Living on the Edge

O U R   N A T U R A L   H E R I T A G E

LIVING ON THE EDGE

Thursday, April 30, 2009

It probably doesn’t sound very flattering to say that Prince Edward County is like a lopsided layer cake.  However, it is an apt description when one looks at the geology of Prince Edward County. It is a limestone plateau from which the retreating glaciers managed to strip all or most of the overburden. Just ask any farmer who has ever tried to scratch a living from the shallow soils in some parts of the county.  Not much soil in places.  Some 250 metres below though is the basement upon which all this rests – Precambrian rock, some 950 million years old, made up mainly of complex igneous and metamorphic rock types. Travel straight north to Madoc and you begin to see this Precambrian rock quite clearly as it rises to the surface where we can’t help but be aware of its presence. It is true north country – something that doesn’t exist down here in Prince Edward County. Or  does it? Take a drive along Victoria Road, and there it is, an impressive granite rock knob, reaching boldly out onto the shoulder of the road. It’s not much for grandeur, for it’s just a fleeting glimpse as we speed by, but it’s there. It is a unique geological feature that can be found nowhere else in the County, unless one digs down 250 metres. It’s called the Ameliasburgh Inlier, but we’ll discuss that in another column.

To make this layer cake lopsided, we just have to look at the physiography of the County as a whole. Much of the north and east sides of the county are marked by a limestone plateau ranging from just a few metres to more than 75 metres high in some cases. On a map you can trace it along as it follows Victoria Road, Mountain View, County Road 14 (hence the nickname High Shore Road), levelling out a bit, then becoming  more pronounced as it travels along another appropriately named High shore Road from Green Point to Picton. It continues, impressively, to Lake on the Mountain and meanders along the south shore of the North Marysburgh Peninsula, leaving behind such spectacular landmarks as Cape Vesey, otherwise known as The Rock and, farther along, McMahon’s Bluff. As the high plateau peters out, it comes to an exhausting conclusion at Little Bluff, but not before another encore or two along the north shore of Long Point.

Compare that now to the south and west shoreline of Prince Edward County  which is relatively flat, sloping ever so gently into Lake Ontario and reaching far out into the shallows of the lake, quite noticeably, as limestone shelves.  We see this curious feature too on Main Duck Island , 19 km out in Lake Ontario, as though someone had taken a large piece of Prince Edward County, and lowered it ever so gently into the water – high, limestone cliffs on the island’s north shore, tapering south and becoming level with Lake Ontario on the island’s south shore – almost identical to the lopsided topography of  much of Prince Edward County.

An unusual topographical feature of this limestone plain in Prince Edward County, is the location of three lakes, all of them along, or very near to, the edge of the plateau. We know them as Roblin Lake, Fish Lake and, of course, legendary Lake on the Mountain. It is pretty much accepted now that Lake on the Mountain was formed by a geological phenomenon known as a collapse doline, where over many thousands of years, underground springs slowly dissolved the carbonate limestone rock, causing the roof to cave in and form a basin which became Lake on the Mountain.

Could Roblin Lake have been formed by the same process?  Check out a depth chart some time and you will see that Roblin Lake for all its size is actually quite shallow, but lo and behold, right in the middle at the lake’s west end is a basin about 15 metres in depth. A collapse doline on a smaller scale? It could very well be, for how else did this basin get there? Perhaps the same is true for Fish Lake.
Whatever the answer might be, to have these lakes living on the edge of a high plateau is a feature of Prince Edward County that we should not take for granted, as one would have to travel afar to see such an unusual physiographical feature elsewhere.

It is these natural features, and mysteries concerning how they came to be, that contributes to Prince Edward County being one of the richest natural areas in the province.

This is the fifth 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 .