Glaciation at and on the escarpment: carving the layer cake
Glaciation, a nice break from the heat
Long after the last of the sediment that became the Escarpment was deposited in a tropical environment, and still a long time after those sediments were lithified into rock, Earth’s climate progressed into a period of regularly alternating warm and cold periods, the cycles of glaciation. Our little planet has been an icy planet for most of the Quaternary Period (~2.58 m.yr.) and remains so now, even if it doesn’t seem that way mid-summer. For the past 780 k.yr., Earth has regularly alternated between warm periods similar to what we live in now, and cold periods where snow and ice accumulated and spread, covering about half of the northern hemisphere. The Great Lakes Region has been glaciated at least six time in the past 780 k.yr.
We call the last glaciation the Wisconsinan in North America, in Europe it is called the Wurmian (or Devensian in Britain), whereas the last interglacial (~125 Ka) is called the Sangamonian in North America and the Eemian in Europe[1].
A bit of context on the climate cycles of Earth
Paleo-climate science is a multi-disciplinary field, but perhaps the most straight-forward explanation for how we can tell what climate was like in the past is through stable isotope analysis. We will just briefly look at this, so don't be alarmed if chemistry gives you anxiety. Marine creatures that make their shells (or tests) from calcium carbonate or silica are inadvertently taking a snap-shot of the ocean’s isotopic composition. The ratio of oxygen with 18 neutrons as opposed to 16 neutrons, and of carbon with 13 as opposed to 12 neutrons can be taken from these shells and used as a very predictive indicator of past climatic conditions. Data from these sources are organized and form Marine Isotope Stages. Additionally, oxygen and hydrogen isotopes are taken from ice cores in Antarctica and Greenland which relate another side of the same story. The take-away is that when Earth is cooler, more water gets trapped in glaciers, and typically continental precipitation has more light (oxygen-16) than heavy (oxygen-18) isotopes; this leaves the oceans holding a higher proportion of the heavier isotope (oxygen-18) which ends up enriched in shells.
Below is a history of oxygen isotopic composition (that’s the squiggly line on the left) from Lake Baikal in Siberia which shows where interglacial periods occur sharply and glacial periods slowly build. During interglacials the oxygen-18 proportion line is to the right, during glacials it makes its way to the left. Select Marine Isotope Stages are labelled and the occurrence of the Sangamonian/Eemian interglacial, Wisconsinan glacial, and the current interglacial periods have been added. Diatom abundance refers to the occurrence of small marine organisms called diatoms which are algal, form the foundation of the food-chain, and make their tests with silica (SiO2).
The Sangamonian, or Sangamon, was as warm as the present climate and one of the best places to study it is right across Lake Ontario from Brock, in the Don Valley. Here is an excerpt from a review of the geologic history of the Great Lakes by Larson & Schaetzl (2001)[2]:
*Note that Jaan Terasmae, cited above, was a Professor of Quaternary Geology in Brock’s Earth Sciences DepartmentThe best evidence for multiple glaciations in the Great Lakes watershed occurs in the Don Valley Brickyard near Toronto (Fig. 5) where a fossiliferous sand (Don Formation) rests on till and is overlain by a thick sequence of sediments associated with the Wisconsin glaciation. The fossils found in the sand include pelecypods and gastropods (Coleman 1933, Baker 1931, Kerr-Lawson et al. 1992), pollen and plant remains (Terasmae 1960, Richard et al. 1999), diatoms (Duthie and Mannada Rani 1967), caddisflies (Williams and Morgan 1977), ostracods (Poplawski and Karrow 1981), and vertebrates (Karrow 1969, Harington 1990). Studies of the diatoms, caddisflies, and ostracods all indicate that the sand was deposited in a freshwater estuary or river mouth environment and in a climate typical of temperate North America today (Duthie and Mannada Rani 1967, Williams and Morgan 1977, Poplawski and Karrow 1981). Studies of the pollen and plant remains indicate the presence of a hardwood forest which was yielding to spruce/pine (Terasmae 1960, Richard et al. 1999). On the basis of the pollen record it has been suggested that the annual mean temperature at the time the sand was deposited was probably 3°C warmer than present (Terasmae 1960). The general consensus is that sand at the brickyard was probably deposited during the Sangamon interglaciation, whereas the underlying till was most likely deposited during the Illinoian glaciation (Terasmae 1960, Karrow 1984, Karrow 1990).
