The basal Silurian Medina Group including the Whirlpool Sandstone, Power Glen Shale, Grimsby Sandstone, and Thorold Sandstone was studied in the outcrop belt along the Niagara escarpment from St. Catharines, Ontario, east to Rochester, New York. These formations represent facies changes in a wave-dominated environment.

The Whirlpool Sandstone and Power Glen Shale are a transgression succession of shoreface to delta-front sediments. Regression followed with deposition of the Grimsby Sandstone in lower to upper delta-plain environments.

The Whirlpool Sandstone contains lower shoreface, upper shoreface, and foreshore deposits. These deposits are clean, well-sorted white orthoquartzites. The overlying Power Glen Shale, deposited in a delta-front environment, contains numerous lenses of storm-deposited, hummocky, cross-stratified, fine-grained sandstone; scattered colonies of crinoids

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and starfish; and thin lenses of limestone. The Grimsby Sandstone overlies the Power Glen Shale. It is composed primarily of red sand and siltstone deposited in foreshore to upper delta-plain environments. The foreshore sediments in the Grimsby are generally clean, white, and well-sorted because clay and silt-size particles were winnowed by wave processes.

The foreshore deposits of the Whirlpool and Grimsby Sandstones contain the cleanest, best sorted sands in the Medina Group, and possess the greatest primary intergranular porosity. Although primary porosity in the Medina has been largely obliterated by quartz overgrowths, these foreshore deposits offer good potential for porosity development and provide gas reservoirs in the Medina Group.

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