Sinkholes of Northeast Michigan
Stunningly Colorful Sunken Forests – Sinkhole Geology
Created when subsurface layers of anhydrite and gypsym dissolve, sinkholes dot the landscape in and around Alpena with the country’s highest concentration of these geologic wonders. A thick layer of Devonian limestone and shale, known as the Traverse Group, lies beneath the surface of Northeast Michigan. Below that, lies the Detroit River Group, which contains anhydrite and gypsum. These minerals dissolve much more readily than limestone, and as the Detroit River Group rocks erode and create caverns, Traverse Group rocks above collapse into the underground caverns, creating sinkholes. Check out the Sinkhole Adventure Guide to learn more!
Sinkholes are fascinating locations to discover Devonian Era fossils from prehistoric life 400+ million years ago. Also keep a close eye out for unique plant-life as the environment in the bottom of some sinkholes is home to flora not found anywhere else in Northern Michigan.While many sinkholes in the area are on private land there are some incredible locations that are open to the public. The Stevens Twin Sinks and Bruski Sink, Rockport State Park sinks, Sunken Lake, Mystery Valley, and Misery Bay sink are all easily accessed via woodland trails or a kayak ride out on the water.
Check out this video for a brief overview of how sinkholes form:
More About Sinkholes
Sinkholes are depressions in the landscape caused by the dissolution and collapse of subsurface limestone, dolomite, or gypsum. The term karst is now used to describe regions throughout the world that have features formed largely by underground drainage. Karst terrains are characterized by caves, steep valleys, sinkholes, and a general lack of surface streams.
As a result of rock solubility and various geological processes operating over time (glaciers, etc.); a number of phenomena and landscapes were formed that gave unique but specific characteristics to this terrain as defined by this term. Karst is frequently defined by dolines (sinkholes-dry and wet), cracks and shafts, poljes (elliptical depressions), caves, ponors (swallow holes), caverns, intermittent springs, submarine springs, disappearing and re-appearing streams and rivers, underground river systems, denuded rocky hills, karst plains, and collapses. It is difficult to give a very concise definition of this word because it is the result of numerous processes that occur in various soluble rock with diverse geological and climatic conditions.
In northeast Michigan, a thick layer of Devonian era limestone and shale, known as the Traverse Group, underlies Alpena and Presque Isle counties. Below that, about 800 feet deep, lies the Detroit River Group, which contains large amount of the evaporites anhydrite and gypsum. These minerals dissolve much more readily than limestone, and as the Detroit River Group rocks erode as a result of water movement and permeation over time, the process creates caverns, Traverse Group rocks above collapse into the caverns creating the “sinkholes” we see from our perspective above ground.
Due to the nature of these rock deposits earth cracks are also often found near sinkholes on bedrock fault lines. Such cracks are caused by “slumping” of rock as the layers beneath erode away. Hammocky terrain results from large rock blocks collapsing and settling, and often shattering in the process. Unique flora can also be found at the bottom of sinkholes a unique biome is created from temperatures changes and light levels. You can find plants growing in the bottom of some sinkholes that are not found anywhere else in the local region.