CURRENT INTERPRETATION RETREATING WISCONSIN ICE SHEETS
The Niagara River, its Gorge and Falls as we see them today had their genesis fifty five hundred years ago when water from the retreating Wisconsin ice sheets ice drained to the sea.
At the Whirlpool, the river incised into a pre-existing abandoned channel and the river waters were concentrated within this narrow chasm. The resulting deluge scoured out this channel – possibly within days.
As water levels settled down these huge, churning rapids subsided into something like the falls we know today.
The Niagara River continued to scour out the channel upstream creating the Niagara Gorge at approximately 1m per year, placing the modern day Niagara Falls 5 kilometres upstream from the Whirlpool.
WARM TROPICAL SEAS
Not only does the erosive power of the Niagara River provide insight into the rate of geologic change but the walls of the gorge itself also record huge amounts of geologic time.
The 90 metres from the top of the falls to the bottom of the gorge represent some 40 million years. The Niagara River tumbles over the hard resistant Lockport dolomite and thunders into the gorge below. The dolomite was deposited in warm tropical seas some 400million years ago in the Middle Silurian Period.
Outcrops or surface exposures of these rocks define the Niagara Escarpment which today stands high above the adjacent Ontario farmland. This hard layer of rock is underlain by a much more easily eroded sequence of shales and sandstones and these in turn are underlain by 440 million year old shales of the Queenston Formation.
As the river cascades over the falls it carves out the softer underlying units resulting in the collapse of the hard layer above. In this way the river erodes rocks that were deposited over a period of 40 million years.
A MODERN PERSPECTIVE
Of course Spencer lacked the geological insights gained by the geological community over the subsequent 100 years after he published his work.
Geoscientists have advanced in glacial geology, geophysics, plate tectonics, stratigraphy and knowledge of sea level change. Spencers work on the Great Lakes had convinced him that the region had stood much higher before the ice age and had been subject to incursions of the sea.
He actually believed that pro-glacial lakes in the vicinity of today’s Great Lakes had been marine with floating ice, not, as we know today, freshwater lakes dammed by ice sheets. With these ice dams we know that water bodies would have been maintained at differing depths.
Even in his 1907 work on Niagara Falls he refers to abrupt up and down movements of the land depositing strong and distinct beach lines, not recognizing the varying lake levels as a result of glaciation.
OF PARTICULAR INTEREST
In describing the formation of Niagara Falls of particular note was the narrowing of the river through the Niagara Gorge with the Whirlpool at the downstream end and the Falls at the upstream end.
As described by Spencer, The modern whirlpool is the result of the re-opening of a fragment of a buried valley after the falls had broken through its side at the present outlet. He went on the say, When the full force of the current was diverted into the smaller pre-glacial channel most of the shale beds were swept away and also some of the thinner layers of limestone.
With the opening of this ancient Niagara River channel the waters were concentrated within a narrow chasm and the river, and its falls eroded the headwall of the gorge upstream.
SPENCER’S INTERPRETATION – GEOLOGICAL CONTROVERSY
During the 1800s there had been much speculation as to the origin of the Whirlpool and the gorge between this point and the falls upstream. In the 1840s James Hall had merely considered the Whirlpool to be an eddy in the course of the river.
Subsequently Lyell had noted changes in the slope of the banks of the Niagara River and attributed this to the river now flowing in the course of an earlier pre-glacial stream.
In 1881, Spencer disagreed with this interpretation and deemed the river to be only a result of modern processes. However with his subsequent borings, he was able to change his mind and the ancient pre- glacial channel was fully described.
- Origins of Niagara, A Geological History. 2010 http://www.niagarafrontier.com/origins.html
- Middleton, G.V. 2004 Joseph William Winthrop Spencer (1851-1921), in Macqueen, R.W., (Ed.) Geoscience Reprint Series 8, p.217 Geological Association of Canada
- Natural Resources Canada, 2008. Geoscape Toronto. The Niagara Escarpment Information Bulletin. http://geoscape.nrcan.gc.ca/toronto/pdf/escarp_activ3_e.pdf
- Niagara Parks Commission, 2010. Niagara Falls Geology Facts & Figures http://www.niagaraparks.com/media/geology-facts-figures.html
- Mesolella, K.J., 1978, Paleogeography of Some Silurian and Devonian Reef Trends, Central Appalachian Basin AAPG Bulletin V. 62, No. 9 (September 1978), P. 1607-1644, 18 Figs.
- Shaw, E.W., 1924, Memorial of Joseph William Winthrop Spencer: Geological Society of America Bulletin, v.35,p. 25-36
- Spencer, J.W., 1907, The Falls of Niagara: Canada, Department of Mines, Geological Survey Branch, 400p. (images 6,7,8,9)
- USGS & GSC., 1913. Topographic Map of the Niagara Gorge, 1:12,000, University of Toronto, Map and Data Library, Toronto, Ontario http://maps.chass.utoronto.ca/cgi