Shell Ice & Sandwich Ice

Shell ice comes in two forms: dry shell ice and wet shell ice (aka slush sandwich). It’s important for wild ice skaters to be familiar with both types as they can cause serious falls.
Most of us are familiar with the dry shell ice commonly observed in mud puddles that have partially drained after a thin layer of ice has formed on the surface. Dry shell ice may form after a significant rain event and/or after warm temperatures create puddles of ponded water on the surface of an existing ice sheet. As temperatures drop below freezing, a thin layer of ice forms on the standing water and before all the water freezes, a portion of the remaining water is absorbed by the underlying ice, leaving a thin air layer. Ultimately, we wind up with an easily identified light-colored layer of eggshell-thin ice (1/16” to 1/8”) elevated above the underlying ice, with a layer of air in between. Always approach areas of dry shell ice with care, as they can slow your forward motion abruptly, causing you to pitch forward and potentially fall. Sometimes but not always, it’s possible to safely glide through a stretch of shell ice by double-poling with your Nordic poles.
Wet shell ice may form on the surface of an existing ice sheet when snow-covered ice becomes flooded owing to a combination of upwelling water and above- freezing temperatures. When the temperature eventually plummets, at first only the surface of the snow-slush mixture freezes. When this happens, we may end up with a layer of water sandwiched between the thin surface ice and the thick underlying ice sheet. Until the top layer of the slush sandwich can support your weight without cracking, a skater runs the risk of dropping a skate blade through the surface layer into the underlying trapped water and tumbling forward.
Often, it’s extremely difficult to predict where wet shell ice will occur, although the glassy areas of formerly pooled water are prime suspects. And, while they are difficult to spot, frequently all that is needed is an additional evening of below- freezing temperatures to create a solid surface.
There are occasions when the layer of slushy water may take several days to completely freeze. In these instances, it’s important to determine if the ice underlying the slush layer can support your weight. Once again, don’t be shy about testing the ice with your ice pole or ice pike.
(Many thanks to Richard Behr for this material.)