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.

As can be seen here, collapse of a sink is also a localized natural hazard!

The Stevens Twin Sinks Preserve is a 30 acre parcel owned by the Michigan Karst Conservancy in Alpena County. It was purchased in 1993 with gifts from William and Archie Stevens and other members and friends of the MKC. The Preserve surrounds two sinkholes separated by a fragile saddle-ridge, each about 200 feet in diameter and 85 feet deep, which give the Preserve its "twin sinks" name. An interpretive nature trail that visits the rims of these sinkholes and other karst and natural features of interest is currently being developed.

Other sinkholes in the area are on private property and may not be visited by the public. The Stevens Twin Sinks Preserve is the only sinkhole site open to the public in this area. All nature lovers trust that the public will treat the sinkholes and other features to help preserve them for public interest and enjoyment now and in the future.

Quite recently the Bruski Sink across Leer Road (photo below) was donated to the Conservancy and made part of the Preserve. This sinkhole has been used for illegal dumping of trash for many years, contributing to the contamination of groundwater in the area. The MKC plans to clean out most of the visible trash and install barriers to future dumping.

    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.sinks.JPEG (532337 bytes)

Sunken Lake and "Mystery Valley"
Legends concerning Sunken and Rainy Lakes have been written about since the 1950's. An early geologist, Newton Winchell, speculated on the presence of an underground drainage system in Alpena and Presque Isle Counties. Early State Geologists, such as Carl Rominger and R.A. Smith also observed the sinkholes and karst features of this area.
    Sunken Lake and its dry-lakebed counterpart, "Mystery Valley", are located in sections 32 and 33 of Presque Isle County, just north of the Alpena-Presque Isle County line, and 2 or 3 miles north of the well-known Leer sinkholes. Sunken Lake is located on the North Branch of the Thunder Bay River. In the 1950's, the Michigan Tourist Council reported that flowers were planted in the floor of Mystery Valley after Sunken Lake has dried up and disappeared one year. During the following year, the lake reappeared as the sinkholes in its bed again became plugged, illustrating how and why these types of lakes drain--they have sediment-plugged sinkholes in their beds that occasionally become "unplugged" of sediment, allowing the water to flush down the sinkhole into subterranean drainageways in the fractured limestone below.
    One year during the early 1900's, it is alleged that lumberers attempted to float logs down the North Branch of the Thunder Bay River at Mystery Valley, but the logs would stop at the large sinkhole in the valley. Since the log jams caused the water which normally flows into the sinkhole to back up, the valley flooded and a diversion dam was consequently built to route the logs away from and around the sinkhole at Sunken Lake. Another popular legend from the lumbering era states that loggers would "ride into the sinkholes" on their logs, then reappear in Misery Bay of Lake Huron, 23 miles to the southeast, "still smoking their pipes".

The Rainy Lake Episode(s)
Rainy Lake, located about four miles east of Shoepac Lake, is composed of five or six sinkholes. The deepest sink typically has up to 100 feet of water in it and acts as a swallow. Rainy Lake has had significant fluctuations of its water level in the past. This is frequently caused by catastrophic drainage due to the temporary loss of the silt and clay "plugs" which seal the sinkholes. The last drop occurred between 1979 and 1982 (compare the aerial photos of Rainy Lake taken in 1981 and 1983), when a 4 to 5 foot vertical drop in the water level occurred in February 1982. Measurements have shown a change in the bottom contours of the lake due to the continuing collapse of sinkholes beneath the lake.
    Although Sunken Lake has disappeared and reappeared several times, historical documentation is more complete for Rainy Lake.  Rainy Lake is a rather large lake in SW Presque Isle County (see below).  This lake, more than a mile in length, is underlain by limestone bedrock, which is obviously full of solution holes, caves, and joints. 
It is well-documented that Rainy Lake drained in 1894, 1925, 1950, and in 1980. By 1983, Rainy Lake has fully recovered from the 1980 drainage event. There are at least five sinkholes in the bed of Rainy Lake, which reportedly drain into a west-east trending subterranean drainage system. In the 1980 event, however, only one sinkhole appeared to be active.
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This subterranean drainage system is reportedly connected to Sunken Lake, Shoepac Lake, Devil’s Lake, and Misery Bay. No one has seen or explored this alleged underground drainageway, but geological evidence supports its presence. According to Will Gregg, during the 1925 event, citizens expected to find dead trout in the dry lake bed, since Rainy Lake is normally a productive trout lake. Much to their surprise, no dead fish were found, yet the following year, when the lake reappeared, the trout also came back. Obviously, the fish had been swept into the subterranean passageways with the water, only to reappear with the water at a later point. In the same account, a story is told of another early attempt to float lumber down the Rainy River. Immediately prior to the log run, the lake drained through the sinkholes. The lumberers consequently built a rail line on the dry lake bed. After the logs were shipped, the Rainy Lake sinkholes again became plugged, and the lake reappeared.
The images below are from a personal collection, and have been reprinted here.  The depict conditions on the dry lakebed of Rainy Lake during the 1925 event.
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Note, in the images below, how the waters have cut a gorge into the soft mud of the lakebed, as they rushed out of the lake.
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More images, this time with an annotation:

