Gley soils represent the most extensive soil cover in Scotland. These soils are found on gentler slopes or in areas of high rainfall where the water does not drain away readily.
All the glacial tills of central Scotland are dominated by gley soils.
Peaty gley soil is waterlogged for all or most of the year. This waterlogging denies the soil the oxygen that the soil organisms need to survive.
The organisms left in the soil extract the oxygen they need to survive from the iron compounds and the soil gradually turns grey, blue or green as the oxygen is depleted.
If only the surface is badly drained, mainly in spring melt water areas, the soil is called a surface water gley.
If the water permeates the soil all year, it is called a ground water gley. If construction work in urban areas disrupts the soil drainage it is called an urban gley.