Thornborough Bridge by Peter Hassett 6 August 2019

Trip Report – Coombs Quarry and Padbury Brook 6 August 2019

The evening started from the Medieval Thornborough Bridge (pictured above) constructed from the local Blisworth Limestone which we were to see in the quarry. The route took us under the busy A421 upstream along the bank of the Padbury Brook (a few members took advantage of much nearer car parking courtesy of the local farmer so missed the walk but had time to set out the demonstration table above the quarry).


The Padbury Brook was a green corridor of reeds and rushes through the parched meadows. Late flowering summer flowers on its banks were still showing well including Great Willow Herb, Marsh Woundwort, Water Forgetmenot, Water Figwort, Angelica and Teasel. 

Two Brown Hares on an arable field opposite offered early excitement. Unfortunately there were few insects in evidence and birds were few and far between but did include Grey Wagtail, Reed Bunting, Swallows, and a large flock of Rooks feeding on stubble.

Joe gave a brief introduction to the rock exposure in the quarry as observed from the viewing platform above it. Mainly drawing on the excellent Bucks Geological Society website and Interpretation Boards he explained that deposition was about 170 million years ago in the Middle Jurassic Period. The area was then at a latitude of about 40 degrees North. He explained that the Blisworth Limestone was deposited in low energy shallow warm marine conditions (as currently found in the Florida Keys). The Blisworth Clay above the limestone was deposited in lagoonal, mud flat and brackish marsh environments. There are fewer fossils but a dinosaur(Theropod) footprint had been found near-by at Thornborough Mill.

Of particular interest was a fault line (subsequent to deposition) which had displaced the younger rubbly Cornbrash seen to the east of the quarry downwards relative to the Blisworth Limestone. This could be clearly seen despite slumping of much later material. 

Jenny then demonstrated how this had happened with a specially baked ‘strata’ cake! (Subsequently the cake was rapidly eroded by participants!)

Most of the group then spent about an hour examining the structural features of the quarry and some Blisworth Limestone samples. We noted that the latter are made up of accumulated fossil shell fragments bound together with a carbonate cement. The fossils that can be found here are of Bivalves, Gastropods, Corals, Ecinoids and Brachiopods. Members identified with some confidence Bivalves and Gastropods including a small number with complete shells. A worm like trace fossil was also found. Others proved more challenging to identify. Calcite crystals (precipitated after deposition) were also in evidence especially with the aid of hand lens.

During the time in the quarry area several members explored the wooded area adjacent to it: a Jay and Tree Creeper were seen and a Green Woodpecker heard. Some also found the ruins of the two 19thcentury lime kilns which operated on the site until 1890 and are now marked by an information board.

This outing was rather different from our normal living wildlife focus but apart from its attractive setting the geology observed was a useful reminder of a past habitat and some of the animals that lived in it.

Text by Jenny Mercer and Joe Clinch

Pictures by Peter Hassett