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The image above is of the dried clays on the bed of Rainy Lake.  The caption handwritten on the back of this picture said:  "The small boy is me, Albert Tennant with my father Del Tennant, at my back and to his right is George Tennant my grandfather and to his right is A.C. Robinson my other grandfather. Note the cracks in the clay bottom. My father has told me that some of those cracks you could put your arm full length in, and that some places you could hear a roar like an under ground current."


Rainy and Sunken Lakes remained geological curiosities for many years. In the late 1960's and 1970's, however, citizens began buying lots on Rainy Lake in several subdivisions. It was an ideal location for a summer cottage until late 1980, when the sinkholes in the bed of Rainy Lake again became unplugged and the lake began draining, after a period of time when the lake was higher than normal. In 1982, the level of Rainy Lake had dropped over 45 feet, and the shoreline had receded over 500 meters in places. During the 1980-1982 incident, water in Rainy Lake was draining at an estimated rate of 10 gallons per second.
    A significant number of cottages and summer homes were built around Rainy Lake between the times of the 1950 and 1980 incidents. The residents were understandably concerned, and after a 30-year dormancy, interest in Rainy Lake was again revived. In an attempt to help bring the lake level up, the Michigan DNR opened a stop-log wildlife flooding dam in 1982 and allowed water to flow approximately six miles down the Rainy River into Rainy Lake. As expected, the resulting increase in lake level was of small magnitude, and only temporary. Rainy Lake has now stabilized, and it is back at its normal level. Late in 1982, the major sinkhole through which the lake drained once again became naturally plugged, allowing the lake to fill.

Post-glacial sinkholes
Postglacial sinkholes are likewise a unique feature of the karst landscape. These sinks are typically nearly perfectly round, about 400 to 500 feet in diameter, with a depth up to 100 feet. Postglacial sinks look very similar to kettle holes with which they are often confused by the untrained eye (sinks are about 5 degrees steeper and are VERY round).
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Very little work has been completed on the relation of karst in northeastern lower Michigan to the structure of the Michigan Basin, prior to the work currently in progress. Basic structural and tectonic processes in the Michigan Basin were reviewed, and almost 500 lineaments in northern Alpena and southern Presque Isle Counties which are discernible on aerial photographs, were mapped. Two predominant average azimuth orientations of photolinears were calculated.
    Many of the linear features discernible in northeastern lower Michigan developed at least in part by ground water solution processes. The primary development of fractures probably took place concurrently with the uplift of the Michigan Basin at the close of the Paleozoic Era. Many of the northwest-southeast and northeast-southwest trending sets of lineaments developed along fracture intersections. Although evidence exists for an upper Devonian karst system, much of the post-Paleozoic karst appears to be oriented along the northwest-southeast linear trend, which is roughly parallel to the regional strike of the bedrock in the area. Much of the subsurface ground water movement in this area probably follows linear zones of weakness which developed along fracture intersections.
    Much has yet to be learned about karst phenomena in northeastern lower Michigan, and what controls the seemingly periodic cycle of lake lowering due to karst, such as has happened at Rainy Lake. Modern technology will not prevent karst lakes from draining in the future, but much insight is to be gained so we can better predict cyclical trends in karst phenomena and be better prepared to deal with them as they develop.

The work of the Michigan Karst Conservancy is carried out by volunteers, who believe in the value to the public of protecting examples of karst features in Michigan for education and scientific uses. Donations to the MKC are tax deductible in accord with federal law. For further information about MKC, and membership in it, write:
Michigan Karst Conservancy, 2805 Gladstone Avenue, Ann Arbor, MI 48104-6432.

(e-mail: mkc@cyberspace.org)

Much thanks to Dennis Hudson and the MSU Center for Remote Sensing and GIS, who supplied some of the text and imagery for this page.

This material has been compiled for educational use only, and may not be reproduced without permission.  One copy may be printed for personal use.  Please contact Randall Schaetzl (soils@msu.edu) for more information or permissions.