The sedimentologic characteristics of the Medina Formation (Silurian) were examined in the sections exposed along the Niagara escarpment in Ontario and New York. The vertical and lateral variations of the textures, fabrics, sedimentary structures, and faunal assemblages indicate a mixed environment of deposition best explained by a deltaic-shallow marine model of sedimentation. The growth of the Medina clastic sequence was related to shifting deltas which, locally, reached the area cut by the natural cross section of the Niagara escarpment. The primary constructive phases of these Medina deltas are well developed in the vicinity of Fulton and between Medina City and Rochester, New York. A third area of direct deltaic clastic input in the Medina sea is present in parts of he DeCew and Niagara Gorge sections. In the Niagara-Ontario area, however, the marine forces were competent enough to rework and redistribute most of the delta-born sediments. The combination of a relatively slow rate of clastic input, a shallow Silurian sea, and a very slow subsidence of the depositional basin yielded a complex interfingering of deltaic topset environments and of prodelta and/or interdeltaic environments. The topset to shoreline environments are characterized by channel sedimentation, beaches, tidal flats--as demonstrated by the presence of Lingula shells still in living position--and high tidal flats to floodplain settings where burrowing organisms flourished. The prodelta parts of the deltas, or the more open-marine environments, are represented by the dolomites and t e gray and red shaly and silty sequences with minor carbonates, which compose the Medina sections in the Hamilton area and which thin eastward and pinch out in the vicinity of Medina City, New York.

The Medina Formation is gas productive directly south of the outcrop section, and the constructed sedimentation model can be of value to predict the best reservoir trends in the subsurface.