Prince Edward County, Ontario, Canada
O U R N A T U R A L H E R I T A G E
MCMAHON BLUFF, A RARE NATURAL GEM
Thursday, July 23, 2009
It is known as a ‘mesa,’ a 97-hectare (240 acres) rocky headland unlike any other in eastern Ontario. Similar flat tableland forms are common in the Guiana Highlands of South America and are known there as a ‘tepuis’ or table top mountains. One of the most outstanding tepuis in South America is Auyantepui, the site of the famous Angel Falls, the world’s tallest waterfall.
The tepui or mesa in Prince Edward County may not boast a famous waterfall, but it has other attributes to offer. Nestled between County Road 13 and the waters of South Bay, McMahon Bluff rises above the water almost 30 metres, or 100 feet. It is an impressive backdrop to the mouth of the Black River. Its flat tabletop plain stretches for more than a kilometre, before sloping steeply to a lower terrace, covered by a rich, mature sugar maple dominated forest.
For visiting botanists, it would not be an exaggeration to say that McMahon Bluff’s spectacle of rare plants has caused many observers to break down and weep openly. Found at this site have been both nationally and provincially significant plants, among them three species of rare sedges. Hairy wood brome grass, hairy wild rye grass and tall millet grass are a few of the regionally rare grasses that have been identified, and other important species have included American ginseng, white vervain, Carolina rose, Pennsylvania blackberry and butternut.
However, the crown jewel was the discovery of the extremely rare four-leaved milkweed which had been assumed to be extirpated from Canada some 50 years previously! And here it was, on McMahon Bluff, its lavender flowers and whorls of four leaves catching the eyes of one informed botanist on the wooded summit, setting it apart from other more common milkweeds in this area. This milkweed member has been ranked by the Natural Heritage Information Centre (NHIC) as “critically imperilled” in the province. The NHIC was established over 15 years ago as a joint venture between the Ontario Ministry of Natural Resources and three other partners, whose role it is to assimilate data on the province’s rare species, vegetation communities, and natural areas.
From a rare plant perspective, McMahon Bluff is indeed very significant. So, how did this unusual landform originate anyway? MNR ecologist at Kingston, Todd Norris, says this drumlin-shaped bedrock knoll is part of a suite of features identified in the area which suggest that late glacial floods beneath the melting ice carved the surface of the limestone bedrock here into spectacular streamlined features. The drumlin that is McMahon Bluff is at the head of one of several such subglacial streams, and exhibits on its flanks the carved forms typical to this kind of environment of formation.
In the past, the land was used for wood cutting and agriculture. However, it was pastured lightly enough and with minimal intervention such as cultivation of non native hay grasses, that the provincially rare and threatened bur oak-shagbark hickory alvar is still present in a condition not far from what it would have been prior to European settlement. Some who have studied the area extensively claim it is the best example of bur oak-shagbark hickory alvar woodland to be found anywhere in eastern Ontario.
Over 50 species of birds have been observed there during the breeding season, and the site is a likely candidate for the bald eagle, if and when the species ever returns to Prince Edward County as a nesting species. Among the warblers found here during summer was a pair of back-throated blue warblers, a species which is usually very much absent from the north shores of Lakes Erie and Ontario. As a neotopical migrant that nests primarily in the mixed forests of northern Ontario, its presence is indicative of the unique and rich variety of habitats found at McMahon Bluff.
The McMahon Bluff mesa is just another example of the incredible diversity of natural features we are privileged to have in Prince Edward County, as part of our natural heritage.
This is the eleventh in a series of columns by Terry Sprague on the natural heritage of Prince Edward County, sponsored by the Prince Edward Stewardship Council. For more information, check out their website at http://www.ontariostewardship.org .