Copenhagen rests on a subsoil of flint-layered limestone deposited in the Danian period some 60 to 66 million years ago. Some greensand from the Selandian is also present. There are a few faults in the area, the most important of which is the Carlsberg fault which runs northwest to southeast through the centre of the city. During the last ice age, glaciers eroded the surface leaving a layer of moraines up to  thick.