The Market Hall Museum displays include a large slab of ironstone, collected a good few years ago from the now disused Edge Hill quarries in the south of the county. The photo above shows this specimen as it was found on a reject stone tip in the quarry, mechanically sawn, but rejected by the stonemasons due to its irregularities. Traditionally, the quarries extracted Hornton Stone; brown or green-coloured sedimentary ironstone that was used for ornamental and building purposes, and as a source of aggregate. Close by, in North Oxfordshire, the rock was once extensively quarried as a source of low-grade iron ore, but that’s another story.
Interestingly, the quarried ironstone beds are underlain by a contrasting rock bed, throughout southern Warwickshire. Geologists would best refer to this rock as ‘conglomerate’, a sedimentary rock containing pebbles and other fragments of pre-existing rocks. The Edge Hill conglomerate consists of a matrix of relatively fine grained ironstone enclosing fossil shells (like the Hornton Stone beds above) but also incorporating chunks of other types of sedimentary rock – mainly sandstone and siltstone.
The fossils, structures and chemistry of the Hornton Stone tells us that it formed as a type of iron-rich sand, in a warm, shallow sea that covered parts of central England during the Jurassic Period, roughly 190 million years ago. The conglomerate layer bears all the hallmarks of even shallower water, and periodic storms ripping up older sea beds to generate the mixed-up chunks of rock. In turn this suggests that Jurassic sea-levels were ever-changing, and comparable changes are known throughout the world, wherever Jurassic sedimentary rocks of marine origin are found.
As today, the causes of these ancient sea-level changes can be due to one of a number of causes. Some were truly global, and caused by oceanic volcanic eruptions building up lava flows beneath the sea and displacing water onto land. Others were relatively local, as earthquakes caused giant blocks of sea bed or adjacent land to either rise or fall. The Jurassic rocks of Warwickshire bear the signatures of both local and global sea-level change.