We are currently in an interglacial, which is a warm period between glaciations. Stadials and interstadials are shorter periods of lower magnitude cooling and warming respectively. In the current climate regime glaciations arrive slowly by gradual cooling over about one-hundred thousand years, then the planet warms over just ten to twenty thousand years before beginning to cool again.
So what defines a glacial period and an interglacial period? In a word, ice, but specifically continental ice sheets and their expansion toward mid-latitudes such as the area of the Niagara Escarpment. The expansion of continental ice depends on climate, primarily global average temperature, but other factors including sea salinity (and thereby circulation), wind patterns, and precipitation patterns have an effect. The fundamental driving force in regular cycles of climate and glaciation is Milankovitch cycles which are resonances in Earth’s orbital parameters of eccentricity, obliquity, and precession, but the effect these orbital cycles have are exacerbated or diminished through the Earth’s atmospheric conditions and continental arrangement. Why do these orbital cycles affect Earth’s climate? Insolation, the amount of energy received on the Earth’s surface, varies with orbital parameters and this is the planet’s source of warmth.
For simplicity we will refer to the end of the Wisconsinan as the Last Glacial Maximum (LGM). It is important to bear in mind that icesheets did not simply plow forward and then recede, they moved in a stuttering fashion, forward and back and forward again[3], and not as a monolith, but in lobes that extended out and often joined. The Laurentide Ice Sheet that covered Canada east of the Rockies was formed from the joining of the Foxe, Keewatin, and Labradorean ice domes which were the last places ice persisted at the end of the glacial period. This map from the Geological Survey of Canada shows the ages of ice sheet retreat and locations of pro-glacial deposits marking the termination of advance, called end-moraines.
as usual, if you'd like to see the full size image, click on it and then select View source file.
The last glaciation receded from Niagara at about 18-14 Ka. The recession of ice sheets is not instant and takes thousands of years. As the ice recedes a new landscape is revealed with new topographic features including drumlins, kames, eskers, and kettle lakes. This is a subject studied primarily by geologists in the intertwined fields of glaciation and Quaternary geoscience.
Erosion & the Great Lakes
The ice sheets abraded the rock of the Escarpment, in some places scouring the surface, leaving bare rock and in others depositing massive amounts of till. That till is sometimes loose unconsolidated sediment, but when it is deposited under the ice and squished (ice sheet are hundreds to thousands of meters thick) it is dense and has its own story to tell. A good example of this in Niagara is on the northern shore of Lake Erie where diamict is being eroded at the water’s edge, but massive quantities of clays and sand are still there and in them are rounded boulders, cobbles, and gravel of both local and exotic origin.
The exotic clasts in glaciated areas are called erratics and they’re great fun for a geologist in southern Ontario as they afford the opportunity to hypothesize about their origin. If you found, for example, a pink boulder made of granite in Niagara, you know that the boulder is exotic because there is no granite anywhere near the surface in Niagara, so at the very least you know that the boulder rode on, in, or under an ice sheet from its place of origin to the north. The more familiar you are with the geology of Ontario, the better guess you can make about the origin of erratics.
Also at the northern shore of Lake Erie, in Rock Point Provincial Park and in Wainfleet Wetlands Conservation Area, you can find textbook examples of striations (or striae) and chatter marks in limestone. Glacial striae are grooves carved in stone by the ice sheet dragging smaller rocks across the surface. Where good contact is made, a long groove records the orientation of ice movement, as do chatter marks where the abrading rocks had less consistent contact. The picture below shows striations on the northern shore of Lake Erie.
Here is a video explaining the measurement of striations on a limestone shelf extending into Lake Erie.
While at first glance glaciation may seem like general piece of geologic history, consider that all infrastructure including roads, gas and water lines, subway tunnels, the foundations of buildings, and also all of our drinking water in most of Canada and the northern United States is on or in glacially altered sediment. Geomorphologists, who study topographic features and materials and who understand the processes forming them are advisors to the engineers and constructors of this infrastructure and to managers of watersheds. Even where few people live there are geologists tracing layered trails of mineral ore in glacial till to locate raw materials that make modern life possible. Without extensive understanding of glacial geomorphology, a geologist would become lost in understanding the dynamics of a planetary surface defined by past glaciation.
The erosion of the escarpment follows which units are more and less resistant to erosion. If you observe that the cap rocks on the escarpment are dolostone and limestone (carbonates) then you can guess which of dolostone/limestone, shale, and sandstone are most resistant. Sandstone and shale are pretty easily eroded by glaciers which scour their surface, pluck broken pieces from them and wear them down until nothing is left. This happens to carbonates, but at a much slower rate. The Escarpment’s cap rock is made of carbonates because they are more resistant to this erosion. However, if water begins splashing up against the sandstones and shales under carbonate layers, well eventually the carbonates will fall.
