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Dominating soil parent material class
Fine-grained till
Medium-grained till
Coarse-grained till
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Medium-grained sediment
Coarse-grained sediment
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Bedrock outcrop
Gyttja
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17/3/2006

Soil parent material

Soil parent material comprises information on genesis and grain-size distribution of the parent material for soil formation. Mineral soil either was deposited as unsorted material, till, or was sorted when transported by water or wind before being deposited as sediment. Till is made from a mixture of grain sizes from boulders to clay. Stones normally are angular, whereas those of the sorted material are rounded by abrasion.


Dominating soil parent material class Fine-grained till Medium-grained till Coarse-grained till
Fine-grained sediment Medium-grained sediment Coarse-grained sediment Peat
Bedrock outcrop Gyttja

The unconsolidated soil layer (regolith) is according to the way it was formed, classified into different types of parent material for soil formation. Type of parent material is assessed in the dug trial pit at the same depth as for the assessment of texture (variable Texture).

Soil parent material is classified according to:

Code

Soil parent material

(1)

Sediment with high degree of sorting (gyttja included)

(2)

Sediment with low degree of sorting

(3)

Till

(4)

Bedrock outcrop

(5)

Peat

 

Soil textural classes describe grain size distribution in the mineral soil, and in the first place refer to the dominating grain size within the fraction with a diameter < 20 mm (quadratic mesh).

Code

Sediment/Till/Others

       

(0)

-/-/Boulders in the pit

       

(1)

Shingle and stone/Boulder rich and stony/Bedrock outcrop

 

>

20

mm

(2)

Gravel/Gravelly /-

20

-

2

mm

(3)

Coarse-grained sand/Sandy/-

2

-

0.6

mm

(4)

Medium-grained sand/Sandy fine sandy/-

0.6

-

0.2

mm

(5)

Fine sand/Sandy fine sandy /-

0.2

-

0.06

mm

(6)

Coarse silt/Coarse silty/-

0.06

-

0.02

mm

(7)

Fine silt/Fine silty/-

0.02

-

0.002

mm

(8)

Clay/Clayey/Gyttja

<

0.002

mm

(9)

-/-/Peat

       

Maps were produced by merging and combining classes for soil parent material and texture according to the following. Bold letters here indicate dominance of this fraction:

Class

Soil parent material

Texture

Fine-grained tills

(3) Till

(6) coarse silty
(7) fine silty
(8) clayey

Medium-grained tills

(3) Till

(4) sandy fine sandy
(5) sandy fine sandy

Coarse-grained tills

(3) Till

(0) boulders in pit
(1) boulder rich and stony
(2) gravelly
(3) sandy

Fine-grained sediments

(1) Sediment with high degree of sorting
(2) Sediment with low degree of sorting

(6) coarse silt
(7) fine silt
(8) clay

Medium-grained sediments

(1) Sediment with high degree of sorting
(2) Sediment with low degree of sorting

(4) medium grained sand
(5) fine sand

Coarse-grained sediments

(1) Sediment with high degree of sorting
(2) Sediment with low degree of sorting

(1) boulder rich
(2) gravel
(3) coarse-grained sand

Bedrock outcrop

(4) Bedrock outcrop

Peat

(5) Peat

When assessing soil parent material type both the way the soil was formed (genesis) and the grain size distribution (texture) is considered. For the development of minerogenic soil parent material, the Inland ice and the melting process were of decisive importance as well as the development of the Baltic Sea. Mineral soils either were deposited as unsorted material, till, or were sorted when transported by water or wind before deposited as sediment. Till is made from a mixture of grain sizes (grades) where stones and gravel normally are angular, whereas clasts of the sorted material normally are more rounded by abrasion. Within the major groups of minerogenic deposits, glacial and postglacial, grain size distribution varies strongly.

The texture describes grain size distribution in the mineral soil, and in the first place refers to the dominating grain size within the fraction with a diameter < 20 mm (quadratic mesh).

