Glacial History of Covey Hill

The abandoned spillway across Covey Hill has long been accepted as one of the overflow channels of the Great Lakes, occupied when "the ice sheet" had receded to a position north of the Adirondacks but still blocked the St. Lawrence drainage below this level, now a thousand feet above present sea level. Current investigation reveals that the area has been glaciated by two Wisconsin ice invasions. The first one came from the northeast and overrode the whole area, whereas the second one came as a readvance from the northwest and only impinged against the north flank of Covey Hill where it built a terminal moraine. It was this latter ice that dammed the St. Lawrence and caused its drainage to overflow at Covey Hill. Radiocarbon dates on peat and on Champlain Sea shells, which overlie this latter till, show it to be pre-Two Creeks, i.e., Port Huron in age. Covey Hill gap is therefore also dated as Port Huron (Mankato) in age. Along the north slope of Covey Hill is a well-developed marine shore line at 525 feet above present sea level, whereas in the lowlands to the north the base of the marine Champlain clays is now at 200 feet above present sea level. These clays lie here on varved lake clays, the top of which show mud cracks, dessica-tion breccia, oxidation, and fluvial erosion, and they demonstrate an episode of dry land which was later flooded by eustatic rise of sea level to make the Champlain Sea. These relations demonstrate that sea level had been lowered at least 325 feet during this part of the Pleistocene.