During July and August 2005 the Great Lakes Division of Sea Cadets, supported by the Noble Odyssey Foundation, began a long-term investigation of a massive geological formation lying beneath the waters of Lake Huron. Called the Mid-Huron Ridge, it is a submerged rampart of limestone that extends for about 100 miles between Alpena, Michigan and Kincardine, Ontario. The feature is little known except to scientists and offshore fishermen. Even less known, the Huron Ridge in prehistoric times formed a land bridge across the lake when water levels were as much as 400 ft. lower than at present. During this time, groundwater, seeping downhill through porous bedrock, leached channels and cavities in the limestone.
Sinkholes and Springs
In some places the land over cavities collapsed, creating sinkholes. Groundwater emerged as flowing springs in coastal sinkholes and at other low-lying points along the ancient Lake Huron shore. Even after the lake refilled, some flows continued up to the present. Today these underwater sinkholes and springs may provide fascinating insights to ancient features of geology, biology, and perhaps even human pre-history.
Middle Island Sinkhole
We began our investigation of the Huron Ridge near Middle Island, a few miles northeast of Alpena, MI. There. a large sinkhole lies just off
the island’s shore. Our divers soon located two underwater springs that flow actively from the base of the sinkhole’s steep walls at a depth of about 70 ft. The spring water proved to be cold, almost devoid of oxygen, and rich in minerals such as sulfur and iron. Because of its higher density the discharge spreads across the floor of the sinkhole, forming a distinct layer. Most interesting, however, are the lush colonies of microorganisms that flourish only in the cold discharge layer. These colonies take several forms, from white hairy growths, to purplish mats, to pale networks of fibers growing on the rocks or sandy bottom.
The Huron Ridge limestone formation in Lake Huron (NOAA image).
Aerial photo of Middle Island sinkhole (T. Black).
NOF divers prepare to explore the sinkhole (L. Clyburn).