The Escarpment in layers
We are about to get into some ages, so a note here that ages recorded in Ma refer to mega-annum meaning million-years, and m.y. refers to an interval of time in millions-of-years. For example, The Silurian Period lasted for 24.6 m.y. from 443.8 Ma to 419.2 Ma.
So what is the Niagara Escarpment made of ?
If you walked north from Lake Erie to Lake Ontario you would walk across bedrock that started in the early Devonian (about 400 Ma) through the Silurian (419 Ma to 443 Ma) and perhaps briefly into the late Ordovician at about 445 Ma. The Escarpment as it is currently revealed by erosion in Niagara is in the middle of the peninsula and in the middle of that time, with almost everything dating to the Silurian Period.
Let’s begin near the bottom of the escarpment rocks present 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. It was in the littoral (near-shore) zone approaching this marine environment that the sand, clay, silt, and limestone that are now rock called the Queenston Formation were deposited. 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[1]. 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 in Bronte Creek which is eroding the red shales in Burlington, ON where you can walk on it near the creek. South of the escarpment the Queenston remains buried.
The sediment that has since been lithified and which we call 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, the Grimsby, 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 may be missing. The Neagha 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). Dolostone is harder and more resistant to weathering than sandstones, it is grey in outcrop, and may have milky white areas. The top of the Reynales is eroded, another uncomformity indicating missing time in our rock history. Whether a section of limestone has been transformed (through a process called diagenesis) into dolostone depends on local conditions, so it is possible within a limestone formation to find pockets of dolostone and vice versa.
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. If conditions were right some of the calcium (Ca) in the limestone was swapped out for magnesium (Mg), forming the mineral dolomite which is the main component in the rock dolostone.
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[2]) 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. 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 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.