College Lake Geology

A former chalk quarry, College Lake is now one of BBOWT's flagship nature reserves. Read about the site's industrial heritage, geology and fossil finds.

 
Author: Neil F. Adams, Curator, Fossil Mammals, Natural History Museum, London.

Industrial heritage

The Tunnel Portland Cement Company began quarrying chalk at the Pitstone Quarry site in 1937. Each year it produced roughly 100,000 tonnes of cement, which required about 190,000 tonnes of chalk. In 1968, Tunnel Portland Cement Company became Tunnel Cement. Production peaked at ~950,000 tonnes per year in the early- to mid-1970s. Later in 1986, Tunnel Cement was re-branded as Castle Cement after merging with several other cement companies. Castle Cement continued quarrying at the site until 1991.

College Lake Nature Reserve is what is left of Quarry 3, where quarrying began in 1969. During quarrying of the chalk in this area of the site, quarry workers came across some very different rocks and sediments cut into the chalk. These were ice age (Pleistocene) river channels, which contained many large fossils, and periglacial slope deposits.

The site was designated a Site of Special Scientific Interest (SSSI) in 1976 for these ice age river channel and slope deposits, as well as the fossils they contained. Because the planning permissions for the quarry had been granted before the site received protected status, quarrying was allowed to continue. Nevertheless, a collaboration between English Nature (now Natural England) and the Tunnel Portland Cement Company ensured some of the scientifically important, fossil-rich channel was conserved as a promontory (the Mammoth Promontory) that would not be quarried.

Thanks to the efforts of Graham Atkins, a former lorry driver at the cement works, the quarry manager was persuaded that Quarry 3 could be restored in a more environmentally friendly way by creating a nature reserve. This eventually led to the development of College Lake Nature Reserve. Originally, Quarry 3 was destined to be restored to agricultural land after quarrying operations had finished. With a small team of volunteers, Graham worked with George Goddard (former general manager of Castle Cement) to transform the quarry. Graham wrote a book entitled ‘Creating a Nature Reserve: The Story of College Lake’ that was published in 2013 about the site’s journey from quarry to nature reserve. The site is now managed by BBOWT and contains diverse habitats including the lakes, woodland and chalk grassland.

The chalk

The oldest rocks exposed at College Lake Nature Reserve date to the Cretaceous, when dinosaurs dominated life on land. The Cretaceous rocks at College Lake are around 100–95 million years old from a period in the Late Cretaceous known as the Cenomanian. During the Cenomanian, large parts of the UK were covered by a relatively shallow ocean, including the area that is now College Lake. The Cenomanian rocks in the nature reserve are chalks. These rocks are soft and greyish and, fittingly, they belong to the Grey Chalk Subgroup – a large group of rocks that can be identified across the country.

The oldest chalks, found at the bottom of the old quarry, belong to the West Melbury Marly Chalk Formation, and above these are somewhat purer, whiter chalks belonging to the Zig-Zag Chalk Formation. The lower rocks are called ‘marly’ chalks because they contain more clay, which gives them their characteristic grey colour. These clays were probably washed into the sea from the nearby land. As sea level rose in the Cenomanian, the distance to the nearest land increased. Less clay reached the area, so the chalks of the Zig-Zag Chalk are purer and contain more calcium carbonate. The calcium carbonate in the chalk derives from the hard skeletons of various types of tiny plankton that once lived in the ocean.

Fossils have been found from the chalk, particularly from the marly chalks that were deposited in shallower seas. The fossils include remains of sea creatures such as ammonites, sea urchins, sea lilies, starfish, bivalves (a group including clams, oysters, and mussels), crustaceans (including lobsters and crabs), sea snails, sponges, and sharks. Some plant material, probably washed into the sea from the nearby land, can also be found as pieces of driftwood and as associated small fragments.

