The Niagara Gorge Geotrail
The beautiful Niagara Gorge is made up of steep walls that rise on either side of the Niagara River, downstream of Niagara Falls, extending to Queenston, Ontario (and Lewiston, New York, depending which side of the river you’re standing on). The gorge itself is a geoscientist’s dream. The colourful rock units that lie one on top of the other may look like layers on a wedding cake but are in fact more like pages of a really good book (see image 1). Our book begins in the Late Ordovician Period, approximately 448 million years ago, at the bottom of the gorge, just above the river’s surface, with a long and juicy chapter called the Queenston Shale. The plot oscillates throughout the book as you read upwards through chapters of dolostone, limestone, sandstone, and shale. The book ends powerfully (note: spoilers ahead) in the Late Silurian Period (about 425 million years ago) with the limestones and dolostones of the Lockport Group, located at the very top of the gorge. The composition of each rock unit can tell us so much information about what the environment was like here, water depth, the types of organisms that lived at the time that the sediment was deposited, and much more. We don’t see giant 11-kilometre-long rock walls like this everywhere, so who or what do we have to thank for this geologic feature? Turns out, it’s just water being water. Southern Ontario has been through its fair share of glacial events. The most recent glacial event, called the Wisconsin Glaciation, lasted roughly 75,000 to 11,000 years ago. The amount of ice that was present in the area cannot be overstated; an ice sheet approximately 1-3 kilometres thick covered the Niagara region! As you can imagine, once this ice started to melt, it produced massive amounts of water. It was all this water that reshaped the water landscape of Ontario, creating new lakes, rivers, and streams, in addition to carving out land features that affect drainage patterns. The Niagara River was formed from this glacial meltwater, as it flowed from early Lake Erie to what is now Lake Ontario (but was the ancient Lake Iroquois at the time). When the Niagara River first started to flow, it would fall over the Niagara Escarpment at its original location, flush with the rest of the escarpment in the Queenston area. It didn’t take long, however, for the water to start eroding away at the escarpment in the river channel (see image 2). The rock units of the escarpment vary in their resistance to erosion. Shale, for example, is more easily eroded by fast-flowing water than limestone. As such, these rock units would erode away first beneath the top Lockport Group layer of dolostone and limestone (we call this top erosion-resistant layer a “caprock”), which would eventually lead to the dolostone/limestone caprock breaking off due to a lack of underlying support. Over the last 12,000 years, Niagara Falls has moved upstream (due to erosion) approximately 11 km! This makes it one of the fastest moving waterfalls in the world (see video below)! Even though Niagara Falls gets all the glory, no trip to the area would be complete without visiting the Whirlpool. Located downstream from the falls, it has a maximum depth of 83 metres and produces such powerful currents that the resulting change in water levels resembles marine tides (see image 3)! When the falls eroded from the rest of the Niagara Escarpment to the location of the Whirlpool, it began to erode through the ancient glacial sediments that had filled in the St. David’s Buried Gorge. These glacial sediments were less resistant to erosion than the surrounding Paleozoic sediments, so more erosion occurred here, forming a wider section of the gorge. This also caused the Niagara River to change direction; this direction change is responsible for the turbulent waters in the Whirlpool (see image 4). The Niagara Whirlpool area has some fantastic geology that can be visited in person, but you have to know where to look. The Niagara Gorge Geotrail, available through GeoscienceINFO.com is a great resource that not only provides exact map-based GPS locations, but it also tells you what geological features you are looking at and how they formed. The Niagara Gorge Geotrail begins by setting the scene beside the Niagara Glen Nature Centre. At this stop you are standing directly on the Wintergreen Flats, which at this location form a promontory – a point of high land that extends out as a headland. Stop #2 provides a fantastic view of the magnificent rock units of the escarpment on the American side. This is best viewed in the early afternoon, as the sun lights up the different coloured layers of shales, sandstones, dolostones, and limestones. When you look at this horizontal rainbow of rocks, think about the different environments that were present at the time they were deposited. From the deeper seas that deposited the shales, to the shallower seas of the sandstones, to the often-busy metropolises of marine organisms that led to the limestone deposits. If fossils are your thing (which I know they are), stops three and five have just what you need. A giant fossilized coral reef allows you to play detective and search for all kinds of little critters from bryozoans to little now-disarticulated crinoid ossicles that once stood tall one on top of each other, holding up the fleshy part of the animal in its calyx at the top. A fallen block of dolostone contains a fossilized trilobite, a class of invertebrates that went completely extinct just before the dinosaurs came on the scene, approximately 250 million years ago (see image 5). The Geotrail wraps up with a large rock block with a strange-looking tunnel in it. It looks like this tunnel was man-made, as it’s exceptionally round and extends right through the rock. This tunnel, actually called a “pothole”, was naturally made through the abrasive action of rocks and water. Potholes form through the repetitive circular movement of
