We are going to learn about sinkholes. The sinkholes at this site are the Stevens Twins Sinkholes. It was purchased in 1993 with gifts from William and Archie Stevens and other members and friends of the MKC. I hope you can walk around the site to view the sinkholes from all angles. There is an earthcrack, just follow the signs. Across Leer Road is the Bruski Sinkhole Preserve. You'll find one large sinkhole with a sad history.
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 about 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.
Because these two sinkholes formed so close together a "saddle" formed between them. A "saddle" is a narrow ridge of rock that separates two sinkholes.
To claim your find please send the answer for the following questions to the cache owner. Although not required, photographs are always welcome. Do not enter the sinkhole. The saddle is narrow so use care when walking on it. If you don't like heights this cache may not be suited for you.
1.) In your estimate what is the difference in elevation from the lowest point of this saddle from the "Bruce Herr Memorial Walk"?
2.) In your estimate what is the narrowest width of the saddle?
Not required but always a pleasure to see, please post your photographs too.
To email your answers please click here to open a new email.
This cache is located within the Stevens Twin Sinks Preserve, which is owned and managed by the Michigan Karst Conservancy. All of the preserves managed by the MKC are free to visit. Parking is allowed only in the designated areas. Always practice Cache In, Trash Out.