Carr is a type of ground that is turning from a bog to a wood. When the conditions are right, reeds and other tall plants grow in bogs, and over the years the soil formed from dead reeds and plants accumulates, causing the ground level to rise above the surrounding water level of the bog.

Trees such as willow and ash then colonise the new ground, and they cause the ground to dry out further, to the point where other types of trees such as oak and beech can become established.