I mispoke – the ice the forms below the detritus is needle ice, not hoar ice. Hoar ice forms below snow and causes avalanches. Needle ice occurs in soil where water seeps to the surface. The water freezes when it comes into contact with the air, but water below keeps pushing.
Here’s a picture from snowshoeing this past weekend. It was the largest heaving I’ve seen where the ice crystals extruded a good 6″ from the ground.
oh, and here are some cool ice ribbons.
But claiming that we saw needle ice just isn’t as much fun as saying hoar ice. Ribbon ice is cool indeed though.