I treasure memories of times paddling the Rideau from Jones Falls into Morton Bay. There, we’d draw our canoes onto the granite shore and explore our way to the top of Rock Dunder. Where sky met horizon to the south-west, we could see the skyline of the historic Limestone City. Turn to the south and those with good eyesight could see the Sky Deck overlooking that Garden of the Great Spirit, the Thousand Islands. Black silhouette of tower. Blue of sky. Green of summer forest. On other occasions in the white of winter and with troops of Scouts, we explored Rock Dunder’s wonderland by day and found welcome shelter by night in a rustic log cabin. Recently, my wife and I enjoyed the privilege, the pleasure and the physical exercise of exploring Rock Dunder again.
Rock Dunder is shown on some geological maps as a pluton. On others it is part of a much larger igneous feature termed Lyndhurst granite. But beyond definition, Rock Dunder is a remarkable formation of beautiful pink granite. It took form deep in the roots of the Grenville Mountains that a billion years ago towered over this part of Laurentia. We can call this core of our continent the Canadian Shield. These massive snow-capped ranges and deep unforested valleys once extended north-east to present-day Labrador and south-west to Kansas. In fiery ovens deep beneath those mountains, plutons such as Westport Mountain and Rock Dunder were formed, their mineral recipes were baked.
Earthquakes in Indonesia remind us that mountains are still being heaved up when and where the plates of our planet’s thin crust jostle and collide. Volcanoes erupt on land and beneath sea. Plutons are forming within the crust as they did a billion years ago.
Looking off the “steep” end of Rock Dunder photo by: Doug Bond
Conversely wind, water and ice eternally wear mountains down even to their very cores. The Ice Ages of the Pleistocene Epoch occurred within the last mere million years of our planet’s history. That choreography of ice did much to sculpt Rock Dunder’s ancient and hard pink granite into its present shape on the Rideau Cataraqui landscape. Indeed, several great advances of continental and alpine glaciers occurred during the Pleistocene across the higher latitudes and altitudes of North America and all other continents. These advances were interspersed with interglacial phases; millennia of climate much like the present. Our most recent Ice Age (the Wisconsin) seems to have begun about 100 000 years ago. Shorter summers failed to melt all the winter snow that fell on the highlands of Quebec and Labrador. Delicate snow flakes packed into layers of bluish ice along with bubbles of air, grains of pollen from distant flora, particles of ash from volcanoes and forest fires half-the-world away. When annual layers of ice accumulate to a depth of about 50 metres, a glacier is born. It starts to flow under its own weight like icing on a cake; oozing and sliding outward and/or southward a few metres to several kilometres per year. From Northern Quebec and Labrador, the Laurentide Ice Sheet crept south into the St. Lawrence Valley, across eastern Ontario and as far south as New York City.
To the west, the Laurentide teamed up with the continental ice sheet that was spawned on the highlands of Keewatin. Together, they bulldozed as far south as Wisconsin. Like Caesar’s army, the Wisconsin Ice Age came. It conquered. It melted away. It melted from its gravels and boulders piled on New York’s Long Island about 12 000 years ago. About the same time it melted from the bouldery moraines it had bulldozed onto Wisconsin. It finished face-lifting Rideau country roughly 10 000 years ago. It melted out of Hudson Bay about 5 to 3 millennia ago. You and I are helping its disappearance from the Arctic now.
The Wisconsin Ice Sheet was not only composed of layers of ice. It carried in its sole fragments of rocks picked up as it slid southward; fragments as tiny as clay platelets; boulders as big as your house. Some fragments were very hard, like pebbles and boulders of quartzite that could survive hundreds of kilometres of transport and grinding. Some diamonds from Inuit lands of the present were exported to the northern U.S.A. Soft soapstone did not survive the trip.
Here in Rideau country, glacial flow was from north-east to south-west. Coincidentally this was the same orientation as the ranges, valleys and roots of the Grenville Mountains of a billion years ago. The Wisconsin Ice Sheet was 2 to 5 kilometres thick. It oozed across the countryside, flowing around and even up and over mere rocky obstacles like the hard granite of Rock Dunder. Conversely, the soft skarn beneath nearby Morton Bay was gouged and deepened. Murphy Bay on the Big Rideau and most local bays and lakes were gouged and deepened, north-east to south-west. Canoe Lake and Loughborough Lake are fine examples of long, narrow lakes sculpted by the ice sheet. The fabled Finger Lakes of New York State became the ancestral waters and shores of the Iroquois. If such deep and narrow basins were adjacent to a sea or an ocean, they would be filled with salt water rather than fresh water. They would be called fjords like you would see along the coasts of Norway, Labrador and the west coast of Newfoundland.
