Nature Carves the Big Rideau

Prepared by Doug Bond, Portland                                               

“T’is a blue gem like none other”. 

We have a treasury of fresh water lakes here in Canada, a legacy of more lakes than on all the rest of our blue-green planet. Just as each person on our planet is unique in features and personality nurtured through their years, so is Big Rideau Lake carved like none other in shape, shoreline and natural history. It has its own spirit: serene to stormy, and its own history molded through millions, indeed billions of years. To fathom the personality of the Big Rideau, we must map its many layers, understand its many facets and cherish its many bounties.  

With a surface area of over 100 sq. km., Big Rideau Lake is the largest body of water in eastern Ontario. Its assembly of bays and islands give it a shape as unique as any fingerprint. With a geological history that spans a billion years, the Big Rideau Lake is a blue gem set among some of the oldest rocks and minerals of our continent. Its basin was fashioned by a collision of continents. Long before the Age of Dinosaurs, even before primitive life forms moved onto land, great snow-capped peaks and treeless valleys extended from present-day Labrador through the Rideau area all the way to Kansas. These ranges have been named the Grenville Mountains. Deep beneath these Himalayan-scale peaks, Mother Nature’s “Shake and Bake” forged a great mix of minerals that are now found beneath and around the Big Rideau. 

Paddle your canoe into Horseshoe Bay along the North Shore. Discover crystals of mica in wave and ice carved rock outcrops. Some theorize that mica provided the molecular template for first life. Beyond theory, it is fact that mica weathers into clay platelets. Those tiny platelets enrich soils. Clays nurtured the great stands of pine and oak that thrived on the banks of the Big Rideau. Indigenous people camped on the Big Rideau’s shores through past millennia and found shelter and resources among the forests.  The clays in our local Farmington loam sustained the crops of pioneer families such as the Chipmans, Boltons and Sheldons who settled within sight of the Big Rideau two centuries ago. The Giles, Chants, Myers, Grahams and other farm families prospered on the fertile lands around Portland after the building of the Rideau Canal. And in the early 1890’s hexagonal “sheets” of mica were mined at Murphy’s Point for the fronts of parlor stoves and insulators in radio tubes. 

How many young of age and heart have shouted with glee when they discovered “Gold” while wading a rocky shore of the Big Rideau?   These cubic crystals sparkle in the summer sun. Sorry, kids! It was only pyrite.  Sorry, too that you will find no diamonds in the Rideau’s rocky ledges and islands. But you can find tiny flakes of diamond’s darker sister, graphite. With a flake of this Cinderella sister, you can sketch your name. Or you can fish in Sheldon Bay with an unbreakable rod made of graphite fibres. 

These are just three of over fifty minerals that you can find in our Rideau area within that great natural laboratory, the Canadian Shield and its branch, the Frontenac Axis. The Grenville Mountains of long ago have been eroded to their very roots, exposing a myriad of minerals beneath and around the Rideau. None are in sufficient quantity or quality to warrant modern mining. But they all tell of the enormous forces and amazing processes of their making.

Geologic Map of Big Rideau Lake - This segment of the Westport Sheet of the Geological Survey of Canada illustrates Big Rideau Lake in white. To its north, the Rideau Lake Laccolith is shown in red. The diversity of colours around this laccolith is just a hint of the very complex minerology and geology of the Canadian Shield. This ancient core of North Continent, of igneous and metamorphic rocks extends around James and Hudson Bays to the Arctic. The geology illustrated on the south side of the Big Rideau is much less colourful and less complex; illustrating the much younger sedimentary strata of the St. Lawrence Plain. Its subdued landscape was more suited for human activity [farming] and infrastructure. 

Then there are the rocks, Nature’s mix of minerals. Paddle your canoe along the North Shore, ponder a cliff of igneous rock, a face of the Rideau Lake laccolith. Within this gigantic rock blister, super-hot magma cooled and crystallized into quartz, feldspar,  micas and other minerals, the stuff of granite. Then paddle into Horseshoe Bay. Peel away the moss at water’s edge. Marvel at a metamorphic [changed under pressure] rock called gneiss where bands of quartz, feldspar and maybe some hornblende prove that even solid rock can be baked and stirred under the enormous heat and pressures of mountain building.  

