Wookey Hole to Split Rock - 2017-09-18

Wookey Hole – Split Rock – Wookey Hole

Led by Jean & Doug

This walk was a little over 3 km, starting in Wookey Hole, and then along Limekiln Lane through Split Rock, past Model Farm and across to Tynings Lane before returning to Wookey Hole.

The walk started in a old quarry in Doug's neighbour’s garden which gives a good section of Dolomitic Conglomerate in which a well displayed normal fault is visible (Fig. 1), displacing the beds by about 1 m. Local houses are built mainly of conglomerate (Fig. 2).

Fig. 1 Fig. 2 Fig. 3
Fig. 1. A small quarry in a neighbour's garden, containing a normal fault, indicated by the red line. This resulted from tension rather than compression forces, as the left side has slipped down rather than being pushed up.Fig. 2. Conglomerate not a good building stone as it can't be cut into neat blocks. Here, Bath stone has been used to obtain straight edges.Fig. 3. Successive slippage of conglomerate down the side of the Mendips forming irregular "terraces" in the field.

Then we proceeded on to Limekiln Lane (Fig 3 - 4) and Split Rock quarry (Fig. 5 -6) exposing the Carbonifeorus Limestone. Karst features are visible here along with mineralised veins (Fig. 7 - 8), representing infilled fractures/faults associated with the Mendip mineralisation and the very early stages in the opening of the Atlantic ocean.

Fig. 4 Fig. 5 Fig. 6
Fig. 4. An old lime kiln, once used to turn limestone (calcium carbonate) into lime (calcium oxide) by driving off carbon dioxide.Fig. 5. Split Rock quarry, exposing carboniferous limestone.Fig. 6A fossilised crinoid stem, seen in cross-section, in the carboniferous limestone.

From here we went across to Tynings lane (Fig. 9), where the Carboniferous limestone on the southern limb of the Pen Hill pericline shows a small fold struture. Then back to Wookey Hole.

Fig. 7 Fig. 8 Fig. 9
A crack in the limestone, caused by tension forces, about 30cm across and infilled with crystalline material. Horizontal crystals show that no vertical movement has taken place on either side of the crack.A narrower crack (arrowed) rearby, also infilled with crystalline material.A horizontal shoulder of conglomerate extends from the side of the Mendips at Penn Hill, but is missing in the Split Quarry area where the limestone extends South of the Mendips and the conglomerate has tumbled down on either side of the ridge.

[Notes by Doug, pictures & captions by John]

Further information

BGS Crinoid fossilsdescription [BGS}
Normal faultdescription [Wikipedia]
Dolomitic Conglomeratedescription [BGS]
Carboniferous Limestonedescription [Wikipedia]