Wells

Vallis Vale 19-06-17

Field Trip to Tedbury Camp Quarry and Vallis Vale Unconformity

Led By Jean & Doug

Tedbury Camp Quarry

Six of us braved the heatwave and made our way to the Tedbury Camp Quarry, a large, flat area of limestone which had once been stripped to be quarried for roadstone, but was then left intact, revealing what was once the sea floor in Jurassic times.

Doug showed us how to recognise the bedding planes, running North-South, and the steeply dipping strata of grey Carboniferous Clifton Down Limestone (Fig. 1), as well as numerous small coral fossils aligned along the bedding planes (Fig. 2) and some places where slippage had taken place (Fig. 4). On the exposed side of the pavement were several burrows (Fig. 3) left by molluscs and worms when sea covered the limestone, later filled in by white Jurassic sediments.

Click on the pictures below to see them full-size.

Fig. 1 Fig. 2 Fig. 3
Fig. 1Fig. 2Fig. 3

In places, the overlying inferior oolitic limestone hadn't been completely removed, and was seen to be full of shell fragments (Fig. 5, 6). At the far end, the Jurassic limestones were exposed and could be seen to be thickly bedded and full of fissures.

Click on the pictures below to see them full-size.

Fig. 4 Fig. 5 Fig. 6
Fig. 4Fig. 5Fig. 6

De La Beche Unconformity

We then walked on to the nearby De La Beche Unconformity, described eponymously in the world's first Geological Survey memoir in 1846. Here, Doug showed us the same horizontally-bedded Jurassic Inferior oolite sitting on top of the steeply dipping grey Carboniferous limestone (an angular unconformity) and told us there was a time gap of some 170 million years between the two layers, during which the Carboniferous limestone had been forced upwards by continental collision in what is now France, then eroded and finally gradually encroached upon by shallow Jurassic seas (Fig. 7).

Click on the picture below to see it full-size.

Close-up view
Fig. 7

Further Information

angular unconformitydefinition [Wikipedia]
Carboniferous limestonedescription [Wikipedia]
De La Beche Unconformitydescription [BGS]
Inferior oolitedescription {Wikipedia]
Tedbury Camp Quarrygeological synthesis [Earth Sciences Teachers Assocn.]

[Notes and photos by John]