Geology Concepts - Original Horizontality

Yesterday I did a program at a school on historical geology.  Students learned about the processes through which rocks form, break down, and reform.  They also learned about several concepts that can be used to determine the relative age of geological features.  One of the concepts was that of Superposition - the idea that rock layers that are found on top of other rock layers are usually younger than the layers below them.  Another concept that helps make the idea of superposition work is the concept of Original Horizontality.

Original Horizontality is the idea that the sediments that form sedimentary rocks were originally deposited horizontally - even if the rocks are no longer in a horizontal position.

To show this concept I want to look at two waterfalls in the Upper Peninsula.

The first two photographs show the horizontal sandstone layers at Tahquamenon Falls in the eastern UP.  The sandstone at Tahquamenon Falls formed during the Cambrian Period (500 - 600 Million Years Ago) and are part of a group of rocks called the Jacobsville Formation.

Upper Tahquamenon Falls - note the horizontal layers of sandstone

Horizontal sandstone formations on the Tahquamenon River

The sandstone layers at Tahquamenon Falls remain in their original horizontal position.  In contrast at Bonanza Falls in the western Upper Peninsula, the bedrock formations have been tilted from the horizontal.  The rock here is a type of shale known as Nonesuch Shale.  Nonesuch Shale is part of the Nonesuch Formation and dates to the Precambrian Era.  The Nonesuch Shale is between 900 million and 1 billion years old!

Bonanza Falls on the Big Iron River - note the layers of shale tilting back toward the falls

Looking upriver from Bonanza Falls - the tilted layers of Nonesuch Shale are easily visible in the river