April 20. Niagara Falls

Niagara Falls drops over a ledge of Middle Silurian Lockport Dolomite, a relatively resistant rock unit about 100 feet thick. The dolomite is underlain by easily-eroded shales of the Medina and Clinton Groups, both Silurian, and the Upper Ordovician Queenston Shale. Near the whirlpool below the falls is a thin beach deposit called the Whirlpool Sandstone, which marks the base of the Silurian. The ledge, the Lockport Dolomite, was laid down in part of that shallow marine shelf we talked about the other day – the low-lying platforms that surrounded the deep pool in the Michigan Basin. 

Long after the Silurian Period, when the entire region of the Midwest and Ontario was lifted above sea level, erosion left the most resistant layers including the Silurian Lockport Dolomite standing as ridges and ledges. Niagara Falls is pretty much a legacy of the last Ice Age, and the falls have developed pretty much in the past 10,000 years because they serve as the outlet for one of the largest volumes of fresh water anywhere on earth – the Great Lakes. That water has to cut through or go around resistant materials, and in this case it’s cutting through the Silurian escarpment that forms the neck of land between Lakes Erie and Ontario.


The technical term for a long, low ridge of resistant rock is a cuesta. It’s a Spanish word meaning the slope of a hill. The Silurian Cuesta, which holds up the ledge over which Niagara Falls flows, can be seen easily on a map of the Great Lakes. Many of the peninsulas around Lakes Michigan and Huron are formed from the resistant Silurian rocks that were laid down around the Michigan Basin. Wisconsin’s Door Peninsula and Michigan’s Garden Peninsula in Lake Michigan, and Drummond Island, Manitoulin Island, and the Bruce Peninsula in Lake Huron are all above lake level because they are underlain by resistant Silurian rocks.

Because of flow controls the rate that Niagara Falls is cutting back through the rock has decreased from something like 7 or more feet per year to three or four feet per year – but eventually, as long as water flows, Niagara Falls will disappear. No need to rush to see it though – we’re talking many more thousands of years before Niagara Falls disappears.

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April 20 is the birthday of a couple of prominent geologists. William Edward Logan was born this day in 1798 at Montreal, Quebec. He established the Geological Survey of Canada in 1842 and had a prestigious career. The highest mountain in Canada (and second highest in North America), Mt. Logan in southwestern Yukon Territory, was named for him.

Also on this day, April 20, 1824, Jules Marcou was born at Salins, France. He worked extensively in Europe but is probably best known for his 1853 Geological Map of the United States, and the British Provinces of North America, probably the first good comprehensive geologic map of the US as it was then known. He also helped establish The Louis Agassiz Museum of Comparative Zoology at Harvard.

—Richard I. Gibson 

References 
Good field trip report
Facts and figures 

Geology

Drawing after G.K. Gilbert (USGS)