Changing environments

grand canyonThese very different rocks stacked atop each other in the Grand Canyon bear witness to a change in climate some 260 million years ago as the supercontinent Pangaea had more or less finished welding together and brought a drier continental climate to the land that became the western USA. The reddish underlying mudstones of the Hermit Shale were deposited in flood plains and tidal flats, and contain fossil plants and animal tracks.

The overlying sandstone of the Cococino formation were deposited in a vast desert as windblown sand, gradually covering the mudstone and drying it out. Some of the fossil cracks in the mudstone have been filled with sand, giving us a snapshot of a moment in time way back in the Permian era. It is mostly made up of well rounded quartz grains of a regular size, with very minor feldspar, which geologists would call a ‘mature’ sediment, as opposed to one made of crystals that are easily weathered, either physically or chemically.

So there you have it, great events recorded in a small patch of rock. It speaks of welding continents, growing mountains and the drying of the land to form a great dune desert a very long time ago, and the best bit is that this message from the past can still be read and interpreted today, and the events that formed them imagined as we let our minds travel away into deep time.

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