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Reading Sandstone

I’ve had a lot of questions about green-grey layers in the Island’s sandstone. So today let’s look at two reasons why our sandstone is sometimes this colour!

PEI’s sandstone is iron-rich. When that iron is exposed to air, it oxidizes (rusts) giving it that distinctive red colour. When sandstone’s sediment is deposited during wet periods, water prevents the iron from contacting air and so it doesn’t oxidize. Additionally, organic matter mixed with sediment can further prevent oxidation (organic material both uses up oxygen as it decays, and changes and slows some of the chemical pathways of the oxidation reaction).

Those green-grey layers you see in our coastal cliffs were often formed during flood periods some 300 million years ago. In Photo 1 you can see an unoxidized layer below some rough, pebbly conglomerate. That pebbly layer is further evidence of prehistoric, faster-flowing flood waters carrying larger pebbles with them.

Unoxidized layers break up and erode just like red sandstone does, and you can find both “normal” sandstone rocks as well as green-grey ones in many places (Photo 2, with Senior Geology Dog Ruairidh and Junior Geology Dog Cuan in summer 2021).

Sometimes the iron didn’t get a chance to oxidize, but sometimes the oxidation was reversed (a process called reduction). Have you ever found a perfectly round grey or greenish spot on a piece of PEI’s red sandstone (Photo 3)? If so, you found a reduction spheroid!

Certain metals (such as uranium, vanadium, copper, cobalt and silver for example) can take the oxygen away from the iron in the rock. At the centre of each of these circles (spheroids) is a tiny bit of metal or mineral. Over many tens of millions of years, it reduced the surrounding sandstone, turning it back to its unoxidized colour and leaving a tell-tale circle. Research on reduction spheroids from across the Strait in Pugwash, Nova Scotia, suggests the process continues to this day.

If you are out marvelling at the changes Hurricane Fiona brought to PEI’s coastline, keep your eye out for these common but cool geological features!

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