Jan Mayen is a small island of recent, volcanic origin lying in the Greenland Sea some 350 miles north of Iceland. The vegetation of the island consists for the most part of a moss mat dominated by Rhacomitrium lanuginosum and R. canescens, in which may occur scattered flowering plants. In some places this moss mat covers wide areas without interruption, but frequently it is broken up by areas of bare ground into patches which form strikingly regular patterns, based especially on polygonal, terraced, and striped forms. Similar features have been previously recorded from other arctic and alpine regions, but no botanical description apparently exists of an area where the patterns are so well marked and diverse, or so clearly related to topography, as they are at Jan Mayen. The patterning results primarily from soil movement due to two forces which may act separately or together: gravitation, which.acts on sloping ground; and solifluction, which comes into play where surface debris overlying a frozen subsoil is subjected to alternate freezing and thawing. Sharp (1942) discusses the various theories put forward to explain the complex effects of solifluction, and gives references to relevant literature. Partial restriction of soil movement by barriers of vegetation may also be involved in the production of patterns; the development of more than a meagre vegetation, however, generally prevents soil movement (and so patterning) by the stabilizing effect of the roots. Such patterning is thus encouraged by immature topography, by frequent freeze-thaw activity, and by poor vegetation; all of which features are characteristic of Jan Mayen. The main types of pattern formed are described below.