In permanently frozen silts, spectacular ice formation takes place. The ice is believed to form by molecular migration of water to the freezing face, thus excluding impurities. The ice is not clear, it is mixed with silt and organic materials. The ice wedges are thought to grow in the following manner. The ground contracts and cracks upon cooling.
Snow blows into the crack, and during breakup thaw water runts into it.
In subsequent years, this process will repeat itself. The original crack, now an ice wedge, is a a weak spot where new cracking takes place. The ice wedge will thus grow as snow and melt water keep filling the fresh cracks. Note that the top of the ice wedge will correspond to the top of the permafrost layer at the time of formation of the ice-wedge, as the portion of the ice filling the crack in seasonally thawed ground will melt each year, and the enclosing ground will slump. Such fossil ice wedges need careful examination, as we now know that some sand and dust wedges are not replacement features, but originally formed where the climate was so dry that the cracks were filled by windblown dust or sand rather than snow or meltwater.