat), which then underwent geological processes (coalification) to transform into coal. The organic material begins as peat, becomes lignite, then subbituminous, bituminous and finally anthracite as it is buried deeper and becomes increasingly concentrated in carbon.
Seatearths provide valuable information about the paleoenvironment (past environment) of coal-forming regions, including the types of vegetation, soil conditions, and climate at the time.
Fireclay is a specific type of seatearth with high temperature resistance which is used to make refractory materials necessary for furnaces, kilns, and other high-temperature processes where maintaining structural stability and heat resistance is crucial.
To Log the EarthCache, please complete the following tasks and send your answers to me via the GC website messaging service or by email to forshaw.chris@gmail.com – thanks!
a) Locate the thin coal seam and describe its appearance. What is its texture - how does it feel? What is its estimated approximate maximum thickness here?
b) Try rubbing some of the rock between your fingers with a little fluid (eg. saliva) - what happens?
c) Compare and contrast this with the underlying seatearth/fireclay.
d) Why do you think the coal seam is so thin here compared with further east, where it was mined?
