The bottom sediments shown on the map are divided into four main classes based on their mode of formation (genesis).
Large parts of the area mapped are covered by moraine, often with a layer of gravel and cobbles uppermost. Moraine or avalanche deposits often underlie marine suspension and bottom current deposits.
Moraines were deposited on the seabed by glaciers at the end of the last Ice Age, either as ground moraines beneath the glaciers, or as marginal moraine ridges in front of (terminal moraines) or along the sides (lateral moraines) of the glaciers. Moraines consist of a mixture of sediments of varying particle size, ranging from clay to large boulders. Moraine with an uppermost layer of sand, gravel and cobbles is the predominant type of sediment on many banks, and similar material can also be found in some of the troughs that have strong currents. The uppermost layer may be a few centimetres to decimetres thick and represents the outwash layer or an erosion remnant left behind after bottom currents have washed away the more fine-grained sediments. It also contains coarse particles dropped by drifting icebergs at the end of the last Ice Age.
Marine suspension deposits are fine-grained sediments deposited from the water column. This type of deposit indicates quiet sedimentation conditions. Marine suspension deposits are found locally in troughs between the banks and in deeper areas, often directly above moraine material. Pockmarks are mainly found where the bottom consists of marine suspension deposits.
Marine bottom current deposits are dominated by sand transported along the bottom by ocean currents. They are found in places where the currents decrease in strength, often on slopes, in the lee of elevations, and locally as sandbanks on the continental slope, in troughs between the banks on the shelf, and occasionally on the banks. Sand waves and sand dunes are common on the sandbanks.
Avalanche material is deposited by debris slides on the seabed and generally consists of a mixture of particle sizes with poor sorting. The avalanche material is divided into various classes depending on how the sediments are deposited. To some extent, the classification also reflects the degree of detail in the data used during the interpretation. Avalanche material is particularly widespread on the continental slope, where avalanches were a common process during the ice ages.
The classification of the seabed into four main types is performed on the basis of the scales of the interpretation maps (1:20 000 1:50 000) and the degree of detail in the data used during the interpretation and map compilation. Terrain forms, particle size, photographs and video films of the bottom, bottom samples taken with grabs, box corers, sledges and multi-corers, and seismic data were used to interpret the mode of formation.