The Escarpment in layers
Even in the Niagara Region there are local variations in which units are present in the escarpment (the Neahga Formation appears in some areas of Niagara and not in others, ditto for the Thorold Formation). Since the rocks of the escarpment are sedimentary, not all were deposited in all places (originally, as sediment), and some of the material that was deposited was eroded away before it could be turned to rock (lithification) leaving a gap in the rock record called an unconformity.
We are about to get into discussing some ages, so a note here that by convention ages are recorded in Ma which means mega-annum or million-years, and intervals of time are recorded in m.yr. or m.y., both referring to million-years. The difference between the two is akin to stating that your study hour begins at 2 o'clock PM, lasts for 1 hour, and ends at 3 o'clock PM. For example, The Silurian Period began at 443.8 Ma and lasted 24.6 m.yr. until 419.2 Ma.
What is the Niagara Escarpment made of?
Information on formations from [1] and [2].
If you walked north from Lake Erie to Lake Ontario you would walk across bedrock that started in the Devonian (about 400 Ma) then the Silurian (419 Ma to 443 Ma) and perhaps briefly into the Ordovician at about 445 Ma. Looking at the chronostratigraphic chart, you will notice that the oldest units are nearest the surface in the north near lake Ontario and the youngest units are present at the surface in the south near Lake Erie. The layers of bedrock in Niagara tilt slightly to the south and are layered chronologically, the visibility of the Silurian age rocks instead of Devonian is due to erosion, with the younger Devonian rocks having been removed in the north. The Niagara Escarpment rocks are almost entirely from the Silurian.
Let’s begin near the bottom in Niagara with the Queenston Formation. The Queenston Formation was deposited when the area was coastal, with a sea to what is now the north. This began toward the end of the Ordovician Period at 443 Ma with clay being deposited in an off-shore environment, Niagara at that time was under the sea and was marine for most of the relevant geological periods. Today this sediment has been turned into a red shale rock which gets thinner to the west.
In Niagara, the Queenston is up to 243 m thick (as revealed by drilling), but on the Bruce Peninsula it is down to about 60 m[3]. Along most of the Niagara Escarpment the Queenston is below the surface, so technically not part of the scarp that we see, but in many places the upper ten to one-hundred feet of the Queenston is revealed, forming the bottom of the escarpment. A good place to see the Queenston is at Bronte Creek which is eroding the red shales in Burlington, ON. You can walk on the Queenston near the creek. South of the escarpment the Queenston remains buried.
The sediment that formed the Queenston Formation arrived here from the east and in fact is the westernmost part of a wedge of sediment called the Taconic clastic wedge. The eastern source of sediment was the Taconic Mountains which at that time were still rising. Today’s Appalachian Mountains around Eastern New York state and New England are a remnant of these mountains which have eroded from great height.
The top of the Queenston is truncated, which is to say it was eroded before another layer was deposited on top of it, leaving an unconformity between it and the next units up which are the Whirlpool and Manitoulin Formations of the Cataract Group. The Cataract Group rocks were deposited early in the Silurian Period. Since rocks deposited early in a period are the lowest rocks of that time stratigraphically, in Niagara the Cataract Group represents the Lower Silurian.
While other formations are visible elsewhere, in Niagara the Cataract Group is composed of the Whirlpool (at the bottom), the Manitoulin, the Cabot Head Formation, the Grimsby Formation, and the Thorold Formations. This group is primarily sandstone, with the Whirlpool being white quartz sandstone, the Cabot Head (also known as the Power Glen) being a red to grey shale and sandstone. The Grimsby and Thorold Formations are stained sandstone with minor shale and are difficult to tell apart. The Grimsby is red-stained and the Thorold can be red, but is primarily grey-green and is not present everywhere in Niagara. The Thorold is part of the overlying Clinton Group, but because it cannot always be distinguished from the Grimsby it is included here.
The Clinton Group has a couple units that are not found everywhere in Niagara, in addition to the Thorold, the Neahga Formation may be missing. The Neahga Formation is a green shale. If the Thorold and Neahga are not present, the first unit discernable above the Cataract Group’s Grimsby-Thorold Formations is the Reynales Formation.
Geology students studying core from Lake Erie may be especially fond of the Reynales Formation because, while studying core samples, the Reynales is the first formation in some time that stands out from the lower units because it is composed of limestone and dolostone or argillaceous dolostone (that is dolostone with clay sediment in it). Dolostone is a carbonate rock in which some of the calcium (Ca) of limestone has been replaced with magnesium (Mg). This, essentially, converts the mineral calcite to the mineral dolomite, and dolostone is named after the dominant mineral in it, dolomite. Whether a section of limestone has been transformed into dolostone depends on local conditions, so it is possible within a limestone formation to find pockets of dolostone and vice versa. Dolostone is harder and more resistant to weathering than sandstones, it is grey in outcrop, we will disucss it more in the next section. The top of the Reynales is eroded, another unconformity indicating missing time in our rock history.