The Ice Age

On top of the chalk around College Lake are sediments that date to the Pleistocene (also known as the Ice Age). The Ice Age began around 2.5 million years ago and ended when the current warm period that we still live in began 11,700 years ago. During the Ice Age, there were repeated shifts in climate between cold periods (known as ‘glacials’) and warm periods (known as ‘interglacials’). These shifts were caused by cycles in the Earth’s orbit. Glacials occurred when the Earth was furthest from the Sun and received least solar energy. Interglacials occurred when the Earth was closest to the Sun and received most solar energy.

The Anglian glaciation

The most severe glacial period of the Ice Age (the ‘Anglian’ glaciation) occurred in Britain around 450,000 years ago. A thick ice sheet covered much of the UK during this time and extended far to the south (see blue dashed line on image below). It is now thought that the margin of this ice sheet was only tens of kilometres to the north of College Lake. The ice sheet transported rocks and sediment from hundreds of miles away into the Vale of Aylesbury.

The far-travelled rocks and sediments transported by the Anglian ice sheet can be found in several river channel deposits that were discovered in the Pitstone Quarry during the 1970s. These channels had cut into the underlying chalk and had deposited the sands and gravels flowing through them.

A series of infilled river channels are preserved in the Pitstone Quarry SSSI and are believed to date to a warm interglacial period 200,000 to 250,000 years ago.

At the start of this warm period, springs developed near the foot of the Chiltern Hills. These springs deposited tufa – a type of limestone that forms when carbonate minerals precipitate out of water.

Excavations through the river sediments recovered nearly 12,000 fossils, including bones and teeth of many different animals. Fossils of plants, slugs and snails, beetles and other invertebrates have also been found. All these fossilised animals and plants allow a detailed insight into the environment at College Lake around 200,000 years ago.

The 150,000-year-old glacial period

After the lower channel sediments were deposited, the climate cooled dramatically into the next glacial period. Another ice sheet developed across Britain during this time, but was located much further north than the Anglian ice sheet.

At this time, around 150,000 years ago, periglacial processes were affecting the area around College Lake. The intense cold froze and cracked the chalk bedrock (by a process known as thermal contraction) and strong winds deposited wind-blown sand across the region. The broken chalk and other pre-existing sediments slumped off the Chiltern Hills as mass movement deposits (known as ‘coombe rock’). These coombe rock deposits can be found across the College Lake site.

Rivers, fuelled by seasonal snowmelt, eroded and reworked existing sediments into sand and gravel channel deposits. In some of the deposits, infilled ‘ice-wedge’ casts can be found. These wedges formed by the thermal contraction and cracking of the ground. Water infilled the cracks and upon freezing the water expanded, widening them. This process could repeat itself many times, enlarging the size of ice wedges to many metres. Upon melting, sediment could infill the voids that were previously filled with ice, leaving behind a wedge infill or cast.

Although few fossils have been found in deposits of this glacial period at the site, a number of mammal fossils were discovered, including:

• Red fox (Vulpes vulpes)
• Grey wolf (Canis lupus)
• Woolly mammoth (Mammuthus primigenius)
• Wild horse (Equus ferus), a small-bodied form
• Wild cattle (Bos or Bison)

A reduced body size appears to be a characteristic of horses from this cold period across the UK. Small-bodied horses of this age are also known from Somerset, Sussex, and Wales. The decrease in body size has been suggested to reflect the adaptation of horses to stressful climatic and vegetation conditions during this cold, glacial period.

The following plant species were also recovered from sediments immediately above the lower channel as microfossils of pollen and spores:

• Lesser clubmoss (Selaginella selaginoides)
• Common clubmoss (Lycopodium clavatum)

These plants may reflect the cold climate that prevailed after the deposition of the lower channel. Today both clubmosses are found in mainly mountainous regions of northern Europe.