The Great Lakes were carved by glaciation of pre-existing bedrock contours, their size and form is a result of erosion by several glaciations[2]. Ice follows bedrock contours, but soon carves those contours out and makes new ones based on the location and direction of ice lobe flow. Melt-water accumulates under and on top of ice sheets, and when conditions are right that water is released with great force. Jökulhlaups are such glacial outburst floods and they can scour everything in their path, deepening channels and even lakes. Sometimes pro-glacial (meaning in contact right at the ice sheet front) lakes get so large that when the ice or land holding them in place gives way the result is continental-scale, such as the Missoula floods in Columbia River gorge.
Follow the water
If a continental ice sheet is melting, you may wonder, where does all of this water end up? After the LGM, meltwater pooled in front of the ice sheet forming freshwater lakes that dwarf what we know as the Great Lakes today. In a series of catastrophic releases of water, changing drainage systems, and fluctuating meltwater input, Ontario and surrounding areas were occupied by several iterations of paleolakes. Geologists name these paleolakes based on evidence of highstands and lowstands, and you can see evidence of this complex history today including remains of a forest beneath the surface of Georgian Bay and the lake sediment on which St. Catharines is built and in which wine grapes are grown in Niagara. Eventually the water is discharged to the oceans, in some instances with such volume it changes Earth’s climate for decades or centuries.
Lake Iroquois is the name of a paleo-lake that occupied the area of Lake Ontario plus a significant area past the present shoreline, and often up to the Escarpment in Niagara. This lake might be thought of as a larger version of Lake Ontario which existed until 13 k.yr. B.P.[4] and which had a surface level as high as 40 meters above that of modern Lake Ontario. To the northeast of this lake was its source, the Laurantide Ice Sheet. Below is a map reconstructing Lake Iroquois with the modern outline of Lake Ontario (dashed white line) by Bird & Kozlowski (2016)[5]. Note that the outlet of this lake crossed what is now New York State; what is now the St. Lawrence Seaway was blocked by the ice sheet.
Full Bird & Kozlowsji Map
Just like what happens today along the shore of Lake Ontario, wave action in Lake Lahontan undercut softer rocks. Lake Lahontan eventually drained to the sea and briefly the Lake basin was lower than it is today. Water flowing from the watershed continued cutting away at the escarpment, forming narrow valleys and deepening large valleys cut by the glacier. Small waterways in large valleys are typical of glaciated regions.
This brings us to our current erosional regime, which you can view in action today, as streams and rivers slowly downcut the tops of valleys further into the Escarpment. A happy effect of this erosion is that the region hosts many beautiful waterfalls such as Beamer Upper Falls in the picture below and the waterfalls out Brock University’s back door (Not being figurative, the first little waterfall behind Brock is just steps from the back door. Earth Sciences students measure stratigraphic sections in Brock’s backyard). Nearby Hamilton is sometimes called the “Waterfall Capital of the World” because while Niagara Falls are quite large, there are over 100 waterfalls in Hamilton.
[1] Lowe, J., Walker, M. (2015) Reconstructing Quaternary Environments (3rd ed). Routledge [a note to Brock ERSC students: this book, available in the library, is impeccably well-referenced and can provide most of the background needed when studying Quaternary Geology. Importantly it gives you the sources of information when studying Quaternary geology.]
[2] Larson, G., Schaetzl, R. (2001) Review: Origin and Evolution of the Great Lakes. Journal of Great Lakes Research 27(4) p518-546
[3] Straw, Allan (1968) Late Pleistocene Glacial Erosion Along the Niagara Escarpment of Southern Ontario. Geological Society of America Bulletin V.79 p. 889-910
[4] Lewis, C.F.M., Anderson, T.W. (2020) A younger glacial Lake Iroquois in the Lake Ontario basin, Ontario and New York: re-examination of pollen stratigraphy and radiocarbon dating. Canadian Journal of Earth Sciences v.57 (4): p453-463. https://doi.org/10.1139/cjes-2019-0076
[5] Bird, B., Kozlowski,A. (2016). Late Quaternary reconstruction of Lake Iroquois in the Ontario basin of New York. Map and Chart 80, New York State Museum. Albany NY (http://www.nysm.nysed.gov/common/nysm/files/mc80_iroquois.pdf)