Park your paddle on the gunwales in Briton Bay or by the shore of Big Island. Look carefully at outcrops of lime silicate. Where freshly scraped, this rock is white and brilliant in sunlight. It is because of this white rock that geologists call our part of Ontario the “White Province”. Where it has been exposed to weather for decades, you can scoop away stained cubic crystals of this soft rock by the handful. This almost pure calcite is the same mineral as in your teeth and bones. Maybe it should be called a marble but no statue of David could be carved from our local granular lime silicate. But pioneers kilned it to make plaster. Though you will find a few tiny crystals of pyrite, graphite and other minerals, this local rock is almost pure calcium carbonate. Some of the Big Rideau basin and shorelines are composed of this white rock. Its origin may pre-date the heaving up of the Grenville Mountains in pre-Cambrian time. One theory holds that it formed as a mud in an ancient sea. Maybe ancient micro-organisms took some of the abundant carbon dioxide out of our early atmosphere and combined it with calcium dissolved in an ancient sea. Later, this primordial mud was baked during the creation of the Grenville Mountains and this unique lime silicate metamorphosed into rock. Being much softer on the Moh Scale of Hardness than the granite walls of the North Shore it is easily eroded. 

These local calcium carbonate rocks bring us a great benefit. In Grade 10 Science, we learned that calcium carbonate neutralizes acids. As we humans pollute our environment with acid rain, the Big Rideau is buffered by Mother Nature’s own form of Bufferin against acidification and sterility.

But there is at least one more of the numerous geological features of the Big Rideau that make it unique among lakes of our country. The Rideau Fault Line clearly defines the North Shore of Big Rideau Lake.  It is as identifiable as a scar on a fingerprint. It extends beneath The Narrows, under Westport and south-westerly beneath a series of other lakes. The Rideau Fault is termed a “normal” fault. One side (north) of this gigantic crack in our Earth’s crust has been forced upwards, exposing the North Shore’s rugged pre-Cambrian face. The down side (south) of this normal fault is masked beneath the much younger sedimentary strata of the St. Lawrence Plain. On the lake you can see these layers of early Paleozoic sandstone and dolomite where waves splash the ledges along Hudson Bay and Sherwood Shores. This beautiful Cambrian sandstone has been masoned into a landmark church, other fine buildings in Portland and the local locks of the Rideau Canal.

The Rideau Lake Fault is unlike other known and active faults such as the San Andreas Fault of California. “Will the Rideau Lake Fault ever shake and quake?” Will terrified citizens of Westport ever bundle kids and precious possessions into cars, abandon the rubble of their community and seek shelter in Delta?   Or will citizens of Portland ever have to search through the shattered rubble of their waterfront homes that have been dumped high and dry by a tsunami? Not likely! Normal Faults tend to happen where and when a continent is being pulled apart.    East Africa? Yes!  Rideau country? Not now! Fortunately for us, the Rideau Lake Fault is very ancient and now quite stable. 

Instead of a lurking source of disaster, the Rideau Lake Fault is a boon. In cross-section, the water body of Big Rideau Lake is wedge-shaped. The deepest water (approx. 125 m.) is near the North Shore, north of the Rideau Fault Line. However, toward the south shore the lake is shallow. This great range of water depths was a bonus for indigenous fishers of past millennia. The Rideau provided a spectrum of aquatic habitats for fish and waterfowl to be caught and cured at migratory seasonal camps. Further, before the Canal was built, the shallows of the south shore provided explorers, fishersand theirfamilies with  “The Landing” , Portland’s original name. This was a place to portage from the Rideau system of lakes and river over to the Gananoque system. More recently, each year as winter approaches and the Big Rideau water invert in temperature, northerly winds push chilled surface water into the shallows of Portland Bay. Here, ice forms early and thick. A century ago, this dependable ice supported vast skids of logs on “Mill Bay”, awaiting sawing at Toffey’s (Bolton’s) Mill.  The sawmill is long gone but now each January speed skaters from near and far come to “Skate the Lake” on the natural ice of Portland Bay. It is safe for the local Zamboni and the throngs of skaters and fans, mugs of hot chocolate clutched by mitted hands. But look to the north. A couple kilometers across the snow-covered ice, steam rises from the ice-free and blue water of the Big Rideau Lake.  

