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Stories in Sandstone

To me, Natural History is the story our land tells about itself. PEI’s oldest stories are told by our sandstone cliffs and go back some 300 million years.

Sandstone is sedimentary, and ours records a time when ancient waters eroded the Appalachian Mountains and carried cobble, pebbles, sand, and mud across the landscape. New layers were deposited on top of older layers and eventually the sediments lithified – they turned to stone.

This process follows a few basic rules. First, more energy is required to move larger particles such as cobbles and pebbles than smaller grains of sand, silt, and clay. So, fast-moving floodwaters can carry larger sediments than gentle streams. Second, sandstone is formed in progressive layers, the oldest at the bottom and youngest at the top. Like tree rings, each layer tells us about the environment in which it was formed. Next, sediments deposited by water are always horizontal. Sandstone layers that are not horizontal tell the story of ancient – and immense – geological forces that moved them after they were formed. Finally, our sandstone is red because of oxidation of iron (rust). Sediments that are their original green-grey colour didn’t get a chance to oxidize, often because they were deposited during wet periods or mixed with organic matter (you can read more about that here: ). Taken together, these rules can help decipher the stories.

Photo 1: The rough-looking layer in the middle of this PEI sandstone cliff is conglomerate, evidence of fast-flowing, prehistoric floodwaters.

Have you ever found rocks along the shore that have some of the colour of our red Island sandstone but a texture more like aggregate concrete? This is called ‘conglomerate’ and – when found in layers in our coastal cliffs (Photo 1) – records a time when torrents of floodwater raged across the landscape, carrying a tumbled mix of cobbles, pebbles, and sediment along with it. The turbulence of the middle layer in Photo 1 stands out in stark contrast to the calmer layers above. This pattern is repeated throughout the cliff, telling the story of a climate with alternating periods of fast-flowing floods and gentler conditions.

Photo 2: The layer behind Geology Dog Cuan is mudstone, evidence of calm, slow-moving prehistoric waters.

In contrast to the monsoonal climate recorded in Photo 1, the mudstone layer in the middle of Photo 2 tells of a much quieter time. Mudstone is made from the finest sediments – silt and clay – and is smoother and softer than sandstone. The sediments that become mudstone are deposited by slow-moving or calm waters, and so this cliff records a time when the area could have been an ancient floodplain or lakeshore. The unoxidized layers above and below point to either standing water or organic matter mixed with the sediment. At this same site, I found lovely ripples in the unoxidized layer, confirming the prehistoric presence of shallow water (Photo 3).

Photo 3: Ripples in unoxidized sandstone, made by moving water when the rock was still sediment.

While individual sandstone layers record the ancient climate, the cliffs themselves show the violence associated with the splitting of the continent and birth of the Atlantic Ocean about 150 million years ago. Imagine the force needed to tilt an entire landscape about 20 degrees (Photo 4)! In some areas of PEI, you can find evidence of earthquakes that occurred much earlier, at the same time our sediments were being deposited. But in this photo, layers laid down over millions of years were all tilted at the same time, and so we know this event happened much later.

Photo 4: Cuan highlights the tilted layers that point to massive geological forces - strong enough to move the entire landscape - that happened after these sediments were deposited and lithified (turned to stone).

A classic movie and the TV series it inspired were known for the famous closing line “There are eight million stories in the naked city. . . “. There are millions of stories in our famous sandstone cliffs, each one a part of PEI untamed!

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