Brent Limestone Cliff
The Brent Limestone Cliff Nature Zone is the small peninsula/point that separates the two "halves" of the campground at Brent. It is a chunk of rock and patch of bush that normally would not garner a second look. But a closer inspection reveals a bush that doesn't quite look right; for example, it is heavy in cedars. When we explored the point on foot, the limestone wasn't terribly evident to our untrained eyes, but it was more evident when viewed from a canoe.
The significance of the Brent Limestone Cliff Natural Zone is that "as far as is known, this is the only limestone cliff within Algonquin Park, which is on the Precambrian Shield." (Natural Heritage Information Centre). Why is that?
One interesting aspect of the Brent Crater is that it contains limestone that was deposited on the floor of the Iapetus Ocean several hundred million years ago (during the interval from the impact of the meteor about 500 million years ago to the receding of the ocean about 200 million years ago). Being protected in the crater, this limestone survived the erosion from the subsequent ice ages that removed the limestone coating from the rest of Algonquin Park.
If you examine the above image carefully (and mix in a healthy amount of imagination) you can see the glacial striations. The striations lead directly from the Brent Crater to the limestone cliff at Brent. Does this mean that the limestone cliff is associated with the Brent Crater? I don't know, but I do know that science doesn't usually rely on coincidence.
The Brent Crater and the Brent limestone cliff both lie within the Ottawa Bonnechere Graben. There are other instances of limestone outcrops within the graben (but not within Algonquin Park). According to L.J. Chapman and D.F. Putman (1973):
... Thus a block 35 miles in width is down-dropped, forming what has been called the Ottawa Bonnechere graben. Within it are several minor breaks. These are three south facing escarpments bounding blocks that are tilted towards the north, and in each case Palaeozoic limestones are preserved on their northern flanks. These are the Muskrat scarp along the northeast side of Muskrat Lake, the Dore scarp extending from Lake Dore past Renfrew towards Arnprior, and the series that can be seen intermittently along the north side of the Bonnechere River upstream from Northcote Station, and northeast of Golden and Round Lakes.
That is, the glaciers were unable to scrape off the limestone on the "lee" slopes of these scarps. Such an explanation would not seem (to me) to apply to the Brent limestone cliff
Another interesting (and possibly relevant) aspect of the Ottawa Bonnechere Graben is that the same processes (no reference, my assumption) that laid down the limestone at the bottom of Brent Crater (and universally across Algonquin park) also intruded limestone into the faults and cracks associated with the Ottawa Bonnechere Graben. This manifests itself with lime-loving plants presently growing adjacent to these geologic features. (The most notable example being all the lime-loving plants of the Barron Canyon.) And other than the Brent limestone cliff, where else do you find white cedars in abundance? While found scattered elsewhere, they are prominent in the vicinity of the Barron canyon, along the Barron River, along the lower Petawawa River from the Natch downstream, at the top of Carcajou Bay, at Eustache Lake and probably at Greenleaf Lake (I can't remember) -- wherever the bedrock has cracked open. (And of course, at the bottom of the Brent Crater.)
Frank Ahern (2006), Algonquin Park, Through Time and Space, Warwick Publishing.
L.J. Chapman and D.F. Putman (1973); The Physiography of Southern Ontario, Second Edition, University of Toronto Press.
John Eastman (1992), The Book of Forest and Thicket, Stackpole Books.
Nick Eyles (2002); Ontario Rocks, Fitzhenry & Whiteside.
Friends of Algonquin Park (undated); Barron Canyon Trail Guide.
Friends of Algonquin Park (undated); Brent Crater Trail Guide.
Carbonatite and Alkaline Igneous Rocks in the Brent Crater, Ontario by K. L. CURRIE & M. SHAFIQULLAH in Letters to Nature