Phantom Formation Is Rock Solid In Corridor

Phantom Formation Is Rock Solid In Corridor. Clark Fork Corridor: The Land. Imagine a rock so old and so deep, that in some places, the bottom has never been found! The mountains you have been driving through are made up of such a rock, the Prichard Formation. Dating back 1.5 billion years, it is one of the oldest exposed sedimentary rocks in western Montana. The Prichard is over 4 miles thick at the confluence of the Clark Fork and Flathead rivers and its base has yet to be discovered., Across the river, the uplifting and shifting of the mountains has tilted the Prichard on its side. If you look closely at the large rocks at this turnoff, you may see ripple marks and mud cracks, evidence of their sedimentary origin., Rock of Many Uses , Because the mudstone layers in the Prichard split apart to form flagstone, it has been quarried to provide colorful construction material for hearths, stone paths and facings for buildings. In addition, a slate-like rock found in the formation was used by early people to make tools and weapons., Ancient Origins , The Prichard formed as layer upon layer of sand, silt and clay was deposited from a river delta into a sea covering parts of what is now Montana, Idaho, Washington and Canada. Over millions of years, heat and pressure turned the sediments into a hard, metamorphic rock, argillite.