Usually this layer is a soluble carbonate rock, such as limestone or its purer form, chalk.

BBC: Who, What, Why: How are sinkholes formed?

Florida lies on bedrock made of limestone or other carbonate rock that can be eaten away by acidic groundwater, forming voids that collapse when the rock can no longer support the weight of what's above it.

CNN: Sinkhole eating family out of house and home

The state lies on bedrock made of limestone or other carbonate rock that can be eaten away by acidic groundwater, forming voids that collapse when the rock can no longer support the weight of what's above it.

CNN: Home of Florida man devoured by sinkhole to be demolished

The second sample is a piece of dolomite (a rock made of calcium magnesium carbonate).

ECONOMIST: Meteorites

Beachrock: ledges of rock formed within the body of the beach consisting of sand grains cemented by calcium carbonate and formed by chemical processes.

UNESCO: L O S S A R Y