What is Mackinac Breccia? First lets define what a Breccia is. Breccia is the name widely applied to a diverse group of rocks that consist of angular, greater than sand size fragments surrounded by a finer grained matrix. The diversity depends upon the fact that breccias have several modes of origin, and consequently, both the fragments and the finer grained matrices may be of just about any composition. Consequently, the properties of breccias differ markedly from rock to rock.
Dissolution of evaporite beds in the Salina Group of Michigan has caused collapse of overlying Silurian and Devonian rock. Mackinac Breccia is a Devonian period rock consisting mainly of broken pieces of dolomite. Mackinac Island represents one of the best exposures of late Silurian (444 to 416 million years) and early Devonian (416 to 359 million years) rocks in the Michigan Basin. The Island contains the best exposures of the Mackinac Breccia in the Straits of Mackinaw area.
Several types of Breccia have been identified. Some of them are:
Collapse - Form where there has been a collapse of rock, typically in a karst landscape.
Hydrothermal - Are formed when hydrothermal fluid fractures a rock mass.
Impact - Formed when the earth is impacted by an asteroid or comet.
Sedimentary - Are formed where rock falls from a cliff and forms a talus or landslide.
Tectonic - Formed by the fragmentation of rocks during faulting.
Volcanic - Are formed by an explosive volcanic eruption.
Mackinac Island represents one of the best exposures of late Silurian and early Devonian rocks in the Michigan Basin. The Island contains the best exposures of the Mackinac Breccia in the Straits of Mackinaw area and this is why I've placed this earthcache here. Dissolution and removal of salt from within the Silurian Pointe aux Chenes formation allowed for collapse of overlying limestone, dolomite, and shale, forming columnar breccia pipes. Percolating groundwater gradually cemented the collapse breccias. Recent emergence due to uplift, glaciation, and isostatic rebound has brought the rocks to the surface. Differential erosion has sculpted the breccias into the forms observed on the island. Such as Arch Rock, Sugar Loaf, Friendship Altar and Robinson Foley.
The spot that I have brought you to is not one the named formations on Mackinac Island. It's just a short column of Mackinac Breccia that I thought had an interesting selection of internal features. Besides the majority of angular pieces of limestone, a rounded rock of two can be seen. Also some interesting voids can be seen where a very dissolvable material once existed. You can see that this small column of Mackinac Breccia has eroded less then the surrounding limestone.
Logging requirements:
1.) Stand at the shoulder of the road and estimate the height of the formation.
2.) Stand at the shoulder of the road and estimate the width of the formation.
3.) Can you see angular pieces in the formation?
4.) Can you see rounded pieces in the formation?
5.) Not required but appreciated, please post a picture of yourself or team with a GPS and the Mackinac Breccia formation in the background.