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|