Sunday, December 6, 2009

Rock at Creek Curve - Part 2

This is the obstacle that forces the straight creek channel to form a curve. In the process, rock is moved and the bank falls away.

When rock at the base of the slope is moved away, the rock above slides down. This is a slow, continual process that changes the face of the bank and the landscape above. You would think that a rock wall would be able to stand against a bit of fast moving water. The bedrock at Blue Jay Barrens isn’t typical of what you’d find other places and that’s where things get interesting.

This is limestone, a type of sedimentary rock. Sedimentary rocks are formed when deposited materials are acted on by a combination of heat, pressure and time. The resulting rock is generally found as solid, horizontal layers. At Blue Jay Barrens the layers are neither solid nor horizontal. This phenomenon is the result of a meteor strike about 300 million years ago that produced what is called the Serpent Mound Disturbance.

The layers in this photo seem to be generally falling from the left to the right. There are also fractures that appear to fall in the opposite direction. The larger pieces of rock look solid, but most contain small fractures that make them easy to break into smaller bits. This is why the water has a fairly easy time of detaching the rock from the bank.

Trying to sort out the original rock layers from the maze of fractures can be confusing. Sometimes the fractures converge at vertices of finely crushed rock particles. Moisture enters the fractures and freezes during the winter. This action loosens the rock pieces and makes them more vulnerable to movement by flood water. You can see a lot of fine particles that have fallen atop the larger pieces. This will be easily moved when the water comes up again.

Looking beneath the overhang you can see large cedar roots that once grew on top of the rock layer. Small roots found their way into fractures in the rock and also helped to destabilize the bank.

Further around the curve the bank is still collapsing. The small point coming out on the right is blocking most of the water from this section of bank. Eventually that point will be lost and these large rocks will find their way into the creek channel.

Some of the large rock pieces are more durable than others. They become a more stable part of the stream bed and act as shelter for stream dwelling organisms. The Streamside Salamander is one that depends on immovable rock slabs for successful breeding. These rocks must be perfect for that salamander since this pool fills with their larva each year.


  1. Do you live near the Serpent Mound?

  2. Chris - I'm about a mile from the mound, towards the center of the meteor impact area.