Above the Reynales (but still deposited in the Lower Silurian) are the last three formations of the Clinton Group, the Irondequoit, the Rochester, and the Decew. The Irondequoit Formation is a limestone with fossils of crinoids, which were marine invertebrates. The limestones deposited in the top of the Clinton Group are formed from the shells (often microscopic) of marine creatures which settled to the ocean floor at the time, providing the calcium-carbonate (CaCO3, calcite) that would become the limestone.
Above the Irondequoit is the is the Rochester Shale (the bottom of which is the Lewiston Member and the top the Burleigh Hill Member in Niagara). The Rochester shale is a dark grey calcareous shale (mudstone) with some limestone layers, and importantly, it is fossiliferous. For trilobite fans, this is the formation you are looking for, however, be aware that fossils differ by region, the Rochester extends for hundreds of kilometers and taking fossils is forbidden in some areas. Through the time of the Rochester’s deposition (as interpreted from outcrops in Niagara and Western New York by C.E. Brett, 1983[4]) sea level rose and shallowed and then shallowed some more!
The result of this ancient sea-level change (eustacy) is a layer cake style of deposition, with carbonates deposited toward the top. The Burleigh Hill Member which forms the top of the formation grades into the Decew Formation, an argillaceous (including clay) dolostone. If you are in Brock’s vicinity then you will find Burleigh Hill just to the north-east and Decew Falls to the south-west, both within 3 km of the university. West of Grimsby the Burleigh Bill Member is replaced by an argillaceous dolostone called the Stoney Creek Member. It is likely that the Rochester was deposited in a lagoon type environment with a relatively stagnant body of water intermittently disturbed by strong storms.
The Decew Formation is the top of the Clinton Group, it is an argillaceous dolostone which Brock’s Earth Sciences students might tell you has some very nice vugs. Vugs are cavities in limestone and dolostone in which nice little crystals of calcite, dolomite, sphalerite, gypsum and other minerals often form. They are also nice homes for small spiders.
And now we arrive on top (in Niagara), at the Lockport Formation, which begins with the Gasport member.
The Gasport Member is blue-grey limestone-dolostone with crinoids and thick beds. If you’re lucky you may find some rough corals in the Gasport which can reach 10 meters thickness. Above the Gasport is the Goat Island Member, named after the island atop Niagara Falls. The Goat Island Member is light brown dolostone with thin beds and some development of chert. The Eramosa Member is the top of the Lockport, it is a bituminous dolostone, meaning a dolostone with some shaley beds that have minor coal development. The Lockport formation in Niagara is the top, it is the resistant layer over which the Niagara Falls flow.
There are a few more units that are higher stratigraphically than the Lockport Formation around south-western Ontario, such as the Guelph Formation which brings us into the Upper Silurian (or Late Silurian chronologically). The Guelph is a tan-brown dolostone with thick beds and a significant gas deposit. On the southern side of the Niagara Peninsula there are Devonian rocks, some of which have particularly fertile fossil beds. You can take a look at these in a former quarry in Port Colborne, now the Wainfleet Wetlands Conservation Area, and at the edge of Lake Erie in Rock Point Provincial Park.
[1] Armstrong, D.K., Dodge, J.E.P. (2007) Miscellaneous Release - Data 219: Paleozoic Geology of Southern Ontario, Project Summary and Technical Document. Ontario Geological Survey Sedimentary Geoscience Section
[2] Armstrong, D.K., Carter, T.R., (2006) Open File Report 6191: An Updated Guide to the subsurface Paleozoic Stratigraphy of Southern Ontario. Ontario Geological Survey, Queen's Printer for Ontario
[3] Brogly, Martini & Middleton (1998) The Queenston Formation: shale-dominated, mixed terrigenous-carbonate deposits of Upper Ordovician, semiarid, muddy shores in Ontario, Canada. Canadian Journal of Earth Sciences, 35 (6),
[4] Brett, C. (1983). Sedimentology, facies and depositional environments of the Rochester Shale (Silurian; Wenlockian) in western New York and Ontario. Journal of Sedimentary Petrology, 53(3), 947–971. https://doi.org/10.1306/212F82F1-2B24-11D7-8648000102C1865D