Moränavlagringar med linser av sorterat material i Stockholmstrakten

Abstract

Till deposits with lenses of sorted material in the vicinity of Stockholm

Lenses of sorted material consisting of fine sand, sand and gravel are common in till deposits. In the vicinity of Stockholm a number of sections in till deposits have been studied. Most of these are located either on the stoss sides or on the lee sides of knobs of bed-rock in relation to the last ice movements.

All the sections on lee sides are characterized by the presence of till with lenses of sorted material. These deposits are complex and of various kinds. The till proper is unconsolidated and commonly includes portions of incompletely sorted material. Sorting of this till is somewhat better than on the stoss sides (figs 3 and 7). The lower percentage of fine-grained fractions shows that the material has not been transported any great distance in the ice. Many sections on the lee sides show till-with-lenses covered by real lodgement till. The latter is compact (hard packed), shows foliation and is lens-free. In all the sections on stoss sides has only the latter type of till been observed.

The genesis of the various deposits in which till-with-lenses has been found is discussed. It is assumed that the till-with-lenses at locality 1 (fig. 2) and other similar sections is due to deposition at the margin of an advancing ice sheet, the lenses of sorted material representing marginal meltwater streams. Subsequently, such deposits were shielded in some way from erosion by the ice and became progressively covered by a hard packed, foliated lodgement till.

The orientation of boulders at locality 1 (fig. 2) shows that the till-with-lenses at this locality has been deposited during an ice movement directed about S 30° E, whilst the upper part of the hard packed and foliated lodgement till has been deposited during an ice movement directed about SSW. According to the examination of glacial striae in the vicinity of Stockholm, the latter movement must be much younger than the former.

At some localities large lenses were found almost in direct contact with the bed-rock. Such may be relict from sorted sedimentary layers deposited outside the ice and later overridden by it. At one locality (10) the sorted material was very fine-grained, hard packed and with contorted strata (fig. 11). The small portion (“boulder”) of till with very fine sand and silt at locality 1 (figs 2 and 15) also shows that fine-grained sedimentary layers have been overridden and partly taken up by the ice. At other localities where overlying lodgement till is lacking, the till with lenses could have been deposited much later, for example at the ice-margin during the shrinkage of the ice. The section shown in fig. 8 is of this type, though one of the lenses here is unusually large.

A more detailed explanation of the genesis of such deposits requires consideration of the following features. 1. Dead-ice and dead-ice deposits in lee places. 2. Uplift of the ice margin in water. 3. Marginal drainage. 4. Lenses as primary or secondary deposits, in the latter case the lenses are “boulders” in the till.

In conjunction with the till deposits with lenses of sorted material on the lee sides in the vicinity of Stockholm some other groups of till deposits, in which lenses are generally met with, are discussed. These include till deposited by dead-ice, super-glacial till, till deposited in conjunction with eskers and till in end moraines. Since lenses of sorted, permeable material in till can be of importance for the present-day circulation of water in the ground, the knowledge of such deposits can be of practical value.

A detail, often seen in till deposits, is a thin (1/2-—1 cm) mantle of sorted sand-fine sand in direct contact with the surface of the boulders. Such mantles are assumed to have been formed in a thin layer of water between the ice and the surface of the boulders. The melting of the ice close to boulders should, in accordance with the physical theory of elasticity, be a result of an increase of pressure in the ice close to boulder surfaces.