Miniature Spits and Embankments on a Lake-Shore, Iceland

Abstract

A SERIES of miniature spits and embankments examined during a Cam? bridge expedition to Iceland in 1937 si ulated to a high degree their greater counterparts round the British coasts. It seems therefore that scale reductions of 10 or 100 to 1 do not materially affect the mechanism of spitbuilding. The lake occupied a mountain-girt hollow in the eastern foothills of Snaefell, East Iceland. It is shown in Fig. 1, taken from a plane-table survey made by Lieut. J. N. Jennings. The bare steep slopes yield large quantities of sand and gravel, and extensive delta flats encroach on the lake. Abundant supplies of shore material are thus made available, and a variety of depositional shore-forms have resulted. The most striking shore feature was the storm beach ridge (Plate 1) which extended the full length of the eastern shore. It was terraced on its lakeward side (Plate 2) by as many as eight lesser beach ridges. It reproduced both in plan and profile countless features possessed by the far greater storm beach ridges round the British Isles, such as the Chesil Bank (Plate 3). The highest ridge rosje 15 inches above lake-level, and the width of the embankment varie'd from 17 to over 30 feet. The back of the bank was corrugated by a series of shingle tongues, formed, like their counterparts at Chesil, Dungeness, etc, where storm waves had broken through the embankment locally, and washed tongues of shingle down the landward side. The profile of such embankments is now largely understood, and can be closely examined in the wave trough of the Cambridge Physiographical Laboratory, on a scale little smaller than that .shown in Plate 2. The close relationship between the height of the embankment and wave size is shown by the heights (in inches) given in Fig. 2. The size of the waves depends on the strength of the wind, and the stretch of water over which it blows. To produce the largest breakers the waves must not be very oblique to the shore or their power is dissipated over a wider extent of shore, and energy also seems to be lost in swinging round to break onshore. Depth of water immediately offshore is also important, but this is chiefly determined by the dominant (or largest) breakers, being deeper where the breakers are large than where they are small. The bank was highest (15 inches) at its southern end where it faced north-west, towards the greatest stretch of water and the maximum exposure to wind. Farther north the bank gradually