Westport and San Francisco are on fault lines. According to Canada’s Geological Survey Centennial Map, the Rideau Lakes Fault shapes and follows the cliffed north shores of the Big Rideau, the Upper Rideau, Westport Sand Lake, beneath Wolfe Lake and Fermoy, Canoe, Knowlton and Holleford Lake. The Holleford coincidence is fascinating as Holleford is also the site of one of the most ancient meteorite craters still visible on our planet. Undoubtedly there were myriad meteors, asteroids and comets that collided with our Earth in its earliest times. Mountain ranges have risen. Rain, wind and glaciers have eroded. Most traces of ancient impacts from heavenly bodies have been erased. But now you can stand on a hill above the tiny hamlet of Holleford (near Sydenham) and look northward the several kilometers across one of our oldest craters, still evident after millions of years of time and trees.
The Rideau Lake Fault is defined as a normal fault, a fracture in our Earth’s fragile crust caused by tension. One side of the fracture has pulled away and sunk down relative to the other. You can see the form of the Rideau Fault from Westport’s Lions Beach, from Rideau Vista School and from many other places along its nearly 50 kilometer length. In the picture, the Westport Mountain side to the right has remained high. The left or south side has relatively slipped down and slightly away. Now hundreds of millions of years after the original fracture, the Rideau Fault Line defines the visible boundary between the rugged Canadian Shield to the north and the flat and fertile St. Lawrence Lowlands to the south. Keep in mind that the Canadian Shield does not suddenly end at the North Shore. Again, the left side of the faulted Shield has slipped down along the fault line. Ancient seas have largely buried it beneath sandstone and other sedimentaries. Lakes such as the Upper Rideau now find basin in the lee of the Rideau Fault. Historically, the Rideau Fault also marks the heritage line between those families who were so fortunate to first settle and farm the Lowlands and those that later faced the challenges of pioneer life on the Shield. In a few places to the south of the Rideau Fault, the Shield pokes through the younger Lowlands. Colonel By and his Sappers and Miners met this granite reality at “the Isthmus”. Folks in Portland and elsewhere on the local Lowlands pump well water from aquifers that run between the hard, ancient Shield and the softer strata left by Paleozoic seas. They mine beautiful Canadian Shield white granite in St.Cloud, Minnesota. Pre-Cambrian rock can be seen at the bottom of the Grand Canyon. So, we might truthfully tell our American friends that Canada’s Shield has been supporting much of the United States for over a billion years.
The Rideau Lake Fault Line is not the only local fracture from Mother Nature’s “Shake and Bake” recipe book. Periodically in our planet’s evolution, continents collide. Compressions cause giant wrinkles in rock. Mountain ranges like the present Himalayas are pushed up as the sub-continent of India, once attached to Antarctica has drifted and now squeezes and pushes under the southern side of the continent of Asia. The result is Mount Everest and its sister peaks, the Tibetan plateau and the quaking of communities and schools far into central China. Likewise a billion years ago, the early continent of Laurasia (Canadian Shield) collided with Amazonia (Brazilian Shield) and the Grenville Mountains were formed. At other times and places tensions tear at the thin veneer of our planet as continents pull apart. Normal faults occur. Sometimes the tearing is slow, the speed at which fingernails grow. Sometimes the movement is fast and quaking. The Petawawa and the Mattawa Faults are two of many normal faults that define the Ottawa Valley. When tension in our planet’s thin crust causes two parallel sets of normal faults, the land between can drop, creating a rift valley. As you drive eastward past Kanata on Hwy. #417, you get a grand view across the Ottawa River to the Laurentians. You are looking across the Ottawa – Bonnechere Rift Valley formed millions of years ago. Just millennia ago, this same Ottawa Rift channeled enormous torrents of water from melting ice sheets. Just centuries ago, the Ottawa Rift was the gateway for French explorers and fur traders to the heart of North America and for Scottish settlers to “the Vallee”.
Many vets of WWII hold memories of the Rhine Valley of Europe, another famous rift valley. There too, the Earth’s crust pulled apart, the central valley dropped and this “rock music” left the Vosges Mountains to the west, the Black Forest to the east. Much European culture, music, fine wine and frothy beer have come from the Rhine Rift. In Africa, the “beat” goes on. Earthquakes, active volcanoes and the spectacular Rift Valley of East Africa are musics of our sphere. Lakes Malawi and Tanganyika lie between the Rift’s parallel faults. Further north, this family of faults forms the matching shores of the Red Sea; into Asia it becomes the Dead Sea and the Jordan Valley in Israel. A million years ago our human ancestors were fashioning tools from Rift rocks. A million years later some of their descendants fire rockets over Rift real estate.
But back to the Rideau Lake Fault. Is it likely to quake? Might it shake life and limb for folks in Fermoy? Should lads at the Lions Beach eagerly wait to surf a quake-induced tsunami? Should good citizens of Westport hastily load a few prized possessions, pets and kids into cars, flee their Shangri-la and seek safe haven in Portland?
The fault lines that lattice eastern Ontario are products of tensions within the Earth’s crust in ancient times long before T-rex. Since then, our part of North America’s real estate has become quite stable. Mountains are neither being pushed up or pulled apart here. If you want to experience a big quake, try a #3 seismic zone like Vancouver Island or San Francisco. If you never want to experience planetary “rock and roll”, try Winnipeg, a #0 seismic site. Eastern Ontario? We’re in a #2 seismic zone. We do get earthquakes but not severe shakers as in California. For one thing, the infamous San Andreas Fault is a nasty shear fault, right at the surface, right beneath schools and hospitals. For two, mountain-building and seismic activity is still going on along the Pacific Coast. For three, the dry California climate causes rocks to stick and snap rather than slide easily under stress. That’s three barrels loaded in the California game of Richter Roulette.
Quakes from our faults like the Rideau Fault are mainly caused by isostatic rebound after the most recent Ice Age. Our part of North America was weighted down beneath the Wisconsin Ice Sheet for many millennia. This continent-wide glacier mostly melted away since twelve thousand years ago. The ground beneath us is still rebounding. The Great Lakes have changed shapes and levels. Whales once frolicked in the Champlain Sea, a branch of the Atlantic Ocean that once reached across our depressed landscape as far as North Gower. That Champlain Sea has drained to the Gulf of St. Lawrence. The floor of Hudson Bay is still rising, abandoning concentric shorelines. With this isostatic rebound, we experience frequent very mild tremors epicentred along our many ancient fault lines, enough “rock and roll” to totter rock statues near Jones Falls, enough to cause planners, architects, insurance brokers and law- makers to add earthquake-proofing to our lives and buildings.
I hope I have allayed most fears that readers might have about living close to the Rideau Lake Fault. Hopefully, you may rest almost assured that the quakes we experience here in eastern Ontario will be cause for casual excitement rather than disaster news on CNN.
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.