Pressure Ridges
Pressure ridges form in ice that’s heating up rapidly, typically on a warm sunny morning. The ice expands horizontally as it warms, and if the lake has a sandy shoreline with a gentle slope, the ice will be forced up the beach, pushing piles of sand and rocks in front of it. (This is called ‘ice shove’.) But if the shoreline is steep and rocky, and the ice is hemmed in by the rockbound shore on all sides, it has nowhere to go but up or down. The ice first bends and then buckles, sometimes with explosive force like an earthquake.
The term ‘pressure ridge’ can be a bit inaccurate. It’s true that many pressure ridges feature plates of ice forced up into the air at near-vertical angles, as in the photo on the right, and they’re easy to see from a distance. But there’s also a 50-50 chance that the ridge will form upside-down instead of right-side-up. Ice can be forced underwater just as easily as it can be forced up into the air. In an upside-down ridge the ice may have been pushed as much as two feet underwater. The puddle on top can be difficult to spot, especially if it has skimmed over with new ice, but a change in color and/or texture usually gives it away. New ice will appear black and shiny, smooth and black, while a deep unfrozen puddle will usually have a greenish or brownish tint as in the photos on the lower right.
The best way to cross a pressure ridge is to sidestep it, using your ice poles to brace yourself on either side (photo below). Of course, you first have to find a safe crossing point. and that can sometimes take a while. Sometimes the ridge abruptly ends in the middle of the lake, and you can just skate around the end of it. If it doesn’t, potential crossing points include the shoreline; a boundary between thinner and thicker ice; a spot where the ridge makes a 90-degree turn or intersects another ridge; a spot where the ridge switches from upside-down to right-side-up; or in the case of overlapping plates, a spot where the plate on top becomes the plate on the bottom. Worst case, you may have to climb off the ice and ‘portage’ along the shore to get around a ridge.
Above: To safely cross a pressure ridge, sidestep it using your ice poles for balance. This photo shows overlapping plates where the ice on the left has slid over the top of the ice on the right, forcing it underwater and forming a shallow puddle along the right-hand side.
Below: This pressure ridge came ashore on an island. Islands, peninsulas and headlands are typical locations where pressure ridges form.
Above: A typical pressure ridge has ice thrust upwards by as much as 5 feet.
This occurs when horizontal expansion pushes one ice sheet over another.
Above: An upside-down pressure ridge with a skimmed-over puddle on top.
Above: A pressure ridge can consist of alternating sections of elevated and submerged ice.
Below: The ice can be safe right up to the very edge of a downfolded ridge. But no guarantees, so always test it with your ice pole!