The Geological Story of Creswell Crags
By Lizzy Peneycad (Creswell Crags volunteer and researcher)
Creswell Crags is renowned for its important record of Ice Age mammals and Palaeolithic hunter-gatherers. However, the history of the site extends back much further than the Last Ice Age. Creswell’s story began in a time before the dinosaurs had evolved, when the continents were joined into the single landmass of Pangaea, and when Britain lay at the edge of a subtropical sea.
Creswell during the Permian Period
During the Late Permian period, between around 272 and 252 million years ago, Britain was located in the subtropics, on the western shore of a warm and highly salty body of water known as the Zechstein Sea. This sea extended all the way from eastern England to Poland. The shallow margins of the sea were home to an abundance of organisms that use calcium carbonate to construct their external skeletons, such as reef-building corals and marine molluscs. Over millions of years, the remains of these organisms accumulated on the shallow shelf of the Zechstein Sea to form limestone.
It is thought that soon after the limestone formed, it was infiltrated with seawater that was rich in magnesium. This led to an alteration in the mineral structure of the rock through a process known as dolomitisation. This process is where the calcium within limestone is partially replaced by magnesium, resulting in the crystallisation of a carbonate mineral called dolomite. Geologists refer to the dolomite-rich rock created through this process as dolostone.
The Late Permian dolostone formed in the Zechstein Sea occurs today as a sequence of rocks known as the Cadeby Formation (formerly known as the Lower Magnesian Limestone). The Cadeby Formation extends from Nottinghamshire in the south, to Northumberland in the north, as a linear strip of bedrock that passes directly through the Creswell area. It is in this bedrock that the famous Creswell Caves can now be found. But how were these caves formed?
The formation of Creswell’s Caves
There are three key factors that have contributed to the formation of Creswell’s Caves. The first factor is the chemistry of the bedrock. The carbonate minerals that occur within dolostone, namely calcite and dolomite, can be dissolved by a weakly acidic solution. Normal rainwater is slightly acidic, with a pH of between 5 and 6. This means that the action of rainwater on dolostone will result in the gradual weathering of the rock over time.
The second factor is the structure of the rock. Creswell Crags is situated in a section of dolostone that has shifted vertically through movement along fractured surfaces in the rock, known as faults. These faults lie at the western and eastern ends of the present-day Creswell gorge. Movement along the faults caused a large number of smaller fractures (joints) to develop within the rock. These joints act as weak points through which rainwater can enter and penetrate the bedrock. Over time, the rainwater gradually dissolves the rock along the joints causing them to enlarge into fissures and caves. Fractures are especially prominent in the western half of Creswell gorge, and this is where the majority of the caves occur today.
The final factor is the release of water from the melting of a large ice sheet. Between approximately 478 and 424 thousand years ago, during a cold period known as the Anglian Glaciation, ice covered much of the British Isles. As the ice retreated, large quantities of meltwater were released into rivers, which then carved into the underlying bedrock. Creswell gorge was created through this process, as a tributary of the River Poulter incised into the uplifted section of dolostone. This incision exposed fissures and caves along the walls of the gorge, enabling them to become filled with sediment once they were drained of water. Evidence from Pin Hole Cave suggests that there may have been several cycles of sediment deposition and erosion within the caves between around 200 and 65 thousand years ago, before they were eventually occupied by animals and Palaeolithic people during the Last Ice Age.
During their occupation of Creswell’s Caves, Palaeolithic hunter-gatherers left abundant evidence of their activities, including the use a variety of stone tools. But how do the materials that these tools were made from relate to the geology of the Creswell area?
The raw materials for Palaeolithic tool-making
The majority of the Palaeolithic stone tools discovered at Creswell Crags are made from two different raw materials: quartzite and flint. Palaeolithic people didn’t use the local dolostone to make their tools, as for obvious reasons, a rock that can easily fracture and dissolve in rainwater is unlikely to make a reliable implement! So where did the quartzite and flint come from?
At Creswell Crags, quartzite was only used as a raw material for tool-making between around 50 and 40 thousand years ago, during the late Middle Palaeolithic. Neanderthals made these quartzite tools using rounded stones, originating from Lower Triassic pebbly sandstone deposits. These deposits, formerly known as the ‘Bunter pebble beds’ (now the Chester Formation), can be found only 5 km to the east of Creswell Crags. Quartzite was therefore a readily available local resource that Neanderthals could use to make their tools.
Flint was also used as a material for tool-making during the Middle Palaeolithic at Creswell Crags, although flint tools from this period are relatively uncommon. However, by the time of the Upper Palaeolithic, flint became the only material used for stone tools at the site. The flint flakes found in Creswell’s Caves have a distinctive light grey colour. This colour is typical of the flints that occur within the Late Cretaceous Chalk of the North Lincolnshire and Yorkshire Wolds. This chalk is located more than 100 km northeast of Creswell Crags, and so it seems unlikely that Upper Palaeolithic people sourced flint directly from the bedrock. Instead, it has been suggested that flint from northeast England was brought to the Midlands area through transport by glaciers during the cold periods that preceded the Last Ice Age. A local source of flint is supported by the discovery of natural flint fragments in Pin Hole Cave by Leslie Armstrong during his excavations in the 1920s.