Return to the Brent Crater
2010 April 28
One of the more interesting ecological features of the crater is the cedar swamp. According to the Natural Heritage Information Centre description of the Brent Crater:
By far the most unusual and interesting plant community in the crater is a White Cedar swamp located between Gilmour and Tecumseh Lakes. Mature cedars of an average dbh of 15 inches are dominant here. The uncommon orchids, blunt-leaf orchid (Habenaria obtusata) and adder's mouth (Malaxis brachypoda) are locally abundant here. Other unusual plants for this locality are pinesap (Monotropa hypopothys), one-flowered pyrola (Moneses uniflora), bulblet bladder fern (Cystopteris bulbifera), and leatherwood (Dirca palustris). The area between Gilmour and Tecumseh Lakes is botanically unusual and possibly unique in Algonquin Park. Large Basswood 25 inches in diameter are common here as well as large White Spruce of about the same diameter. On the lower ground huge White Cedar are dominant. Beneath these, the moss carpet is richer than anything seen so far in the Park. The mosses are species which grow only in calcareous sites and are very uncommon, if not absent, elsewhere in the Park. Bulblet bladder fern (Cystopteris bulbifera) growing abundantly here, has been recorded from no other areas in the Park. This cedar swamp is similar to those of the Bruce Peninsula and the fact that it is found in the Precambrian Shield makes it exceptionally interesting.
One of the goals of this trip was to locate and explore the cedar swamp. Another goal was to locate the "canyon" mentioned by Charles O'Dale.
Our plan was to generally follow the hiking trail, but to diverge from it in the vicinity of the gully to look for the cedar swamp. Farther along the trail, there is a zone of shattered rock dating from the original meteorite impact, and we intended to leave the trail again here to look for the canyon.
It was early spring and the wildflowers were just starting to come out and, fortunately, the bugs not quite yet.
We left the trail at the gully and started following the creek downstream. A little way downstream, Muskwa Creek flows into Little Gilmour Creek, which drains Rana Lake. From here downstream, the creek flows through a cedar swamp. (The hiking trail skirts the southern edge of the swamp). There were many large cedar trees and a few large spruces, but we didn't find any of the large basswoods mentioned in the description from the Natural Heritage Information Centre. However, we believe the cedar swamp to be very extensive and we visited only a very small part.
Thrashing through the swamp wasn't that difficult, but this has been a dry spring. We suspect that in a normal year, it would be much wetter.
After thrashing around in the swamp — with just one soaker (Bob) — we got out our compass and set a course back to the trail near where it visits Tecumseh Lake. It wasn't very far, but that wasn't at all obvious at the time.
We stopped for lunch just beyond the shattered rock site. Our plan for after lunch was to attempt to reach the canyon mentioned by O'Dale (location 'B' of Part II). We figured that if we headed generally eastward, and neither lost nor gained altitude, we would be following the shattered rock zone. The canyon is apparently located where a small creek cuts through this zone. According to our estimates, it wouldn't require more than a few hundred meters of bushwhacking.
After a hundred meters or so, we decided that discretion was the better part of valour; we turned around and returned to the trail. This bushwhack was rougher and more hazardous than in the cedar swamp; the potential for injury or getting lost was much greater. It was a bit much for a pair of senior citizens without additional support.
We returned to the car by backtracking and taking the "shortcut" that connects the two sides of the hiking trail. Although a little longer, this route avoids having to walk along the Brent Road.
The trail was refurbished in 2008 (including the new observation tower) and the trail guidebook was revised. There is little difference in the revised guidebook, but the numbering of the various posts along the trail has changed. If you use an old guidebook you may be confused. Spend the fifty cents; buy a new guidebook!
There are several points of interest illustrated in the above image:
The bottom of the crater, and thus the bottoms of Tecumseh Lake and Gilmour Lake, their shorelines, and the land between them, is glacial drift and consequently of less interest than the walls of the crater. On the floor of the crater, most of the really interesting stuff is deeply buried.
There is an arc of exposed limestone along the southern edge of the bottom of the crater. The cedar swamp is associated with this feature. Most of this area is along Shingeris Creek and its tributaries. Shingeris Creek drains into Gilmour Lake. However Muskwa Creek also cuts through this feature to create the gully visited by the trail. Although we visited only a small part of the swamp, we surmise that it extends almost 3 km in an arc from the bottom of Tecumseh Lake to the bottom of Gilmour Lake adjacent to the limestone outcrop.
There is a ring of shattered rock, roughly coincident with the rim of the crater. The canyon described by O'Dale was caused by a creek cutting through the shattered rock.
One aspect of the crater that I find interesting — although probably trivially obvious to a seasoned geologist (or alternatively, perhaps wrong!) — is that the crater has remained circular and not tilted. I take this to mean that this particular piece of bedrock has remained stable and has been subject only to erosion, and not bending or folding, for almost half a billion years. This is in contrast to the severe bending, folding, faulting, etc. that this piece of bedrock was subject to during the Grenville Orogeny 1.1 billion years ago. The crater is located near the edge of the Ottawa Bonnechere Graben which formed 600 million year ago (~150 million years before the Brent Crater) during the breakup of the supercontinent Rodinia If the graben was reactivated 200 million years ago during the breakup of the supercontinent Pangea, it did not significantly affect the Brent Crater. Other than perhaps that, this ground has been quiet for a long time. Of course, the multiple ice-ages were somewhat disruptive. (Eyles and Wikipedia disagree about the chronology here a little; I am following Eyles).
- Friends of Algonquin Park (2008); Brent Crater Trail Guide.
- Nick Eyles (2002), Ontario Rocks, Fitzhenry & Whiteside.
- O'Dale, My Aerial Explorations of Terrestrial Meteorite Craters - Brent Meteorite Crater
- Natural Areas Report: ALGONQUIN PROVINCIAL PARK - BRENT CRATER
- The melt rocks at Brent Crater, Ontario, Canada by Grieve, R. A. F., In: Lunar and Planetary Science Conference, 9th, Houston, Tex., March 13-17, 1978, Proceedings. Volume 2. (A79-39176 16-91) New York, Pergamon Press, Inc., 1978, p. 2579-2608.