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Quaternary deposits of Sweden

The soil parent material in Sweden was formed during the youngest period of the Earth's history, the Quaternary period, which started 2-3 million years ago. This period is characterised by impacts of vast ice masses, Inland ices, with thickness up to 3,000 metres, periodically covering large parts of e.g. Northern Europe. Between times with glacial climate there were ice-free warmer interglacial intervals, similar to the current climate or warmer. The latest ice age started c. 115,000 years ago and during this glacial time and the following post-glacial time, nearly all unconsolidated deposits (regolith) were formed. When the Inland glacier had its largest extension, around 23,000 years ago, Scandinavia was covered by an ice cap of a couple thousand metres thickness. At that time the ice front reached its maximum position in western Denmark, northern Germany and Poland. Around 16,000 years ago the deglaciation of the southernmost parts of Sweden had started. The distribution between land, water and ice changed continuously depending on the melting Inland ice, the isostatic land uplift and the changes of the sea surface (eustatic movement). Around 10,000 years ago the southern part of Sweden was free from ice and to a large extent covered by the sea. Later, after 3,000 years i.e. 7,000 years BP, almost the whole ice cap had vanished. Minerogenic deposits left by the Inland glacier and its melt water are now covering the bedrock and this regolith is making up the parent material for soil formation. The Inland glacier, which moved like a slowly flowing mass over the sub glacial bed removed older soils, crushed and abraded the bedrock and deposited the mixture as till, a badly sorted or well graded sediment. The water from the melting ice accumulated into cracks and tunnels within and under the ice. These water streams transported and sorted the material that melted from the ice, particles of all sizes from large boulders to fine clay. Depending on the transporting capacity of the water the coarse particles were deposited within ice tunnels or outside the ice front as glaciofluvial deposits. The fine material was suspended in the water and deposited at some distance from the ice front as glacial clay or silt.

Parts of the Swedish regolith were formed after the deglaciation as postglacial sediments and this formation is till going on. Sand, silt and clay are transported by rivers and are deposited as fluvial or alluvial sediments. Clay and gyttja make up marine or lacustrine deposits. Organogenic deposits accumulate from decaying vegetation more or less in situ. Sediments that are formed in place, without transportation are said to be sedentary. Examples are gravel formed by disintegration of the underlying rock (alvar ground) or peat formed by the accumulation of organic material.

Much neoformation of regolith is going on along recent beaches and coastlines. Gravel and sands are eroded, transported and deposited by wave action and intense winds transport and build sand dunes. Another phenomena very much characterising Swedish minerogenic deposits is explained by the ongoing land elevation and subsequent development of different stages of the Baltic Sea. Ancient raised beaches with sorted material and even shingle terraces are found on high altitudes above the recent sea level. The highest located shoreline markings are called the highest shoreline (HK). These are located at different altitudes in different parts of Sweden depending; for example, on how large the isostatic uplift has been in the area and when it was deglaciated. This HK is said to be metachronous, meaning that it was not formed at the same time for the whole of Sweden. The altitude for the HK is an important boundary in the landscape for soil formation. This will be explained further on.

By soil parent material type is understood unconsolidated deposits (regolith) found on the bedrock. Most Soil parent materials are made up from weathered and disintegrated rock material. In addition there are organic soil material types that have developed from plant and animal remnants.

Classification of soil parent material types is based on the way soils were formed, their environment and their grain size distribution, i.e. texture. Depending on the way soils were formed (genesis), two types are recognised: soils formed from transported material, so-called sediments, and soils formed "in situ" from the bedrock by weathering. Transported material has most often also been sorted into different grain sizes. These soils therefore have a relatively uniform grain size distribution, determined by the environment where it finally was deposited. Sediment transported by water is, for example, coarser with higher water speed at the locations where the material was deposited.

Characteristic for most Swedish soil parent material types are that they were formed during glacial and interglacial times and are therefore considered glacial soil parent material types. Globally, soils formed through weathering are the ones most common.

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Classification of the regolith

The classification of Swedish regolith is based on the mode of formation and the environment. The two major groups are glacial and postglacial deposits. Glacial deposits have been formed directly from the Inland ice or the melt water from the ice. Postglacial deposits are formed chiefly by redeposition or neoformation after the deglaciation.

Thus the terms glacial and postglacial are used in the context of formation and environment, but not as strict chronological concepts. Beside these minerogenic deposits there are also organogenic deposits, in terrestrial systems often being peat.

