A hummock is a hillock or small hill of broken ice which has been forced upwards by pressure. The ice may be fresh or weathered. The image above is Shackleton's ship the 'Endurance' being gripped in a pressure ridge and was taken by Frank Hurley.
A bummock, on the other hand, is the submerged counterpart of a hummock and one should not do as I did before posting, leave the 'o' out of counterpart. Freudian slip, ahem.