Used only in the plural, zigzag rocks refers to one or to a series of low chevron-shaped dams, open at their apex and reaching, usually, from bank to bank across a river or stream. They were built by Native Americans, notably in the Northwest and on rivers along the fall line of the Appalachian Mountains. Constructed on hard river bottoms swept free of soft sediments, the converging lines of boulders and cobbles were designed to funnel ﬁsh toward weirs—traps—constructed at the apexes. Some structures were laid out in the shape of a W, to trap both anadromous ﬁsh swimming upstream to spawn and catadromous ﬁsh swimming downstream. A staggered sequence of such separate weirs might extend a half mile or more downriver, as was once the case on a stretch of the Susquehanna near Harrisburg, Pennsylvania. Long-abandoned zigzag rocks, one of the most distinctive marks of human interaction with the Earth, can still be seen in some places.