When a rock falls off a mountain and jams in a narrow passage, that’s a jammed boulder. When rocks are driven down a watercourse by a ﬂood and pile up in such a way as to block the normal ﬂow of water, that’s a boulder jam. Boulder jams are mostly of interest only to kayakers and other creek travelers who come upon them suddenly. Exasperated hikers ﬁnd the jammed boulder a nuisance or worse, depending on how successful they are in getting around, under, or over it. A safety note: Never travel in jammed boulder country alone.