The Grimsby Sandstone, on the south shore of Lake Ontario, is a hematitic quartz sandstone with subordinate graywacke, siltstone, and shale. Red color dominates with some green-gray mottles and layers. Samples from 15 measured sections showed increasing mean grain size from Clappison Corners, Ontario (0.0364 mm), east to Lockport, N.Y. (0.1051 mm), and from there decreasing east to Fulton, N.Y. (0.0652 mm). This corresponds directly to thickening of the formation from 5 feet 4 inches at Clappison Corners to 24 feet at Lockport, thence a thinning to 7 feet 4 inches at Fulton. Deltaic subaerial to nearshore environment of deposition is indicated by the gross lens shape of the formation, its red coloration, variable clastic lithology, current and interference ripple marks, shale pebble conglomerates, swash and wash marks, and mud cracks.