I’ve highlighted just a few of the wonders of the Big Rideau, legacies from the past billion years and more. Consider that a tiny fraction of that, a million years, is but a mere moment in our planet’s history. But within thatsaga of The Big Rideau, great sheets of ice accumulated on the highlands of northern Quebec and Labrador. Repeatedly they oozed and scrapped across our landscape, sculpturing it from the north-east toward the south-west. By coincidence of geology and time, this was also the grain of the ancient Grenville Mountains, eroded to their ‘roots’ by a billion years of wind and rain. The recurring ice ages polished the hard ridges and gouged soft cavities from these remnant roots. Look at a topographic map or an aerial photo of the Big Rideau. Note that the ridges, the islands and the bays have a lineation, like a table top of polished  oak. There is a grain to Rideau’s landscape; an orientation, N.E. to S.W.; a sense of direction recognized and used on foggy mornings by legendary fishing guides of the past century like George Carr and Hermie Morris. It is also one of the many ways the Spirit of the Rideau spoke to the original indigenous people.

The Blue and the Green; the Paradise of The Big Rideau. 

Looking westward toward the Narrows Lock seen in the distant centre near the horizon.  Note the lineation N.E. to S.W. of the islands in the foreground, a legacy from the Grenville Mountains a billion years ago and from the Pleistocene Ice Ages a mere million years ago. Further, this picture is testament that after two centuries of accelerated human impact, the blues and greens of Nature still dominate the Spirit of the Big Rideau.  

What’s more, the most recent ice sheet melted and retreated from Rideau Country a mere 10 000 years ago. It exposed a legacy of drumlin hills around Forfar, ground moraine, mineral-rich parent material for a myriad of local soils, sustenance for a further myriad of mid-latitude forests. Its icy melt-waters shaped our landscape leaving raised beaches, spillways, ponds and lakes which provided habitat for many fish and waterfowl found in Rideau country today. These are some of the legacies-in-landscape of the Pleistocene Ice Age that you can see and appreciate beyond the shorelines of the Big Rideau. And there are glacial landscapes that you cannot readily see.  Scuba divers tell of amazing bottom-scapes of beaches and ledges hidden beneath Rideau’s blue water and white caps.  

Perceptions of Big Rideau Lake have been expressed in saga and song through the ages. Indigenous people told of watery highways and byways, of living in harmony with their environment. They were awed by the spirit of the Big Rideau, a spirit that was at times violent, but always bountiful. In the 1700s, maps of the Rideau were drawn on birch bark and parchment by European explorers, missionaries and traders. In 1802, Reuben Sherwoodsurveyed a town line across the Big Rideau. Others followed with circumferentors and chains, measuring and mapping the Rideau countryside and shorelines into lots and concessions. Only then could pioneer families legally settle its shores, pay taxes and accelerate the human impact on the Big Rideau and its hinterland. Forests were cleared. Soil were broken. A great Canal was built, altering lake, shoreline and hinterland. Fields widened and farms prospered. Soils and nutrients flushed into the Rideau. A succession of maps have been drawn chronicling sites of seasonal cottages of families who treasured the Rideau environs, of year-round residences and the infrastructures of roads and railroads, power and telephone lines, stores and marinas to support settlement.

A century ago, the Garretts hosted guests at their island fishing lodge and Viscount Kingsmill spent summers, even some winters at his Shangri’la on Grindstone Island. The Atlas of the Rideau chronicles our “fast-forward” into the era of human impact. Now more than ever, we must understand the many facets of this gem, natural and human, past and present. Only by appreciating our unique legacy of the Big Rideau, a billion years-in-the-making, may we realize our responsibility to preserve and protect our gem and its many facets.  

Indeed! The Big Rideau is a blue gem like none other.  

References and other suggested readings:

• Interpretive Structural Map of Westport Area, Fig. 21, Centennial Publication. Geological Survey of Canada, 1967
• Surficial Materials and Terrain Features, Ottawa-Hull, Map 1425A, Geological Survey of Canada, 1977
• Map 2054 Gananoque Area, Ontario Dep’t. of Mines, 1964
• Geological Highway Map of Southern Ontario, Map 2441, Ontario Ministry of Natural Resources 1979
• Putman, L.J. & Chapman, D.F.; The Physiography of Southern Ontario, Second Edition, Univ. of Toronto Press, 1966
• Gillespie,J., Wicklow, R. & Miller, M., Soils of Leeds County, The Ontario Department of Agriculture and Food 1968
• Berry, L.G. & Mason, B., Mineralogy, Concepts, Descriptions & Determinations W.H. Freeman & Co. 1959

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This article, like all of the How the Lake Works series, is presented in a blog format. We welcome and encourage your comments - if you would like to engage in the discussion, please post your comments below.