Potholes in Connection with Plastic Scouring Forms

Abstract

The study is based on observations on the Norwegian Skagerrak coast.

In lee sides which are more or less obliquely orientated in relation to the main direction of ice movement, potholes, plastic scouring forms and striae appear closely together and are supposed to be parts of one form system, developed simultaneously by a scouring substance which flowed along the lee side due to a “lee-side effect”, caused by the subglacial pressure conditions.

Lee-side potholes have two parts. The lower part, the pothole proper, is cut vertically down into the rock, having spiral-formed grooves in the wall, and elongated crescent-like depressions and a crest-like asymmetrical elevation in the bottom. The upper part is half-cylindrical, and more or less bent in distal direction, with a characteristic difference in shape between the proximal and the distal rim.

Lee-side potholes have probably been developed by a deflection of the scouring material along vertically trending grooves, in lee of or in front of protrusions of the rock surface. An eddy, developing the lower part, must have come into being where the flow bent from a horizontal to a vertical direction. The upper pothole part has probably been developed by the nearly vertical flow, bending in distal direction to join the general flow along the lee side.

Potholes in stoss sides and top surfaces have often wall and bottom forms like those found in the lower part of lee-side potholes.

In the potholes studied, the flow of the scouring substance in the eddies did most probably go spirally downwards at the walls and start to ascend near the centre at once it had reached the bottom. Taking this, and the striae following the sinuous courses along the lee sides, into account, it seems reasonable to infer that the scouring substance was fine-grained and rather homogeneous, and that it was viscous or plastic, like a soaked ground moraine, or possibly a mixture of water and ice particles carrying rock material (as suggested in an earlier paper by the present writer, Gjessing 1966a).

The rotation of the scouring substance was probably set about by an asymmetry in the flow of the scouring substance or the ice passing over the hole.

The hydrostatic pressure in the scouring substance, exerted on the bedrock surface, must have been of importance for the erosional effect.

Descriptions by other authors of pothole forms developed subglacially and in present-day rivers, and of laboratory experiments, seem to be in good accordance with the results of the present study.