Tool marks are reported from ca. 1750 Ma storm deposits in the upper Cottee Formation, McArthur basin, Northern Territory, Australia. The tool marks are distinct from the common forms produced by bouncing and skimming intraclasts and pebbles, and it is interpreted that they were produced by drifting elongate strands and clusters of strands of a flexible material gently dragging across the sea floor. The only known natural substances with these properties are organic. It is interpreted that the marks were produced by drifting filaments of eukaryotic algae significantly larger than any known from actual fossils in rocks of this antiquity.