Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, it becomes marlstone. It is created in marine or freshwater environments. Usually it is found through the activities of algae.
Marl makes up the lower part of the cliffs of Dover. The Channel Tunnel follows these marl layers between France and the United Kingdom. Marl is also a common sediment in post-glacial lakes. An example of this would be the marl ponds of the northeastern United States.