By soil types are meant the unconsolidated deposits found above the solid bedrock. Most soil types consist of weathered and fragmented rock material (= mineral soil materials). In addition, there are organic soil types, which have formed from dead plant and animal remains.
The classification of soil types is based on their mode of formation and depositional environment, as well as on their grain-size distribution (texture). Based on mode of formation, a primary distinction is made between transported soils, so-called sediments, and soils that have formed in situ from the underlying bedrock. In transported soil types, sorting into different grain sizes has often occurred. These soils therefore tend to have a relatively uniform grain-size distribution, which is determined by the environment in which the material was deposited. For example, a sediment transported by water is coarser the higher the water velocity was at the time of deposition.
In the formation of Sweden’s mineral soil types, the inland ice sheet and its melting have been of decisive importance. Mineral soil has either been deposited as till (generally unsorted soil material) or as sediment (soil material sorted by water or wind). An important natural boundary in this context is the highest coastline (HK), which marks the highest level reached by the Late Glacial sea. It is expressed in metres above present sea level. Mineral soils may show major differences above and below the HK.
Globally, so-called weathering soils, formed through in-place weathering of bedrock, are the most common. What characterises most Swedish soil types is that they were formed in connection with an ice age and are therefore classified as glacial soils. Soil materials formed after the most recent ice age are referred to as postglacial soils.
Sweden’s glacial soil types
Glacial soil types were formed during periods when the country was covered by ice. A distinction can be made between soils deposited directly from the inland ice and soils deposited from meltwater released from the ice.
Till
Till refers to soils that were transported and deposited directly by the inland ice. They consist of material that the ice detached from the bedrock and abraded to varying degrees. The constituents therefore vary greatly in size. Till types vary strongly at the local scale, which makes a concise geographical description difficult. Below follows a description of some factors that influence the distribution of different tills.
The source material of a till naturally has great importance for its character. Depending on the resistance of the bedrock to glacial abrasion, the till has acquired varying degrees of blockiness and different grain-size compositions. Since till is a mixture of bedrock material from the surrounding area, influences from small bedrock occurrences may be visible over very large areas.
Geographically, the position relative to the highest coastline is important for till composition. The highest coastline, HK, indicates how high the sea level reached during the period following deglaciation. Above the HK, till may have remained largely unchanged since deposition from the ice, whereas below the HK, tills in exposed positions (slopes) have been reworked (wave-washed), with finer grain-size fractions being removed over shorter or longer distances. The remaining coarser material then forms more or less affected, wave-washed till, while the reworked (removed) material has formed various sediments. Topography has often played a major role. At elevated positions in the terrain below the HK, strong wave washing has often occurred, visible as a surface layer of pebbles overlying the till. At elevated positions above the HK, finer-textured tills are instead common, as these tills were often transported longer distances within the ice.
If till is sufficiently strongly wave-washed and reworked, a sediment is formed. Along the HK or just below it, it may be difficult to distinguish wave-washed till from sediment (poorly sorted gravelly sand). The general rule is that the soil material is classified as wave-washed till if the wave-washed layer is, on average, thinner than 0.5 metres. If wave washing has reached deeper, the soil material is classified as sediment. This rule may, however, be difficult to apply in an individual soil pit.
In terrain sections where tills and sediments alternate, tills generally occupy the higher positions, while sediments are located in the lowest-lying parts of the terrain.
The appearance of till may also be linked to the ice-flow pattern in various ways. Examples include the many till landforms that have developed, partly through interaction between the ice and the substrate (drumlins, Rogen moraine and Veiki moraine), and partly at the ice margin (terminal moraines, marginal moraines and De Geer moraines).
Glaciofluvial soil types
When meltwater from an ice sheet transports and deposits soil material, glaciofluvial deposits, or glaciofluvial soil materials, are formed. Because melting ice sheets produced very large volumes of water, these deposits may be very thick.
When water within the ice sheet became concentrated, powerful flows—so-called subglacial rivers—formed, rushing through tunnels beneath the ice. Sand, gravel and stones accumulated at the base of these tunnels, forming features visible today as eskers. Subglacial water was concentrated in the lower parts of the landscape, and most eskers are therefore found in such positions.
Where a subglacial tunnel emerged at the ice margin, material could no longer accumulate as ridges but instead formed different landforms. When the tunnel emerged on land, flat, extensive outwash plains of sandy glaciofluvial material formed in front of the ice. Such formations are known as sandur and are rare in Sweden. When the tunnel instead emerged into the sea, glaciofluvial deltas were formed. These are found mainly around the highest coastline and are referred to as HK deltas.
The soil types described so far are relatively coarse-textured. Finer fractions, such as clay, very fine silt and silt, were instead transported further with the meltwater and deposited under calmer conditions on the seabed beyond the ice margin. An example is glacial clay, which is characterised by its banded appearance (varving). This structure originates from annual variations in meltwater discharge from glacial rivers.
Sweden’s postglacial soils
Postglacial soils, as the name implies, formed after the most recent ice age. This occurred through transport and reworking of glacial soils, primarily by water and, in some places, also by wind.
At the end of the ice age, large parts of Sweden lay below sea level. Material transported by rivers from land could be deposited on the seabed and form marine sediments, which were later exposed through land uplift. In low-lying seabed environments, conditions were calm, allowing the finest soil particles to settle. An example is postglacial clay, which, unlike glacial clay, is not varved. In a similar way, fine-textured soils could also form on lake bottoms as lacustrine sediments.
As sea level gradually fell, the underlying till was exposed to wave action, which accumulated material into so-called wave-washed sediments. These are found especially in areas that first emerged from the sea.
Another type of soil material is fluvial sediments, associated with rivers. As a river flows towards the sea, soil material is eroded where the erosive force of the water is sufficient and redeposited where the flow becomes calmer. In this way, soil material is continuously redistributed depending on flow strength. During this process, grain-size sorting occurs, resulting in deposits with very uniform texture. During periods of very high discharge, for example during spring floods, rivers may overflow, forming overbank sediments along their margins.
Water-sorted sediments are usually deposited in a way that produces smooth and level terrain; an exception is esker gravel, which often forms pronounced ridges.
Wind-deposited sediments in Sweden mainly consist of aeolian sand dunes, either active or fossil. Aeolian sediments consist primarily of sand and silt, partly because finer material is removed and partly because wind is unable to transport coarser material. Fossil dunes formed mainly on glaciofluvial deltas, where large amounts of sand were available and where katabatic winds from the ice sheet promoted dune formation. Active dunes are forming today in coastal areas and in sandy soils that have lost their protective vegetation cover.
Note that both water- and wind-deposited sediments may also occur above the HK (for example, water-laid sediments deposited in ice-dammed lakes).
Organic soils have a relatively wide distribution in Sweden. They consist largely of peat soils, which are mainly found in wetlands. These have formed through lake infilling or in locations where wet conditions have favoured the growth of mosses and sedges, for example where water has continuously flowed along a slope.