Another basis for classification of minerogenic deposits is the grain size distribution. The vast majority of soils are composed of rock fragments (clasts) and mineral grains of different sizes. Historically the scale by Atterberg has been used but recently this grain size scale has been updated according to international standards. The earlier used Swedish terms "grovmo", "finmo" and "mjäla" has been replaced by a ternary differentiation, where "grovmo" is replaced by fine sand. Thus there are three classes of sand: coarse sand; medium sand and fine sand, comprising material 2.0-0.06 mm. In a similar way, "finmo" and "mjäla" make up silt, with the classes: coarse silt (finmo); medium silt and fine silt, comprising material 0.06-0.002 mm in size. The mesh of the sieves design is quadratic and called free mesh size, contradictory to the original circular holes. Also the boundaries for gravel have been changed but in this case an upper boundary at 20 mm is still used.

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Glacial deposits

Till

Till is composed of material transported and deposited directly by the Inland ice. Most of the till material is reworked older sediments and residues from a weathered bedrock, taken up by the Inland glacier. Some of the materials were also detached from the underlying bedrock by corrasion and more or less abraded by the ice. The components therefore vary strongly in size and characters. Locally there is a large variation in till type and a brief geographical description is therefore difficult to make. The most important properties could be summed up in the following sentences. Till is made up of all grain sizes (grades) and the quantitative proportions are strongly variable. It is a mixture of well-graded material without layering. The clasts are mostly angular. Elongated clasts are oriented parallel to the direction of the ice movement and they bear witness of the ice movement direction. A glacier or an Inland ice has deposited it. Below is a brief description of some of the factors influencing the distribution of various tills.

Of great significance for the final character of a till is of course the parent material and mode of deglaciation. Depending on the bedrock resistance to ice abrasion, the abundance of boulders in till and grain size distribution varies strongly. Because till is a mixture of old regolith and bedrock material, contributions from distant bedrock can be seen but chiefly the local bedrock material dominates. Of paramount importance is the relationship between the retreating ice and the highest shoreline (HK, Högsta Kustlinjen , in Swedish). Due to the pressure from the Inland ice, the surface of the Earth’s crust was pressed down. A consequence of this was that lowlands were submerged during the deglaciation and marine conditions prevailed in southwestern Sweden while fresh water or water with low salinity occurred in the Baltic basin up to the Bothnian bay. The HK is defined as the altitude where the highest located shoreline markings are found. These are located at different altitudes in different parts of Sweden depending; for example, on how large the isostatic rebound or uplift has been in the area and when it was deglaciated. When the ice retreated from an environment situated below the HK, the mode of deglaciation is called sub aquatic. This mode is characterised by a steep retreating ice front rising from the sea surface and where icebergs are detached (calved) from the front of the glacier into the sea. At the sole of the glacier, till is smeared to the bedrock and thus forming basal till or lodgement till. This is a very firm type of till. In an environment above the HK the deglaciation mode is said to be supraaquatic or subaerial. In this case the melting of the ice appears from the surface, and the ice is down wasting like a collapsing soufflé. In this case the till material is enriched on the surface of the ice, resulting in an ablation till. This is loosely consolidated rock debris, formerly in or on a glacier that accumulated in place as the surface ice was removed by ablation.

Another concept worth mentioning characteristic for the zone of the highest shoreline is the reworking by wave abrasion that can occur after the till has been deposited. Above the HK the till in principle is not influenced by any wave action from the sea after it was deposited from the ice, whereas below the HK, the fine-grained materials could have been removed from the surface layer by wave washing. Moreover topography may have had a significant influence. At exposed areas, there has often been a strong surge, seen by a superficial layer of residual boulders or shingle on top of the till. The residual material thereby is a more or less affected, wave washed till, while the finer redistributed material form washed out sediments. In such places it should be possible to find primary deposited, unaffected till under a cover of water-sorted sediments. Above the highest shoreline in central parts of the country more fine-grained tills are often found, because these tills were often transported long distances inside the ice before deposition. This is not always true because the character of the texture is very dependent on the mode of deglaciation and what type of till there is.

There may also be a relationship between till appearance and ice movement patterns. Many morphological characters are developed, partly as a result of the interaction between the ice and the basement ("drumlins", "Rogen - and Veiki-moraines", "Kalix-till", "leeside moraine"), partly at the front or the sides of the ice ("terminal moraines", "marginal moraines","de Geer moraines", "lateral moraines"). In this case it is relevant to talk about a variety of moraine types.

