The 50-acre (20 ha.) quarry 1.5 mi (2.4 km) northwest of Rockford, west-central Ohio, exposes a dozen or more dolomitized reef structures of Silurian age that became a single, coalesced complex during their upward and laterally expansive growth through the 110-ft (33.5 m) vertical interval of observation. These structures illustrate well the manner and stratigraphic interval in which many Silurian reefs began to grow at the edge of, and on, the Midwestern platform area between the proto-Appalachian, -Michigan, and -Illinois basins. The Rockford complex is related to the Fort Wayne bank (a barrier-like bank that fringed the proto-Michigan basin), either as an initial stage of the bank itself or as an early stage of pinnacle growth directly basinward of the bank.

The Rockford complex is in the Lockport Group and ranges into the stratigraphic position of the Guelph (late Middle Silurian to early Late Silurian). Further, it represents a basin-edge Lockport facies of the basinward Salina Group (Greenfield Dolomite and higher). In Indiana terms, the complex is stratigraphically and structurally similar to some reefs originating in the upper part of the Salamonie Dolomite and extending upward through the Louisville Limestone. The Rockford development thus correlates with the bottom part of the more than 400-ft (122 m) Indiana reef stratigraphy.

The reefs proper are biolithically zoned in laterally arranged, time-independent, intergrading units: (1) core rock dominated by bluish-gray carbonate mud that is poor in identifiable fossils; (2) distal core rock dominated by in-place stromatoporoids and tabulate corals; (3) proximal flank rock dominated by bioclastic rubble, some in-place reef-frame builders, and many reef dwellers; and (4) distal-flank rock dominated by poorly sorted carbonate sand. The overall general fourfold vertical zonation is, in ascending order: (1) near-white, coarse-grained, skeletally derived, thick-bedded mostly nonreef dolomite; (2) grayish coarse-grained, bioclastic, irregularly bedded, near-reef dolomite containing the lower parts of most of the reef cores; (3) bluish-gray, dense, carbonate-mud rock i cores; and (4) light-colored dolomite ranging from flank-positioned bioclastic rubble to more mature core rock having an increased proportion of frame-builder fossils.

Most flank beds thicken and rise toward the cores, but in two examples lower beds dip toward the cores, suggesting in-mass settling of rigid structures. Individual growth centers are upward-expanding, inverted-cone structures. One core area became dominant and shed large amounts of flanking debris that smothered less favorably situated growth centers and brought about general coalescence into an integrated complex. Some structures may have originated as nonrigid carbonate-mud mounds, but the mature appearance of the overall reef structures and fossil communities suggests relatively high-energy conditions just below surf level.

More than 110 identifiable species make up the reef community, more than are known for any other Midwestern Silurian reef exposure. The fauna has strong Guelph affinities, and some elements, such as the echinoderms, display abundance, diversity, and good preservation that are rare for Silurian reefs.

Information from the Rockford reef site adds significantly to an evolving synthesis of Silurian history of the southern Great Lakes area. Salient features include basin-to-shelf contemporaneity of deposition of evaporite-bearing and reef-bearing sequences, the manner of evaporite deposition in the basins and its coordination with platform events, the assignment of a Cayugan age to much of the reef-bearing sequence, and paleoenvironmental interpretation in the light of plate tectonics.