Beef Rock
Age: Jurassic – around 195 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 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.
Chert
Age: Cretaceous - around 100 million years old
Composition: Silica and sand
Origin: Top of the cliffs and river
The most common stone on the beach.
Chert is very hard stone composed of silica. Some chert pebbles are sugary like sand and that’s because chert is formed from sandstone. It is usually a sandy or orange colour. We usually don't find any fossils in these rocks.
Flint
Age: Cretaceous - around 100 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 a very hard rock composed of silica and is similar to chert. It is typically black with a white ‘rind’ and is mainly found in Chalk rocks. It forms from the slimy remains of certain microscopic sea creatures that have been chemically changed over millions of years.
Limestone
Age: Jurassic – around 195 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.
Iron Pyrite
Age: Jurassic – around 195 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.
Shale
Age: Jurassic – around 195 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.