Earth cracks do exist in the area, but some are covered with soil which is why it's important to stay on the trails. You wouldn't want to step into one.
We are going to learn about earth cracks. This cache is located at the Mystery Valley Preserve. A mowed path will lead you to the earth crack area. The earth crack system here is somewhat complex in that there are several cracks that run together. At one point they cross. Please be careful not to step into one of them. Care should also be exercised when standing close to the edge of one. This is one place where it's a real good idea to not go alone.
Alpena and Presque Isle counties are underlain by a thick sequence of Devonian limestone's and shale's, called the Traverse Group. At a depth of about 800 feet at the Preserve occurs the Detroit River Group, which includes considerable evaporates such as 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 were created by the collapse of Traverse Group rocks into the cavities created by the dissolving of Detroit River Group evaporates. The result is the settling and collapse of large blocks of bedrock.
The water that dissolved the evaporates 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. Here the joints run in an east-west direction as can be seen in the large sink hole to the north and the sinkholes in Sunken Lake.
Evidence for the dissolving of the evaporates is found where the water that goes underground returns to the surface from submerged sinkholes in Lake Huron, twenty miles to the east. The resurgent water is saturated with gypsum.
Now that you know how the nearby sinkholes were formed it's time to understand how they influenced the formation of these earth cracks. The rocks beneath your feet have collapsed, under and around the sinkholes. This causes the surface rocks to slump towards the sinkholes, forming these earth cracks. Other cracks are sure to exist in the area but they are covered with soil which is why it's important to stay on the trails.
Send me your answers to the following questions through my profile.
Logging requirements are as follows:
1.) At the posted waypoint how deep would you estimate the earth crack to be?
2.) At the posted waypoint how wide would you estimate the earth crack to be?
3.) At the posted waypoint how long would you estimate the earth crack to be? Don't walk either direction, just estimate the length by looking left, then right.
Not a logging requirement, but always appreciated is a photo showing your GPS and/or your face with one of the earth cracks in the background.
This cache is located within the Mystery Valley Preserve, which is owned and managed by the Michigan Karst Conservancy. All of the preserves managed by the MKC are free to visit. Always practice Cache In, Trash Out.