We are going to learn about earth cracks. This cache is located at the Stevens Twin Sinks Preserve. A mowed path will lead you to the earth crack area. The earth crack is not very wide or deep, but still please be careful not to step into one of them.
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 evaporates - 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 evaporates. 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 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, as indicated by the grouping of the Twin and Bruski sinkholes.
Evidence for the dissolving of the evaporates 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.
Now that you know how the nearby sinkholes were formed it's time to understand how this earth crack formed. The rocks beneath your feet have collapsed, under and around the sinkholes. This causes the surface rocks to slump towards the sinkholes, forming this earth crack. Other cracks are sure to exist in the area but they are covered with soil.
To claim your find please send the answers for the following questions to the cache owner with the link below. You will need to bring a tape measure of at least 10 feet with you. Although not required, photographs are always welcome.
1.) How many opening did you see at this earth crack?
2.) This is a multi part question. I don't want the depth. Tell me the longest surface measurement for each of the openings you saw in the earth crack?
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.