Return to the Brent Crater

While Bob had visited the floor of the Brent Crater a couple of times (see: Day Trip into the Brent Crater), Diana never had. So in mid April 2010, Bob and Diana portaged a canoe down to Tecumseh and Gilmour Lakes and essentially repeated the trip referenced above. However, there were sufficient details that we wished to check out further that we returned the following week. In particular, we wished to examine more closely the gully cut by Muskwa Creek, and also to locate the cedar swamp that lies "between Gilmour and Tecumseh Lakes". Additonally we were interested in finding a "canyon" that we had learned about on the web. This time we didn't bring the canoe -- many of the more interesting geological features are best viewed on foot (and fortunately the bugs weren't out yet).

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.

Brent Crater Trail in Algonquin Park

(photographed: 2010-04-28 - map - explore

Near the beginning, the trail passes through a typical Algonquin mixed forest growing on a thin veneer of glacial till over precambrian bedrock.

Brent Crater Trail in Algonquin Park

(photographed: 2010-04-28 - map - explore

Soon the forest opens up into a very pleasant hardwood bush.

It was early spring and the wildflowers were just starting to come out and, fortunately, the bugs not quite yet.

red trillium along Brent Crater Trail in Algonquin Park

(photographed: 2010-04-28 - map - explore

Red trillium

white violet along Brent Crater Trail in Algonquin Park

(photographed: 2010-04-28 - map - explore

White violet

Muskwa Creek along Brent Crater Trail in Algonquin Park

(photographed: 2010-04-28 - map - explore

Muskwa Creek flows through the hardwood bush. At this point, it is a very tidy creek, cutting througn the soil of the hardwood bush.

along Brent Crater Trail in Algonquin Park

(photographed: 2010-04-28 - map - explore

This area has been previously burned and many of the charred stumps remain.

Muskwa Creek along Brent Crater Trail in Algonquin Park

(photographed: 2010-04-28 - map - explore

As Muskwa Creek continues its journey down into the crater, it begins to carve out a path in the forest floor.

gully along Muskwa Creek in the Brent Crater in Algonquin Park

(photographed: 2010-04-28 - map - explore

The gully (post 4 in the 2008 trail guide). Here Muskwa Creek has managed to cut itself a significant ravine. This is very unusual for an Algonquin creek, because usually the bedrock is, to all intents and purposes, impervious to significant erosion by flowing water.

rocks in gully along Muskwa Creek in the Brent Crater in Algonquin Park

(photographed: 2010-04-28 - map - explore

However, the bedrock here is not insoluble granite or gneiss, but a conglomerate cemented together by gritty limestone, as shown in this and the two photos following. The embedded stones are the remains of mechanically eroded talus that fell from the original steep walls of the newly formed crater. The talus was reworked by the waves of a tropical sea and then embedded in the mud of the sea bottom.

rocks in gully along Muskwa Creek in the Brent Crater in Algonquin Park

(photographed: 2010-04-28 - map - explore

rocks in gully along Muskwa Creek in the Brent Crater in Algonquin Park

(photographed: 2010-04-28 - map - explore

rocks in gully along Muskwa Creek in the Brent Crater in Algonquin Park

(photographed: 2010-04-28 - map - explore

Bob, checking out some of the rocks. The rocks were in fact quite crumbly; that is, the gritty limestone is not a very strong cement.

fern in gully along Muskwa Creek in the Brent Crater in Algonquin Park

(photographed: 2010-04-28 - map - explore

The trail brochure makes much of the bulblet bladder ferns that grow in this limestone-based environment. As it was very early in the growing season, any ferns were just emerging. We don't know if this is the fern in question or not. Bob suspects it is; Diana is less sure.

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.

cedar swamp in the Brent Crater in Algonquin Park

(photographed: 2010-04-28 - map - explore

A large white cedar near the edge of the swamp.

cedar swamp in the Brent Crater in Algonquin Park

(photographed: 2010-04-28 - map - explore

Cedars beside the creek, farther into the swamp.

cedar swamp in the Brent Crater in Algonquin Park

(photographed: 2010-04-28 - map - explore

The creek water is very clear.

cedar swamp in the Brent Crater in Algonquin Park

(photographed: 2010-04-28 - map - explore

Cedar trees and standing water in the swamp.

cedar swamp in the Brent Crater in Algonquin Park

(photographed: 2010-04-28 - map - explore

Bob climbing over a the trunk of a fallen white cedar. This gives an idea of the size of many of the trees in the swamp.

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.

Tecumseh Lake in the Brent Crater in Algonquin Park

(photographed: 2010-04-28 - map - explore

Launch site on Tecumseh lake. Note the prominent red clump of pitcher plants in the foreground. While pitcher plants are a common Algonquin plant, the ones that grow here are particularly vigorous and brightly coloured.

Brent Crater Trail in Algonquin Park

(photographed: 2010-04-28 - map - explore

Looking through the trees from near the shattered rock zone. The north rim of the crater defines the horizon in the distance.

shatter zone along the Brent Crater Trail in Algonquin Park

(photographed: 2010-04-28 - map - explore

shatter zone along the Brent Crater Trail in Algonquin Park

(photographed: 2010-04-28 - map - explore

shatter zone along the Brent Crater Trail in Algonquin Park

(photographed: 2010-04-28 - map - explore

The above three photos show some of the rocks in the shatter zone (post 7 in the 2008 trail guide).

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.

Brent Crater Trail in Algonquin Park

(photographed: 2010-04-28 - map - explore

Along our bushwhacking route. What this photo doesn't show is how steeply the ground drops off to the left and rises to the right.

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.

Notes

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!

geology of Brent Crater in Algonquin Park

(photographed: 2010-04-28 - explore

This coloured satellite view of the Brent Crater shows the various geological features of the crater based upon data presented in the paper by R.A.F. Grieve (1978). The precambrian bedrock is shown in pink, glacial till in yellow and limestone in blue. The zone of shattered bedrock is indicated by the "spider lines". (Note that these lines are schematic and only indicate the shattered area; they are not a mapping of actual cracks.) The current rim of the crater is roughly coincident with the outer edge of the shattered zone. Tecumseh Lake is the eastern lake within the crater while Gilmour Lake is the western one. The Brent Road is shown as the brown line cutting across the southeast quadrant, while the walking trail is shown in red. The "gully" is near the north west corner of the walking trail whereas the location highlighting the shattered rock is near the north east corner of the trail.

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).

Sources