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Primary and secondary polygons

Published online by Cambridge University Press:  30 January 2017

R. W. Groves
R. W. Groves*
Affiliation:
University College of Townsville, Pimlico, Townsville, Queensland, Australia
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Information

Type
Correspondence
Information
Journal of Glaciology , Volume 7 , Issue 49 , 1968, pp. 130 - 132
Copyright
Copyright © International Glaciological Society 1968

Sir,

During a visit to Iceland in the summer of 1966 a species of patterned ground, possibly hitherto undescribed, was observed. It was discovered on the small summit plateau of the 998 m peak on the Thingeyri peninsula, Vestfirthir, north-west Iceland. A large area of this plateau was covered with large sorted polygons inside each of which several smaller sorted polygons were situated. The primary polygons measured 5–6 ft (1.5–1.8 m) and the secondary polygons 1–2 ft (0.3–0.6 m), respectively, across their internal diameters (Figs. 1 and 2).

Fig. 1.

A single primary polygon containing many secondary polygons, north-west Iceland

Fig. 2.

Several secondary polygons within a primary polygon, north-west Iceland

According to Reference WashburnWashburn (1956), secondary sorted circles within large primary ones have been described by Poser, but such secondary polygon development is not mentioned in Washburn’s review.

If any readers are acquainted with further records of such patterned ground or have suggestions as to its origin, I should be pleased to hear of them.

In addition, a simple laboratory experiment was devised to demonstrate stone stripe formation. This illustrated that forces analogous to alternating contraction and expansion could result in stripes.

A large polythene bag (2 ft 6 in by 4. ft; 0.76 by 1.2 m) was anchored at the base. It was filled to a depth of about 1ft 9 in (0.53 m) with soil containing a number of small flat stones up to 0.5 in (1.25 cm) across. The neck was gently pulled up about 6 in (15 cm) and allowed to descend repeatedly (Fig. 3a and b).

Fig. 3.

Experiment devised to produce stone patterns in a polythene bag (a, b). Surface view of stone patterns after about 50 oscillalions (c)

This compressed and relaxed the soil round the edges, simulating contraction and expansion. Since the bag was a simple one consisting of two polythene sheets scaled together, the forces were only operative perpendicular to the sides of the bag. After about 20 elevations, lines of stones appeared, trending parallel to the sides of the bag (Fig. 3c), and perpendicular to the applied force. A round-bottomed bag would yield polygons.

1 June 1967

References

Washburn, A. L. 1956. Classification of patterned ground and review of suggested origins. Bulletin of the Geological Society of America, Vol. 67, No. 7, p. 823–65.CrossRefGoogle Scholar
Figure 0

Fig. 1.A single primary polygon containing many secondary polygons, north-west Iceland

Figure 1

Fig. 2.Several secondary polygons within a primary polygon, north-west Iceland

Figure 2

Fig. 3.Experiment devised to produce stone patterns in a polythene bag (a, b). Surface view of stone patterns after about 50 oscillalions (c)