NIAGARA FALLS Escarpment
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FIELD TRIP STOP - NIAGARA FALLS (Differential Erosion and Escarpment Formation)
LOCATION: The border of New York State and Ontario, Canada.
GEOLOGIC FEATURES: Cap Rock: Differential Erosion
DESCRIPTION: Niagara Falls (actually composed of three named waterfalls - ther American Falls, Bridal Veil Falls, and Horseshoe (Canadian) Falls) is not the highest waterfall in the world, but it has the highest flow rate of any waterfall in North America. After glaciers, with thickness more than one mile, carved out the Great Lakes during the last ice age (Wisconsin Glaciation about 10,000 years ago), water flowing from the Great Lakes carved a path through the preexisting Niagara Escarpment to form the Falls.
The Escarpment is not a result of a fault; it is, rather, the result of the unequal erosion between the resistant Lockport Dolomite (Middle Silurian) that forms the river-bottom at top of the Falls and the underlying softer and more rapidly eroded Lower Silurian Rochester Shales (and limestones). These two rock formations are presently exposed from the top of the falls to river level (up to 188 ft. at Horseshoe Falls). They are being eroded upstream (to the north) at a rate of about 1 ft./yr.
STUDENT QUESTIONS:
(1) Why does the Rochester Shale erode faster than the Lockport Dolomite? Explain.
(2) What is the specific definition of an Escarpment?
(3) What is a major economical use of Niagara Falls?
(4) CHALLENGE – The Falls has receded upstream nearly 7 miles over the past 12000 years. What is this rate per year? Why might the current rate be only about 1 ft/yr?
SELECTED REFERENCES:
-NYSM, Niagara Falls. Accessed on Jan. 3, 2020: http://www.nysm.nysed.gov/research-collections/geology/resources/niagara-falls
PHOTOS:
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Figure 1 - The American Falls falling over the Niagara Escarpment
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Figure 2 - American Falls (left) and Horeshoe (Canadian) Falls to the center. Exposed rocks on either side of the falls are capped by the Lockport Dolomite (Upper Silurian) which is underlain by the Rochester Fm. (mainly shales with some interbedded limestone).
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Figure 3 - American Falls (to the far left), the smaller isolated Bridal Veil Falls (between the American Falls and the extended rock exposure) and Horseshoe Falls (its western edge seen in the far right of the photo). The Lockport Dolomite forms the uppermost edge of the exposed rock. The Rochester Fm. can be observed to be more deeply eroded under the Lockport Dolomite.
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Figure 4 - Horseshoe (Canadian) Falls.