sea ice deformation results from the interaction between ice floes, as they are driven against each other. the end result may be of three types of features: 1) rafted ice, when one piece is overriding another; 2) pressure ridges, a line of broken ice forced downward (to make up the "keel") and upward (to make the "sail"); and 3) "hummock", an hillock of broken ice that forms an uneven surface. a "shear ridge" is a pressure ridge that formed under shear – it tends to be more linear than a ridge induced only by compression. a "new ridge" is a recent feature – it is sharp-crested, with its side sloping at an angle exceeding 40 degrees. in contrast, a "weathered ridge" is one with a rounded crest and with sides sloping at less than 40 degrees. stamukhi are yet another type of pile-up but these are grounded and are therefore relatively stationary. they result from the interaction between fast ice and the drifting pack ice.