Logging Tasks:
- Do you think the size of the ash layer is increasing, decreasing, or staying the same? Please explain why you think so.
- What is the approximate thickness of the ash layer?
- Describe the texture and color of the ash layer and compare it to the limestone that is below the ash layer.
- Observe the fossils in the ash layer. What mineral(s) have likely influenced the color of the fossils?
- Take a photo of yourself, a signature item, or your GPS with the ash layer in the background.
Lesson:
During the Devonian Period, approximately 390 million years ago, a shallow tropical sea covered the area at the posted coordinates. Numerous volcanic eruptions occurred to the east. These eruptions sent volcanic ash into the atmosphere, which then fell into the Devonian sea. The ash descended to the sea floor. While the ash was just a thin layer, it had a profound impact on the sea floor. The porous and reactive nature of volcanic ash caused it to rapidly react with the seawater, leading to the dissolution of most of its silica and the formation of a claylike layer known as smectite, or a K-bentonite layer.
Smectite clays create conditions favorable for fossilization. Their fine, porous structure provides an excellent matrix for encapsulating and preserving organic remains. The small particle size and high surface area of smectites help to significantly reduce sediment permeability and limit microbial activity, which would otherwise degrade organic matter and prevent fossilization. With the organic material protected by the clay, the smectite layer solidied into sedimentary rock. The organic remains transitioned into fossils.
To get to the posted coordinates, you walked across a large bed of light colored (tan, gray) limestone. But at GZ, you will see a layer that is much darker ... dark gray even bordering on black. This is the volcanic altered ash layer. In the satellite image below, you can identify the ash layer by it's darker color.
The minerals in the volcanic ash have a significant impact on the fossils in this layer. The silica rich ash causes the fossils to look entirely different than fossils in the limestone. Many have a sparkly quartz appearance. You will also see fossils that have an orange or red hue. This is from iron that has oxidized (rusted) to tint the silica red, orange, or even yellow.
Snail fossil in ash layer
Resources:
Smectite - Wikipedia
The role of clay minerals in the preservation of Precambrian organic-walled microfossils
KET - Falls of the Ohio / Kentucky Field Trips
Note: Access by foot is possible during low water periods, generally August through October. Ground Zero can also be accessed via boat when river conditions allow. Call McAlpine Dam 502-775-5056 and listen to the short recording.
- Foot Access: If the upper dam spillway is open at all, foot access is not possible.
- Boat Access: If the lower gauge is 20 feet or higher, the cache is under water.