Text Box:      In NE lower Michigan is an area of concentrated sinkholes, specifically in eastern Alpena and Presque Isle Counties. Many are in county parks or, as seen here, preserved for future study.  Alpena and Presque Isle counties are underlain by a thick sequence of Devonian limestones and some shales, called the Traverse Group. At a depth of ca. 800 feet at the Preserve occurs the Detroit River Group, which includes considerable evaporites - anhydrite and gypsum. These dissolve much more readily than limestones and have been totally removed further north by water circulating at depth. The sinkholes of the area are created by collapse of Traverse Group rocks into the cavities created by the dissolving of Detroit River Group evaporites. The result is the settling and collapse of large rock blocks with some shattering, leading to a hummocky terrain, such as can be seen in and to the east of the Preserve. Also present on the Preserve are "earth cracks", resulting from the slumping of the rocks .     The water that dissolved the evaporites found its way underground along joints and especially joint intersections. These points of greatest water input created the earliest and largest voids in the Detroit River rocks, which allowed the rocks above to collapse - all the way to the surface, to form the sinkholes visible today. They tend to be aligned along joint trends, as indicated by the grouping of the Twin and Bruski sinkholes. Evidence for the dissolving of the evaporites is found where the water that goes underground in the sinkholes returns to the surface - from submerged sinkholes in Lake Huron, twenty miles to the east. The resurgent water is saturated with gypsum.     This area of Michigan was glaciated and a considerable amount of glacial outwash - sand and gravel - mantles the limestone, which can be seen from the parking area. That the sinkholes are not also filled with outwash shows that they increased in depth following the melting of the glaciers (it is not known what depth of glacial outwash fills the bottom of the sinkholes). The exact relations between the glacial deposits and the sinkhole formation have not been elucidated yet and is an area for future research.     Unfortunately, ignorance of how these sinks formed, and their links to groundwater, facilitated past uses of them as dumps. Shown above is the Bruski Sink located near Posen, which had been used as a dump/landfill for years. Still other sinks in the area are located downslope from cattle yards, and pose a different (and ongoing) threat to groundwater quality. Shoepac Lake    Silt and clay brought in by small streams have thoroughly sealed the bottom of some sinkholes, thereby creating sink lakes. Shoepac, and several nearby lakes, is made up of one or more sinkholes completely sealed off from underground drainage. Recent, 1976 and 1994, active karst collapse can be seen at the eastern edge of Shoepac Lake, providing evidence of the ongoing collapse of the sinkholes beneath the lake.

Karst Overview 3

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