When the climate started to warm again, around 130,000 years ago, the permafrost below ground began to melt. This filled the periglacial sediments, including the coombe rock, with water. There is a difference in density between (1) the coombe rock and (2) the sands and other sediments above it. When the coombe rock became wet and saturated with meltwater, the heavier, denser sands were able to sink into the finer-grained, less dense coombe rock, while the coombe rock was forced upwards into the sands. This process deformed the original layers of sediment, creating warped features known as involutions.

Some potential ways to explain how involutions form include:

• Lava lamps, where the ‘lava’/wax moves around based on differences in densities caused by heating.
• Sinking in quicksand, where a dense object sinks into saturated, wet sand and displaces the sand below, forcing it upwards (perhaps the best analogue for involution formation).
• Feet sinking into mud on wet mudflats (e.g., muddy beaches). When dry, a person can stand on the mud without sinking, but the adding of water (e.g., by incoming tides, etc.) initiates sinking due to density contrasts. Toes sink into the mud and mud is forced up between the toes (similar to sinking in quicksand).
• Something frozen at the base (e.g., ice cream, analogue for permafrost), topped with soft ice cream (e.g., Mr Whippy, analogue for the melted active layer that is like sludge) topped with biscuits which sink into the soft ice cream (analogue for sediment sinking).
• Marble cake – does not imply process, but might be good for visualising the mixing of two different materials, with similar deformation-like structures.

The 125,000-year-old interglacial

What makes the sediments at College Lake unique is that they preserve river deposits from two warm interglacial periods: one around 200,000 years ago and one around 125,000 years ago.

Another river channel, known as the ‘upper channel’ was found cut into the periglacial deposits in an area of Pitstone Quarry that has now been completely excavated away. Because the sediments in this channel were on top of the cold-climate periglacial sediments, they must have been younger and are likely to represent the period of warm interglacial climate 125,000 years ago. A rescue dig through the upper channel was able to recover an important assemblage of fossil mammals, including:

• European water vole (Arvicola terrestris cantiana)
• Straight-tusked elephant (Palaeoloxodon antiquus)
• Narrow-nosed rhinoceros (Stephanorhinus hemitoechus)
• Common hippopotamus (Hippopotamus amphibius)
• Red deer (Cervus elaphus)
• Fallow deer (Dama dama)
• Giant deer (Megaloceros giganteus)
• Steppe bison (Bison priscus)

These mammals suggest warm conditions prevailed, with a mosaic of woodland and grassland habitats. Freshwater aquatic environments are inferred for hippopotamus. The grazing activities of large mammals (hippopotamus, straight-tusked elephant, narrow-nosed rhinoceros) are likely to have maintained much open ground.

The apparent lack of human fossils or archaeology from the upper channel is unsurprising because it is believed that humans were largely or entirely absent from Britain during this interglacial.

Collections – fossils and archives

A relatively small number of mammal fossils from the upper channel (~125,000 years old) is held in the collections of the Natural History Museum in London. The majority of the fossil collections from the site, as well as archive documents, are held in the Bucks County Museum at the Museum Resource Centre in Halton, Buckinghamshire. A collection of fossils from the chalk and ice age deposits are kept on site. The ice age fossils include:

• a bear canine (probably brown bear, Ursus arctos)
• a lion vertebra (probably lion, Panthera leo)
• a horse astragalus and calcaneum (ankle and heel bones), phalanges (toe bones), and teeth (probably wild horse, Equus ferus)
• mammoth molars (probably woolly mammoth, Mammuthus primigenius)
• a wild cattle molar (probably Bos or Bison)
• several smaller ‘carnivore’ teeth (possibly red fox, Vulpes vulpes)
• a possible rhino tooth (possibly narrow-nosed rhinoceros, Stephanorhinus hemitoechus)
• some small mammal remains, including phalanges/toe bones (possibly rabbit/hare, indeterminate Leporidae, or voles, indeterminate Arvicolinae)

These fossils were found during small works on site, e.g. during fence post installation, as well as from washouts and some were found during the original excavations.