Text Box: KARST COUNTRY OF THE NE LOWER PENINSULA A unique landscape can be found in the Northeastern Lower Peninsula of Michigan about fifteen miles north of Atlanta in southwestern Presque Isle County. Here are located sinkholes, also known as sinks or swallow holes, because they "swallow" streams. The bedrock underlying this general area is limestone, which dissolves in weak acids, such as those in rainwater. Sinkholes are formed when large circular caves in the limestone collapse. The small group of sinkholes in NE Lower Michigan is only part of a larger Karst system extending eastward to Lake Huron. Karst refers to a limestone region with many sinkholes, abrupt ridges, caverns, and disappearing and underground streams. Some sinks, like Shoepac Lake, are filled with water while others are dry. The bottom of the first sinkhole, just east of Shoepac Lake, is more than 100 feet lower than the surface of the lake, an unusual occurrence.     The Karst in Michigan lacks sufficient water pressure to flush out the overwhelming load of sand, clay and broken rocks from 100-140 feet of glacial overburden which collapsed into the system sometime after the last glacier left the area. Most of the sinks are round, approximately 80 to 100 feet deep, and have walls sloping up to 850. The bedrock extends down several hundred more feet to the top of a shale layer, which has likewise collapsed along with other formations to a depth of 900 feet.    The Michigan basin contains limestone and evaporite rocks, which are soluble in weak acids. In NE lower Michigan, Devonian limestones of the Traverse Formation are exposed, as at the calcite Quarry in Rogers City. Infiltrating rainwater and snowmelt, which picks up weak carbonic acid from its short-term contact with the carbon dioxide in the air, has dissolved some of these rocks, at preferred sites. The dissolved rocks may collapse to form sinkholes, or the solution cavities may remain open, as caves.   The presence of karst and karst lakes in northeastern lower Michigan has been known since early surveys of the state, and has been documented by several technical and popular authors.  Sinkholes, sinks, or dolines are areas where surface water can easily infiltrate to the groundwater, and can do so very rapidly with very little "filtering" by soils and overlying rocks. Thus, sinks represent "highways" of rapid transit of water from the surface to the subsurface. Sinks are, therefore, areas of great concern for they represent sites of potential contamination of groundwater, from non-point source (such as fertilizers) and point-source (such as cattle yards) pollutants. Types of Sinkhole Lakes Many lakes in northeastern lower Michigan developed in conjunction with sinkholes and other ground water solution features. There appear to be three types of karst lakes in northeastern lower Michigan: sinkhole lakes, sinkhole-controlled lakes, and solution lakes.     Sinkhole lakes are generally round in shape and cover geographically-small areas. The small lakes in northeastern Otsego County, the small intermittent lakes near Shoepac Lake of Presque Isle County, and small lakes near Leer of Alpena County fall into this category.     Rainy Lake and Sunken Lake are sinkhole-controlled lakes, and are characterized by sinkholes in the lake basins which control the lake level. These lakes also have surface inlets and outlets such as streams that enter and leave the lake.     The third type of karst lake are solution lakes, which develop along linear ground water solution features in soluble bedrock. Devil’s Lake, north of Alpena, is a solution lake, as are Trapp, Mindock Fitzgerald, and Long Lakes in northern Alpena and southern Presque Isle Counties. Some solution lakes may also be sinkhole-controlled lakes, such as Devil’s Lake, which occasionally drains through a sinkhole rather than through Long Lake Creek, the surface outlet.     There are many linear features and crevasses throughout this area which are not associated with lakes. Some of these appear as bedrock cracks which can be less than 10 to over 100 feet in depth. These features are common around Devil’s, Trapp, Mindock, and Fitzgerald Lakes. Some of them are parallel to the regional strike of the bedrock formations, and some are parallel to the trend of fracture intersections.

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