June – Alum Shale

Alum Shale occurs within the upper 35 metres or so of the Whitby Mudstone Formation. A suite of rocks that originated as soft sediment accumulating on the floor of an ancient sea (the Tethys) which occupied this area between c.188 million and c.182 million years ago during the late Lower Jurassic phase of Earth’s geological history.

Alum Shale is an unremarkable, grey, thinly-bedded, pyritic mudrock that weathers readily to thin crumbly flakes, the detritus often forming steep talus slopes below the working faces in numerous alum quarries that today reside peacefully along the coast and hills of Cleveland and North Yorkshire. The quarries and boiling houses operated for over 260 years here, commencing around 1600, in the only district in Britain where rock suitable for the important industry of alum-making was, and still is, able to be extracted.

The Tethys Sea supported a diverse fauna of, now mostly-extinct, creatures amongst which can be counted a wide-range of ammonite species, belemnites, fish, and a number of large reptiles including crocodiles, ichthyosaurs and plesiosaurs. At the end of their lives, the remains of these creatures would settle on the sea-floor and occasionally become buried and preserved as fossils. Given the rock’s mode and time of creation, who amongst us could have imagined that the remains of these leviathans, tokens of antiquity from a long lost world millions of years after their lives had ended, would once again see the light of day by way of a quarryman’s hands.

Ammonites achieved their evolutionary zenith during the Jurassic as a result of which some species are only found within a very small stratigraphic range. The usefulness of this to geologists in ascertaining the relative ages of strata was noticed by alum-maker’s son Louis Hunton (1814-1838), who collected data at coastal quarries and made valuable contributions to the young science of biostratigraphy in the 19th century.

The large reptile fossils began to come to light during the 18th century at a time when the science of geology was in its infancy. They provided some of the earliest, best preserved, fossils to be examined by early palaeontologists and found their way to academic establishments across the world. Specimens of these and many more fossils can be seen today on display in Pannett Park Museum, Whitby.

The image above shows Lower Jurassic Alum Shale (grey) making up the lower part of the cliff at Rosedale Wyke, Port Mulgrave. The overlying yellow-brown sandstone belongs to the Middle Jurassic Saltwick Formation.