Mid-Michigan is not known for its dramatic geologic features. This part of the state is covered with glacial moraines and outwash, covering anything interesting under a thick layer of soil and rock fragments.
Fortunately other parts of the state are home to more dramatic formations - including fault lines. One large fault bisects the Keweenaw Peninsula and continues west through Ontonagon and Gogebic Counties. A portion of this fault can be seen at Bonanza Falls on the Big Iron River. Here layers of tilted Nonesuch Shale cut across the river, which has eroded down through the shale.
In the pictures below you can see how the layers of shale slope downward (from the downstream to the upstream) into the river bed. When the river is low, the water flows through only a few gaps in the upper layers of shale, pools on some of the tilting lower layers and runs parallel to direction of the fault, before finding a gap in the edge of the lower layer and flowing downstream.
|Note how the layers of shale tile down into the river (the view is upstream)|
|The layers of shale are easily seen from the downstream side of the falls|
|Water eventually erodes a gap along the upper lip of the falls|
|Water pooling on a tilted ledge of shale|