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Glaciofluvial deposits

Glacial coarse-grained sediments

Glaciofluvial deposits are formed when melt waters from the Inland glacier transport and deposit materials. Since the melting inland glacier produced huge amounts of water these deposits may be quite voluminous. The melt water was concentrated into strong streams in cracks from the surface and tunnels beneath the ice. At the bottom of these, sand, gravel and stones were transported at a high hydrostatic pressure and when the transporting capacity of the water ceased the material accumulated. Today they are recognised as eskers in the landscape. These are ridge-shaped glaciofluvial deposits elongating in approximately the direction the glacier has moved. The water beneath the ice was concentrated to the lower parts of the landscape and this is where most of the eskers are found today. This is especially obvious in the valley trains in Norrland.

Outside the front of the ice, transported materials no longer could accumulate into ridges. Discharging at land, flat, extensive plumes formed in front of the ice, consisting of sandy material from the ice-rivers. Such formations are called sandur and could only be found above the HK. Sandur fields are characterised by numerous irregular channels, a braided river system. They are quite rare in Sweden but similar features could be seen in the Sarek national park, NW Sweden. Discharging at sea, ice river deltas formed. When they were built up to the altitude of the highest shoreline, they are called HK deltas. Initially the level of the delta surface made the basis for the levelling of the HK altitude. Eskers, in areas above the HK are called supraaquatic and are characterised by a very pronounced sharp summit (goat´s back like morphology) while those in areas below the HK have been flattened by surges during the land uplift. They are called subaquatic eskers. The reworked material, littoral deposit or outwash sand, both occur on top of the esker and outside the esker proper. In this case glacial clay could be found under the littoral sand. Generally, this type of esker has a flattened morphology and they are usually covered by younger, postglacial sediments.Glaciofluvial material is characterised by sorting of the material according to grain sizes, separated into layers of only one or a couple of grades in each layer. The coarse clasts are often rounded as a result of the water transport. The character of the minerogenic material so far mentioned is relatively coarse-grained.

Glacial fine-grained sediments

Clay

Fine-grained fractions, like clay, fine and coarse silt were suspended in the melt water and was deposited where conditions were favourable, viz. at relatively great depth on bottom of the sea. Example of such sediment is the glacial clay or varved clay. These sediments are characterised in large areas of Sweden to be regularly built up of silt and clay layers. This rhythmic layering is explained by seasonal variations of the water discharge speed from the melting ice. Depending on the transporting capacity the annually deposited material built varves of variable thickness and also of gradation in grain size. Generally the thicknesses of the varves are greatest in the bottom of the sequence and then decrease upwards. This is also the case with the grain size distribution, with diminishing grains upwards in the section. Also the distance from the glaciofluvial deposit (mouth of the ice river) influences the thickness of the varves and the grain size distribution, with decreasing trends in more distal position. Often the varves are made up from pure clay and then the layering appear as changes in colour from lighter lower layers and darker upper layers in each varve. Only, glacial clay deposited in fresh water is varved. One varve corresponds to sedimentation during one year. The thickness of the varves was governed by flow intensity from the ice-rivers. In those areas where the source bedrock is calcareous there is an important contribution of calcium carbonate to the clay.

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Postglacial deposits

Postglacial deposits are classified according to four major groups: sea- and lake deposits (marine and lacustrine, respectively); river and flood sediment (fluvial and alluvial, respectively); wind sediment (eolian); and peat deposits.

Marine and lacustrine deposits

The post-glacial deposits were, as the name implies, formed after the latest glaciation, through wave washing, transport and redeposition of already settled glacial soils, mainly by water but at some places also by wind. At the end of the glaciations, large parts of Sweden were covered by sea. Material transported by the watercourses from land areas was deposited at the seabed and formed marine or lacustrine sediments, depending on the environment. Through the land uplift (isostatic movement) these sediments now are revealed at the land surface. Wave-washed sediments are found mainly in conjunction with glaciofluvial deposits. Coarse marine and lacustrine sediments have been deposited as outwash littoral sediments. These are shingle, littoral gravel and littoral sand with decreasing grain sizes with increasing distance to the beach and the depth of the water. The thickness of these sediments is quite variable, depending on the exposure and access to suitable minerogenic material during the surging. Often there are problems at classification as to differentiate between wave-washed till and littoral gravel because there is a continuous gradient between these classes. Often the judgement will be "till with a wave-washed surface layer". In this case it is possible to find a primary, firm till below the water-sorted sediment (cf. Glacial deposit: till).

