Dan Roland and Peter Sheldon took a trip to the past in a Cambridge gravel pit for the April 2000 Open University/BBC programme Open Minds.

Dan and Peter visited a gravel quarry near Cambridge to find out where the gravel on your garden path may originally have come from.

The material being quarried here is a layer of sand and gravel which is estimated, from radiocarbon dating of plant remains within it, to be around 40,000 years old. At that time the climate of Britain was very cold and the ground of East Anglia was permanently frozen, like the Siberian tundra is today. Further north in Britain there were glaciers and ice sheets. Spring meltwaters would rush over the frozen ground, carrying sand and gravel and depositing them on the flat plains of East Anglia.

Under this gravel is a thicker layer of grey clays, laid down much earlier in the Jurassic Period, about 160 million years ago. At that time Britain was at the latitude of Greece or Majorca today and was covered by a warm, shallow sea in which mud accumulated. The huge gap in time between these two adjacent layers is due to a long period of erosion of the rocks deposited on top of the Jurassic clays. As is often the case elsewhere, the grey Jurassic clays are delightfully rich in fossils.

Dan and Peter found many fossilised animal remains, as well as examples of all three main types of rock:

Sedimentary rocks - laid down in layers in rivers, on the sea floor, etc.

Igneous rocks - originally molten magma.

Metamorphic rocks - rocks that have been altered by heat and pressure.

Items found in the gravel:

Flint
Most of the gravel pieces are flint, a hard material left behind during the erosion of Chalk, a deposit of fine-grained limestone laid down in the Cretaceous Period about 100 million years ago. The flint was formed within the sediment, and was derived mainly from sponges that acculumated in the soft ooze that formed the sea floor at the time. Being made of silica (quartz), the dead sponges tended to dissolve in the alkaline environment of the sediment around them. The dissolved silica then reacted with dilute sulphuric acid formed when gases coming off decaying organic matter met oxygen coming down burrows from above. The silica started to form a solid again, especially around the animal burrows, which is why flints often have such strange shapes.

Granite
Igneous rock. This kind of rock is found far away from Cambridge, such as in Scotland, and was probably transported first by ice and then by rivers.

Dark banded rock (amphibolite)
A metamorphic rock, formed when great heat and pressure altered some pre-existing type of rock. This isn't usually found in England at all, and was also probably transported from Scotland or even Scandinavia by ice sheets and rivers.  

Limestone
A sedimentary rock, made largely from shells that accumulated on the sea floor, which are still visible in the rock today. It was probably formed in the Jurassic Period.   

Jurassic oyster shell
On the inside of many oyster shells like this one you can still see the scar where muscles originally held the two valves together. Bits of oyster shells like this are very common in gravel because the shells were originally very abundant, and being large, very thick and moderately hard, they have not been readily worn away. 

Rhomb porphyry
An igneous rock, once spewed out of a volcano. It's a fine-grained, purplish rock containing large crystals of feldspar. We can tell from studying detailed features of the rock (such as its crystals and chemical composition) that such pieces must have come from a particular part of Norway, near Oslo - the only place with similar rock outcrops. We can therefore deduce that it must have been transported across the North Sea to Britain by ice. It was later moved by rivers to where it's being dug up now. 

Mammoth tooth
Mammoths were elephants adapted to the cold conditions of the Quaternary Ice Age, which began about 2 million years ago. Most mammoth teeth found in Britain are between 20,000 and 100,000 years old. At any one time, the mammoth would have been using four teeth for chewing its vegetarian diet (one tooth above and one below on each side of its jaw). If it lived to old age, it got through six sets of teeth during its lifetime.

Items found in the Jurassic clay:
Ammonite A spiral shelled, marine animal which was swimming in the sea in the Jurassic Period when the dinosaurs were on land. It is related to today's nautilus and other cephalopods like squid and the extinct belemnites. Unlike the spiral shell of a snail, ammonites had their shell divided into chambers, and the main body of the ammonite lived in the outermost chamber which was open to the seawater. This ammonite is not very well preserved and the shell is also flattened. The greenish-black mineral replacing the shell, and forming irregular growths on it, is iron pyrites. Crystals of pyrites elsewhere are often golden, hence the mineral's nickname - fool's gold. 

Belemnite
This comes from another marine animal from the Jurassic Period, similar to a cuttlefish or squid. The fossilized part was made of the mineral calcite. The bullet-shaped fossil was about a third of the length of the whole living animal and acted as a counterbalance to the weight of the soft tissue surrounding it. Like today's squid, the belemnite probably travelled backwards by squirting water through a funnel near its head. Both belemnites and ammonites became extinct at the end of the Cretaceous Period when so many other animals, including the dinosaurs, died out.

Plesiosaur vertebra
This large vertebra is part of the backbone of a plesiosaur, a huge, carnivorous reptile that was quite common swimming in the seas round Britain in the Jurassic Period. This particular one came from a type of short-necked, large-headed plesiosaur called a pliosaur. The bone is now far heavier than it would have been when the animal was alive because, during the fosilization process, all the spaces in the bone have been filled up with additional minerals.