When the frozen ground reaches very low temperatures, it contracts, splitting up to form a pattern of polygonal cracks. Girard et al. (2013) Geophys. Res. Letters 40, 9, 1748 find that: liquid water content has an important impact on freezing-induced rock damage; sustained freezing can yield much stronger damage than repeated freeze-thaw cycling; and that frost cracking occurs over the full range of temperatures measured extending from 0°C down to –15°C. Anderson (1998) Arct. & Alp. Res. 30, 4 defines the frost cracking window—the optimal thermal range for frost cracking—as between –3º and –10º C. Vegetation and accumulations of organic matter may reduce the likelihood of frost cracking (Murton and Kolstrup (2003) PPG 26, 2).