Littoral sediments are often underlain by glacial clay but these can also be covered by younger, postglacial clay.

In the lowest lying areas of the seabed, conditions were calm and the most fine-grained soil particles settled. An example of such a soil type is the postglacial clay, which apart from glacial clay is not varved. Fine-grained soils have formed in a similar way at the bottom of lakes as lacustrine sediments.

Postglacial clays are often differentiated into "postglacial clay" proper and "gyttja". Also here, there is a continuous change from postglacial clay with a quite low content of organic matter to pure gyttja with a content of organic matter exceeding 30 % by mass (dry weight). This 30 % boundary is at issue at the Geological Survey of Sweden. Gyttja, etymologically is a Swedish word, pronounced [yut-tya]. It is defined as a nutrient-rich sedimentary peat consisting mainly of plankton, other plant and animal residues, and mud. It is deposited as organic mud of finely divided condition in an eutrophic lake.

Fluvial and alluvial sediments

Additional types of soil parent material are river sediments, associated with the watercourses. A watercourse flowing towards the sea may, depending on its erosive strength, force the release of soil materials. These materials settle again when the water moves more slowly. Depending on the strength of the water flow soil material is in this way continuously displaced. During this process soil materials are sorted according to grain size, which with respect to texture, result in quite even deposits and even good sorting, fluvial sediments. During periods of extremely large water flows, for example the spring flood, it is common that watercourses overflow. Along the watercourses wave-washed deposits are settled, alluvial sediments. These sediments are often characterised by a certain amount of organic material, i.e. sedge, leaves and twigs

.

Eolian deposits

Wind sediments (eolian sediments) found in Sweden are mainly shifting sand dunes, either fossil or active. Wind sediments are well sorted and consist mainly of medium sand and fine sand. The proportions are nearly 1:1. Coarser particles cannot be transported by wind. The more fine-grained material is sorted out and redistributed as wind-blown silt in remote areas at higher altitudes. Such deposits are quite rare in Sweden. In particular, fossil dunes were formed at glaciofluvial deltas, where large amounts of sand were deposited, and where katabatic winds from the land ice favoured development of shifting sand dunes. Active shifting sand dunes are today developing in coastal areas and in sandy soils having lost their vegetation cover.

Organic deposits

Organogenic deposits are defined as products of organic activity. They are classified into two major groups: 1) sedimentary peat soils or organogenic sediments, 2) sedentary peat soils or true peat. Organogenic sediments are produced from transported and sedimented fragments (detritus) of higher plants, plankton and living organisms in or upon the bottom mud. Further, minerogenic material and chemically precipitated substances are included. Peat soils are formed above the water surface, but in the mires sediments such as gyttja, silt etc are also formed under the surface in lakes or ponds.

Organic soils are widely spread in Sweden. The majority of these are peat soils found in fens, bogs and raised bogs. Fens mainly form from overgrowth of lakes and from peat formation (paludification) on mineral land. Discharging groundwater from surrounding uplands influences fens and wet conditions favour growth of mosses and sedges, for example at slopes with constantly running water. This causes quite rich nutrient conditions in relation to the other main peat land type, i.e. the bog. This type, often developed as a raised bog, is ombrogenous, meaning that the input of water coming only from atmosphere deposition, i.e. precipitation of rain and snow. These types are restricted to areas with high precipitation and humidity.

Chemical deposits

Calcareous sinter, calc sinter or tufas are loose, ashy deposits of calcium carbonate formed by precipitation from groundwater discharge in areas with either limestone bedrock or calcareous regolith. In Sweden these deposits are restricted to regions with Cambrian-Silurian or Cretaceous-Tertiary limestones e.g. the provinces of Skåne, Östergötland, Västergötland and Jämtland as well as the large islands in the Baltic, Gotland and Öland.

References

  • NAS, National Atlas of Sweden 1994. Geology. Almqvist & Wiksell International, Stockholm.

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