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Sedimentary structures formed by the progressive primary infilling of thermal contraction cracks with sand are termed primary sand veins and sand wedges. In addition to simple vein- or wedge-shapes irregularities can be caused by sand veins branching from their sides and toes. Primary sand wedges form widely in sandy polar deserts, locally in sandy areas of tundra and probably in seasonally frozen ground. There are no unique criteria for distinguishing primary sand veins and wedges from sand veins and wedges of other origins. Identification of the former depends on the occurrence of distinctive features not always present and on evaluation of their lithofacies and palaeoenvironmental contexts. Care and caution are advocated in the use of ancient/relict primary sand veins and wedges as quantitative palaeoenvironmental indicators because modern active wedge distribution is poorly known and hence inferred thermal climatic threshold values are questionable. It is suggested that ancient/relict primary sand wedges exceeding 2 m in depth and with well-developed vertical lamination probably indicate the former occurrence of continuous permafrost, whereas sand veins and narrow sand wedges (frost cracks) are potentially ambiguous as they may form not only in the active layer above and within continuous permafrost but also in seasonally frozen ground in non-permafrost areas. (C) 2000 Elsevier Science Ltd. All rights reserved.