The Wisconsin Ice sheet oozed over the hard granite of Rock Dunder like a massive sheet of sandpaper. It polished the north-east (stoss) end and its flanks. In some places, hard rocks in the sole of the ice sheet chipped half-moon shapes in the granite, crescents called chatter marks, notes in the “sole music” of the Ice Age. At Dunder’s south-west end something quite different happened. Even the hardest of granite has cracks called joints, some very visible, some microscopic. As the ice sheet made its way over the lee end of Dunder, it tugged and pulled on these lines of weakness much the same as you might fan a deck of cards with your thumb. Blocks of Dunder’s hard pink granite were pulled away by the oozing ice and were then used to gouge and polish more southerly lands. This left the south-west end as a very sheer cliff many metres high.
Cliffed at the lee end, gently polished at the stoss end; these are features of a glacial landform called a roche moutonnée; a “stone sheep” in French. Rock Dunder is one heck of a big sheep! And a pink sheep at that! Maybe the term came about because flocks and shepherds found refuge from cold winds and blizzards sweeping down Alpine valleys. Maybe the cliffed lees of roche moutonnées were giant motherly ewes of stone. Indeed, at the base of the shear south-west end of Rock Dunder we discovered, not sheep but an amazing ecosystem. Here trees and smaller plants typically found far to the south are protected from cold north winds and nurtured with extra warmth by sun’s energy re-radiated from its granite backdrop. In this tiny Garden of Eden we found American Linden trees at the very northern limit of their habitat, with leaves big enough that Adam and Eve might have started a clothing boutique here by Morton Bay. On the opposite or north-east end of Dunder, more exposed to wintry north winds and more oblique to the sun’s rays, we found trees and mosses typical of more northern latitudes. This tells of the incredible biodiversity to be found on every hill, every island in Rideau country.
Willows Mountain near Philipsville is another example of a large roche moutonee. Here hard quartzite is gently polished on its north-east end and steeply plucked on its south-west end. Quartzite can be even more resistant to time and erosion than granite. Hard rocks in the sole of the Wisconsin Ice Sheet left long scratches that are still visible in the quartzite, scratches called striations, natural compass lines, N.E. – S.W. Glacial flutings are exceptionally large and long striations or grooves usually evident in softer bedrock such as can be seen in the lime silicate rock just south of the iron bridge over Lyndhurst Creek downstream from Lyndhurst Lake and in Central Park in New York City.
Drive the North Shore Road or hike the Rideau Trail from Narrows Lock to Westport over the granite landscape of Westport Mountain. There you can see many smaller examples of Ice Age sculpturing, pink bedrock formations polished to the north-east, plucked to the south-west end. Maybe it was among such smaller roche moutonnées of gray granite that French shepherds found confusion between sheep and shape. Maybe it was the falling snow. Maybe it was the misty mornings. Maybe it was the French wine.
“Ís og Eldur” (Ice and Fire) - the motto of Iceland, an incredible laboratory of active volcanoes, of glaciers and of human persistence.
“Rock Dunder” – fire and ice - a remarkable formation born in fire, shaped by ice and is now preserved for all of us through the persistence of the Rideau Land Trust. Thanks to all of the Rideau Land Trust who made it public! Thanks to all who help make Dunder accessible to us Sunday afternoon explorers. Pack appropriate footwear and clothing, your binoculars and your camera. Travel Highway #15 to just south of Morton. Take Stanley Lash Lane to the Rock Dunder parking lot. Put on your hiking shoes. Feel awe in the centre of panorama in the green of summer. Find inspiration in the unforgettable carpet of colours that unfolds below you in autumn. Treasure the Spirit of Dunder!
Embleton, C. and King, C.A.M., Glacial and Periglacial Geomorphology (1968)
Wright, H.E. and Frey, David G., Quaternary of the United States (1965)
Map 2054, Gananoque Area, Ontario Dept. of Mines (1963)
Geology/Natural History tours are sponsored annually by the Bastard and S. Burgess Heritage Society on the third Sunday afternoon of September (subject to weather) when we look at some of the sites of significance in our local and very rich natural history.