CORDUROY TERRAIN NEAR CINCINNATI: MEGA-SCALE GLACIAL GROOVES?

A well developed set of regularly spaced, northwest-southeast trending narrow ridges and valleys is visible on a relief map made from a 1/3 arcsecond DEM of the Ohio/Kentucky/Indiana (OKI) region (4,675 km2). Numerous linear features, 'invisible' on aerial photographs and satellite images, could be easily traced on this DEM map. The ridges range from 200m to 2km and are spaced 100-300m apart. Relief between the ridge crests and adjacent valley bottoms is 10-30m. The ridges are carved in a nearly horizontal sequence of predominantly shale units. The even spacing and parallelism of the ridges gives the terrain a distinctive “corduroy-like” appearance.

Longitudinal profiles of the valleys are nearly straight and often unoccupied by streams suggesting they are not fluvially formed. Less well developed corduroy terrain, with a north-south trend, is present in the northern region of the OKI map. The difference in orientation between the two corduroy terrains may relate to the different flow directions of the glacial lobes that etched the topography.

The generally morphology of the two terrains resemble mega-scale glacial lineations (MSGLs) described as “…a highly-grooved surface of till produced by keels of the glacier plowing through soft sediment and squeezing sediment into intervening ridges.” Although the corduroy ridges near Cincinnati are not formed in soft sediment, the fact that they are not found south of the glacial terminus suggests they are related to glaciation. Less well developed linear terrain is found in the northeastern OKI region, with a northeast-southwest trend.

The corduroy terrains of the OKI area differ from the MSGLs in that the plowing has involved dominantly shale bedrock rather than soft sediments. This soft bedrock probably excludes prominent bedrock ridges as a likely cause of glacial keel formation and subsequent grooving. It is more likely that ice keels were created in the main trunk of fast-moving ice fed by a large fan-shaped onset zone. This would have provided a strong velocity contrast between slow moving ice in the onset zone allowing formation of keels that moved rapidly downstream.