Inverted topography (aka inverted relief, or topographic inversion) refers to a formation where the rocks on the surface are older than the rock layers below. It most often occurs when low areas of a landscape become filled with lava or sediment that hardens into material that is more resistant to erosion than the material that surrounds it.
This outcrop formation is located within what is known as the Belt-Purcell Supergroup.
The Belt Supergroup is an accumulation of primarily fine-grained sedimentary rocks and intrusive rocks of late Precambrian age. It is more than 15 kilometres thick, covers an area of some 200,000 km2, and is considered to be one of the world's best-exposed and most accessible sequences of Mesoproterozoic rocks. It was named after the Big Belt Mountains in west-central Montana. It is present in western Montana and northern Idaho, with minor occurrences in northeastern Washington and western Wyoming. It extends into Canada where the equivalent rocks, which are called the Purcell Supergroup, are exposed in southeastern British Columbia and southwestern Alberta.
The following is a quote from Don Winston, of the University of Montana. "... The Belt was deposited in a huge intracratonic basin that stretched across western Montana , northern Idaho and into eastern Washington and Canada . It was filled by sand, silt, clay, and carbonate sediments that locally reach more than 18 km thick. The sedimentary structures of these rocks are beautifully preserved, since they were deposited 1.5 to 1.4 billion years ago. They reveal a world of pure sedimentary processes unfettered by plants and animals. "
These sedimentary rocks are thought to be the oldest in Southern British Columbia.
The upper layers of this formation consist of sandstone rich with quartz and interbedded with argillite and were likely deposited within deep water. A fault movement then thrust this ocean floor on top of the surface and the shallow water rocks that were much younger, creating an inverted topography. This is an irregularity because the surface rocks are typically younger than the underlying rocks.
To log this earthcache, please message me the answers to the following questions and post your MANDATORY photo with your cache log:
1) Describe the layering you see in front of you, including colours, average layer thickness, and is it mostly horizontal or are the layers angled compared to the road?
2) What is the approximate height of the formation at the posted coordinates?
3) What is another name for inverted topography? (read cache description)
4) Post a MANDATORY photo of you or your geocaching name at the posted coordinates when you log the EarthCache.
Please note this earthcache is available in the winter but due to snow plowing, parking may be blocked due to piled snow on the sides of the roads.
Sources:
https://en.wikipedia.org/wiki/Inverted_relief
https://en.wikipedia.org/wiki/Belt_Supergroup
https://en.wikipedia.org/wiki/Purcell_Supergroup
https://www.isu.edu/digitalgeologyidaho/belt/