Sea snails and sediments

If you’ve read Making gneiss, the previous post in this series, you’ve seen why we’re lucky to have billion-year-old Proterozoic era rock at the surface in the capital region. And you’ve learned how the next layer up is 500 million years younger, from a time when tropical sea sediment settled directly on top of the exposed billion-year-old rock.

When that sediment was settling, North America was near the equator and this part of the continent was underwater. In this tropical sea, sand from beaches and ground limestone settled in alternate layers. That time was the Paleozoic era (Ordovician period), and the second type of rock visible in the capital region is from that period.

Trilobite time

Between around 500 and 440 million years ago, the Ordovician is the time of trilobites, corals, snails and other very successful but still relatively simple sea creatures. The first land plants had only just appeared. Fish are still to come. Dinosaurs are 200 million years away in the future. So you may be thinking: so much has already happened, and we’re still only in trilobite time. So yes, rocks are old: mind-blowingly ancient. And life as we know it is very, very new.

The bedrock just under the city of Ottawa is rock that settled and hardened layer on layer during the Ordovician, building up for millions of years, without getting squashed and folded into mountains by tectonic plate collisions or melted into a new kind of rock. Which makes it what kind of rock? Yup, sedimentary, from the sediments settling.

Amazingly, it’s still found today lying mostly flat in the same position it formed 450 million years ago. And all of those awesome sea creatures ended up as countless fossils in the rock, which you can go look for wherever you see exposed rock.

Life goes on

The rock layers just kept stacking up, as continents drifted, sea levels rose and fell, and life got fancier. The first fish, trees and plants lived and died. Then dinosaurs, fur, feathers, and opposable thumbs. And during the time of the dinosaurs, about 175 million years ago, a double fault created the future Ottawa Valley when the central block between the two faults slid down about a kilometre against the sides. Today we see one side of that fault in the Eardley escarpment.

Another wrinkle in time

Then it got cold again around 2.5 million years ago. The Laurentide ice sheet grew up to three kilometres thick, advancing and retreating like an indecisive cat through your front door. This giant millstone ground the rock record of complex life to stone flour over most of Canada. Only the west was spared, which is why you can find dinosaur bones in Alberta.

Remember Snowball earth? Same idea. But this second unconformity spans the rock record for the entire development of complex life, from shrimp to your dog. We just have those flat Ordovician sedimentary layers with some snails, and directly overtop is the next layer from the Holocene, which is more or less today.

Ice writing

Those scouring ice sheets ground everything up, but especially the higher points on either side of the Ottawa valley. That’s why we have much older rock exposed in the Gatineau Hills and Carp, and younger rocks in the valley, where less erosion happened.

The roaming ice has also left its mark all over the place. Imagine the pressure of ice several kilometres thick in places, the bottom encrusted with stones of different sizes, in constant slow motion. When you’re out and about and staring at rocks, try to spot some glacial striae (scrapes from moving ice sheets), or some hydraulic scouring (carved by high pressure subglacial meltwater carrying rocks), or chatter marks where a boulder carried in the ice sheet has dragged and chunked over a rock surface.

Ottawa’s favourite rock

Even though it’s just hardened sediments, Ordovician rock can be very hard. In fact, Ordovician rocks form the top of Mount Everest.

One “well-cemented” local type is called Nepean sandstone and made of nearly pure quartz sand. Settlers used it to build the 1916 parliament buildings, the Canadian Museum of Nature’s Victoria building, the Royal Canadian Mint, the Dominion Observatory, the Langevin Building, and many other storied buildings. You can visit it in its natural habitat on the Old Quarry Trail in Kanata.

You don’t have to go far to see the layered Ordovician rock around Ottawa and Gatineau. In fact, you can even go see where it isn’t: since limestone is softer and more easily eroded than sandstone, its disappearance has led to sinkholes and caverns in the region.