A gravel stop is a sheet metal fabrication used at the perimeter of built up roofing.
Traditional built up roofing (informally called "tar-and-gravel" roofing) consists of multiple layers of roofing felt mopped with layers of hot liquid bitumen, and surfaced with gravel. At the edges of the roof, some means is required of terminating the roofing, preventing water from running underneath the roofing, and preventing the gravel surfacing from washing off in heavy rains. The traditional method is a sheet metal fabrication called a gravel stop.
Architectural Sheet Metal Manual, 6th Edition, Sheet Metal and Air Conditioning Contractors National Association (SMACNA), 2003