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From the posted coordinates, look for a distinct layer of Old Red Sandstone. The photo below shows the area you should be concentrating on. In this EarthCache you will learn how this layer formed its distinctive appearance.
Logging Requirements:
To log this Earthcache, please send us a message with answers to the following questions. There is no need to wait for a reply before logging. If there is a problem with your answers, we will contact you.
- Estimate the average thickness of the individual rippled layers in the red sandstone section indicated by the grey box in the above photo.
- How many rippled layers do you observe, counting from the bottom of the formation covered by the grey box to the top?
- In your opinion, do the ripples look like they were formed by water activity or wind activity? Explain why.
- Please attach a photo to your log of you, your GPS, or something personal to prove you were here. Please do not show any spoilers in your photo. This is a mandatory requirement.
Any logs without the answers being sent and a photo in the log will be deleted.
The Old Red Sandstone (ORS) layers in Portmahomack, Scotland often exhibit ripple marks due to the sedimentary environment in which they were deposited. These ripples are formed by the action of water or wind on the sediment, creating patterns that are preserved in the rock layers. The presence of these ripple marks indicates that the area was likely a shallow water environment or a desert setting where water or wind could create such patterns in the sand. The wavy layers, or cross-bedding, are formed on the sediment surface and these features are then preserved as the sediments are buried and lithified into rock. These features are significant as they provide insight into the depositional environment and the conditions that existed when the sediment was laid down approximately 400 million years ago during the Devonian period.
During the Devonian period, Scotland was part of the ancient continent, Laurentia, and was situated to the south of the equator in a semi-arid environment. The region was entirely above sea level and characterized by significant mountainous terrain. The climate in Scotland during the Devonian period varied from warm and humid to dry and arid. This variability led to the formation of the distinctive red and brown sandstones. The red color of the Old Red Sandstone, which is due to the presence of iron oxide, is a common feature of these rocks.
Over millions of years, the tectonic plates on which the continents rested slowly drifted, collided, and formed new continents causing the landmass to shift from its equatorial position to its current latitude. As Laurentia drifted northward, it carried the Old Red Sandstone along with it.
Source: earthwise.bgs.ac.uk
Geological Significance
The ripple marks and other sedimentary structures in the Old Red Sandstone are not only aesthetically interesting but also geologically significant. They provide valuable information about the paleoenvironment and the processes that shaped the landscape during the Devonian period:
- Environmental Indicators: The presence of ripple marks can help geologists determine the type of environment in which the sediments were deposited. For example, the size and orientation of the ripples can indicate whether the environment was fluvial (river), lacustrine (lakes), or eolian (wind) as ripples formed by water tend to be smaller than those formed by wind.
- Current Analysis: By studying the orientation of ripple marks, geologists can infer the direction of water or wind flow. This information can be used to reconstruct the paleogeography of the region and understand the movement of tectonic plates during the Devonian period.
- Fossil Preservation: The Old Red Sandstone is known for its rich fossil content, including fish and plant remains. The sedimentary structures, including ripple marks, can help preserve these fossils by providing a stable environment for their burial and mineralization.
By examining the ripple-like features in the Old Red Sandstone layers of Portmahomack, geologists and researchers can gain valuable insights into the geological and environmental history of the region. These features are a testament to the dynamic processes that shaped the Earth's surface millions of years ago.
Sources:
earthwise.bgs.ac.uk
geoguide.scottishgeologytrust.org
geologyistheway.com
nature.scot
sciencedirect.com
york.ac.uk