Trent & Peak Archaeology / The University of Nottingham Nottingham Castle Sandstone - the geological secret of the caves All of Nottingham’s caves are cut in to a bedrock of sandstone known as Sherwood Sandstone. This sandstone spreads immediately north of the Trent floodplain, covers much of Nottingham city centre and is subdivided in to three main rock types. The Lenton Sandstone is a fine grained sandstone, about 30m thick and is the oldest and lowest of the units. Above this is the coarser Nottingham Castle Sandstone which is around 60m thick.  Towards the east of Nottingham city centre lies the Mercia Mudstone which overlays the Sherwood Sandstone. This is a much younger outcrop and consists of much finer sandstones. The majority of Nottingham’s caves are cut in the Nottingham Castle Sandstone. This sandstone is beige in colour and consists mainly of medium to coarse grains of quartz sand. Within this are thin layers of pebbles and quartzite flakes.  All of these sandstones were formed during the Triassic period approximately 245 million years ago. Around this time the sea advanced and retreated across the midlands many times depositing sands and pebbles which were subsequently buried and cemented underneath a thick layer of mud and silt.  The physical properties of the Sherwood Sandstone make it ideal for excavation and therefore for producing caves. The Sherwood Sandstone consists of millions of tiny grains of sand which are loosely cemented together by a weak clay.  Between the grains are lots of pores which retain substances such as water, oil and gas. The sandstone is described as weak due to a weak clay cement between the quartz grains. The strength of the rock is also greatly reduced when it becomes saturated.However, a notable lack of fractures and bedding planes within the sandstone ensures the structural stability of cave roof spans.  The softness of the sandstone makes it easy to excavate with hand tools, and the structural stability means that excavated caves are safe to use, even with buildings above them. The exposed cliff of the sandstone outcrop made this an obvious place for the early citizens of Nottingham to make their home. Find out more about Geology The British Geological Survey (BGS) was founded in 1835 and is the world’s oldest national geological survey and the UK’s premier centre for earth science information and expertise. You can learn more about the geology of Nottingham and the rest of the country by visiting the BGS website