6585844927

Sweden

116

It was maximum approximately 285 m hlgher than at present. This 1evel is of najor significance for the distributlon of sedimentary soils in Sweden.

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The most common sol 1 In Sweden is till covering about 2/3 of the total area of the country. It consists of earlier deposits and decomposed rocks which have been eroded, transported and redeposi-ted by the 1ce. The geographical distributlon of the t11l is shown 1n Fig. 3a. The till has partly been heavily Consolidated by the ice to a hard bottom till. The till in the uppermost part of the ice was deposlted during the melting as surface till. It has frequently a high content of boulders and a Iow density compared to the bottom till. The surface and the bottom tills do not necessarlly have the same petrographical composi-tion or origlnate from the same site. It is there-fore frequently dlfficult or impossible to esti-mate the boulder content of the bottom till from the surface composition. However, the boulder content generałly decreases notably with depth.

It can be sald in generał that the graln size decreases, the clay content increases and the boulder content decreases with an increasing transport di stance. Hard rocks, such as grani te, result 1n a sandy and sil ty till rich in boulders, whlle shales g1ve a Iow boulder content.

The till can have been pushed together by the brim of the ice to form terminal moraines indica-ting the location of the brim during a winter (annual moraines) or during a longer period (plurannual moraines). The largest ridges in Sweden are those located in the middle part of Sweden which are part of the Ra-SalpausselkS linę (the Fennoscandian terminal moraines) and a zonę along the west coast. In Sweden the till may to a certain degree be replaced by gravel and other sediments. Terminal moraines have generally a Iow density and occur only below the highest coast linę. Other types of moralne are the "drumlins" which are a type of bottom till.

The sediments are normally unsorted. However, in river vallies in the inner part of Sweden, layers of sand and gravel which are inbedded in the till have been found. The glacial deposits can also occur as isolated lenses interbedded in sedimentary sand and gravel layers and even in clay deposits. Such isolated beds are treatcherous for foundations.

The till is generally located dlrectly on rock In some locations above the highest coast linę the till is covered by gravel, clay and sil ty ice lakę sediments. Below the highest coast linę the composition of the surface soli primarily depends on (1) the conditions prevailing during the de-glaciation and (2) resedimentation caused by the wave action during the apparent lowering of the sea level during the land upheava! (Fig. 4).

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During the melting of the ice sheet large quanti-ties of water were transported along cracks or fissures in the ice or under the ice as ice ri-vers terminatina at the brim of the ice sheet.Fven boulders were transported in addition to stones, gravel, sand, silt and clay size particles. The materiał became rounded due to attrition during transport. When the materiał was deposited, a sorting occurred at the mouth of the ice river because of the decreasing water velocity. The coarsest materiał was deposited close to the mouth of the river in the form of ridges as the brim of the ice receeded each summer when the melting was at 1ts maximum. These ridges,called eskers, can have a length of tens or even hund-reds of kilometers. The eskers are the most characteristic type of glaciofluvial deposit in Sweden. At the edge of the eskers one usually finds sand and silt layers. Above and beyond these the glacial clay gradually has been deposited. The crest of the esker has generally been eroded by the waves during the land upheava1. Thereby the clay at the slope of the eskers has frequently been covered by sand or grave1. Clay can also be found in depressions formed by ice blocks inbedded In the esker during the deposi-tion. These clay filled depressions covered by fluvial outwash can cause trouble in connection with foundation work if not detected during the soil investigation.

The "classical" eskers were formed while the nouths of the ice rivers were located below sea level. If the brim of the ice sheet was statio-nary for a long period large volumes of debris were deposited at the mouth of the ice river.The esker then developed 1nto delta deposits which are particularly frequent at the highest coast linę. The distributlon of the glaciofluvial deposits is shown 1n Fig. 3b.

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Close to the mouths of the ice rivers the sediments consist of glaciofluvial gravel and sand. Further out this coarse materiał 1s overlain by silt and by glacial clay. These glaciofluvial sediments are generally very well sorted.Glacial clay and silt which have been deposited in fresh water are often varved. They consist of regular alternating relatively thick, light and coarse grained "summer layers" and relatively thin and dark "winter layers". In the salt water at the West Coast of Sweden the clay has sedimented rapidly due to flocculation wlthout leaving visible varves. Along the coast of Norrland in the northem part of Sweden the varved clays are absent. They have to a large extent been replaced by varved fine silt because of the relat1ve-ly high velocity of the ice rivers.

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The sand, silt and clay which deposited after the glaciation period are known as postglacial deposits. The extent of these are smaller than that of the glaciofluv1al sediments. The most important postglacial deposits developed when the glacial deposits were reworked by the waves and then resedimented. The fi ner fractions of the till have in many locations been partly washed out by the wave action (washed drift). Also glaciofluvial deposits have been reworked by the waves in the same way.

Clay sedimented both in shallow and in deep