Chert

Age: Cretaceous ~100 million years old

Composition: Silica and sand

Origin: Top of the cliff and river

The most common stone on the beach.

Chert is very hard stone made of silica. Some chert pebbles are sugary like sand and that’s because Chert is formed from sandstone.

Flint

Age: Cretaceous 100 – 65 million years old

Composition: Silica

Origin: Eroded from Chalk hills inland and brought down by the river

Can take strange shapes resembling fossils

Flint is another very hard silica rock. It is typically black with a white ‘rind’ and is only found in Chalk rocks. It forms from the slimy remains of certain microscopic sea creatures.

Limestone

Age: Jurassic ~ 185 million years old

Composition: Calcium Carbonate and mud

Origin: From thin beds of Limestone within the cliff

Limestone can contain beautiful fossils preserved in three dimensions

Limestone usually forms in warm shallow seas where dissolved Calcium Carbonate is concentrated in the seawater by evaporation. It is a hard stone and can form pebbles and boulders on the beach. Some limestone is almost pure Calcium Carbonate, like Chalk, which is white. Limestone at Charmouth is grey because it contains mud.

Shale

Age: Jurassic ~185 million years old

Composition: clay or mud and very fine sand or silt

Origin: Mud layers in the cliff

The name Shale has a specific meaning in geology but it is really just a type of mudstone.

Shale is usually less sticky than other mud rocks because it has some coarser grained material in it like fine sand or silt. Geologists identify silt from mudstone by grinding a little between their teeth! If it’s slightly gritty then its shale!

Iron Pyrite

Age: Jurassic ~185 million years old

Composition: Mineral made of Iron and Sulphur

Origin: From the layers of mud in the cliff

Iron Pyrite is also known as ‘fools gold’ because of its shiny golden colour.

Pyrite or ‘fools gold’ is a diagenetic mineral, which means it formed AFTER the layers of sediment had been buried. The crystals grew underground within the mud layers to form nodules of pyrite. The iron in the nodules causes them to take a rusty brown colour very quickly once exposed to the air but fresh shiny golden pieces are quite easy to find.

Beef Rock

Age: Jurassic ~185 million years old

Composition: Calcite and mud (crystal form of calcium carbonate)

Origin: From the layers of mud in the cliff

Beef rock gets its name because it looks a little like beef steak.

Beef rock is another diagenetic mineral. It forms when calcite crystals grow like fibres usually forming flat slabs in the layers of mud. The slabs are easily broken up to pebble sizes on the beach. You can see the fibres of calcite in the edges of these flat pebbles.