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Your rock is probably a specimen of limestone. Limestone
can be cream, white or grey and often has small bits of fossil shells in it. It
formed when limey mud was deposited in warm shallow seas, often around coral
reefs. The high abundance of animals on the coral reefs resulted in lots of bits
of shell and coral being present in the limey mud which was eventually squashed
into rock. Limestone is forming today in parts of the Caribbean. Limestone from
the lower Carboniferous period (approximately 320 million years ago) can be found in the
Derbyshire Peak District.
Oolitic limestone can also be found in some British rocks. It is made up of lots
of small (1 -2mm) cream-coloured spheres which formed when grains of
calcite were rolled around on the sea bed by gentle currents and built up layers
of limey mud. The spheres, called ooliths, were then squashed together to form
rock. Limestone is often used as a building material, but it can be slowly
dissolved by acid rain.
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