In human time, how four decades fly by! Seems it was only months ago when I was browsing the shelves in a campus book store at University of New South Wales and by habit I stopped under "Geology". As I fanned some Australian editions, my fingers froze when I caught a glimpse of this picture, a familiar scene from the home side of the planet. There in the Down Under, tyros of geology were being introduced to an angular unconformity north of Kingston, Ontario.
Over the years, I've come across illustrations of Elgin's angular unconformity in a number of geology texts. This photo taken by A.S. MacLaren has been seen round the world. Hundreds of high school and university "rock jocks" have clambered up the tilted layers of Elgin's unconformity. Now, nature's greenery is taking over. You might hike among the peaks of the Rockies and look in awe at much grander examples. Indeed, some question whether this Elgin site fits the exact definition of an angular unconformity. But few unconformities are as conveniently visible as this one by the side of Highway 15. It tells a story in the four dimensions of length, width, height and of time.
What is an angular unconformity? Normally, sedimentary rocks are deposited in horizontal strata as in the Philipsville cliff. There, sands washed and sorted on Paleozoic sea coasts more than half-a-billion years ago were cemented into layers. An especially thick stratum of this sandstone was quarried on Henry Halladay's farm to build the Great Arched Dam at Jones Falls. Some of Ottawa's finest architecture was masoned from this Nepean sandstone. There is no doubt about the upper and horizontal half of the Elgin site. "It's sedimentary, my dear Watson!"
But the saga told in the lower half of Elgin's unconformity is very different. For its narrative, we must go back over a billion years. Then, the mighty Grenville Mountains towered over our edge of the Canadian Shield. Himalyan-scale forces squeezed. Crests quaked. Volcanoes made ashes of themselves. Glaciers gouged. Rivers etched. Winds blasted. Now, as you drive westward from Calgary you see the awesome wall of the Rockies rising before you. But you are seeing only part of the picture. Great mountain ranges such as the present Rockies or the past Grenvilles are like great long icebergs floating on the hot and more plastic interior of our planet. For any great mass of mountains that stands above the mean surface of the Earth, there is an equal mass below.
British mapmakers unmasked this phenomenon when they surveyed the foothills of the Himalyas during the Raj in the 1800s. Earlier, while surveying in the Highlands of Scotland, they noticed the plumb bobs on their instruments were attracted slightly to the side by the gravity of adjacent mountains. But the deviation caused by Mt. Everest and her sisters exceeded all expectations. Imperial cartographers concluded that where there was a visible mass of mountains like the Himalyas above the landscape, there had to be a hidden mass below, like an iceberg floating in the ocean. Modern hi-tech instruments confirm their explanation. India and Asia are continents in collision. They pile up (and down) a mountain range like the Himalyas. Strata once at the surface are plunged deep, buckled and boiled. And even as mountain ranges are being pushed up, erosion is wearing them away. The higher the mountains, the more active are the glaciers and rivers. If the top of an iceberg is removed, it will buoy up. Likewise, once formed, mountain masses are worn down only to rise again, only to be eroded down again and again in an epic competition between uplift and erosion, a seesaw battle through time. The goal line is sea level.
Eventually when a mountain range is eroded into its very roots, it becomes isostatically stable, never to rise again or until the next round of continental collision in the neighbourhood. Much of the Outback of Australia is now a stable peneplain. High places have been worn toward sea level by flood and wind. Tilted strata of rock and bands of minerals have been planed off. A few hard monoliths like Uluru (Ayers Rock) persist. Low basins like Lake Eyre are being filled in. By the way, the local names Eyre and Chaffey are prominent "Down Under" too. When an ancient mountain system is eroded to equilibrium, it may be called a Shield, as the Western Australia Shield or the Canadian Shield. It has taken millions of years for the Outback to become flat, stable and near sea level. It probably took millions of years for the Grenville Mountains to become flat, stable and approaching sea level back in pre-Cambrian time.
There are three families of rocks. The sedimentary family includes sandstone and limestone formed and usually found at the Earth's surface. The igneous family includes the granites of Westport Mountain and Rock Dunder, formed deep beneath a mountain range and then raised to the surface. But the rock bands you see in the lower half of the Elgin unconformity are of neither the sedimentary family of rocks nor igneous. They are metasedimentary gneisses and paragneisses of that third and incredibly diverse family called metamorphic rocks. These are rocks that have been changed by conditions of heat, pressure and time; conditions found deep beneath mountain ranges like the Himalyas or Grenvilles. There is no doubt that the families and ages of the rocks in the upper and lower parts of this Elgin road cut do not "conform". However there is a question about the exact kind of rock and its origin in the lower part of Elgin's unconformity.
But to me, the most significant feature of Elgin's unconformity is the in-between, the least obvious zone. If you climb to the top of the sloping metasediments, you will find a thin layer of sandy, gravelly rock called a basal conglomerate, a special stratum of pebbles and sands only a few centimeters thick. Along the Sandstone Trail at Charleston Lake Provincial Park, this gravelly conglomerate is deep enough for you to stand in its notch. This layer of hard pebbles is all that's left from that several hundreds million years of erosion between the initial uplift of the Grenville Mountains and the flooding by Paleozoic seas. Now, that is "UNCONFORMITY"! On that pre-Cambrian peneplain beneath Elgin, everything that was dissolvable was washed and wafted away by stream and wind over eons of time. Hard sands and pebbles of quartz persisted to tell of a time when there was little free oxygen in our Earth's atmosphere. There was not yet enough free oxygen to form a protective shield in our atmosphere, that layer of ozone that we now threaten. Seven hundred million years ago the seas provided sun screen for ancient life from the harmful rays of our star. But the peneplained land beneath Elgin was devoid of roots, worms and humus. Maybe some bacteria inhabited the cracks in the rock. It was a sterile land somewhere between an Earthly desert of to-day and the surface of the moon.
Then about 600 million years ago, Paleozoic seas flooded this sterile and flattened landscape. Winds and waves of the Cambrian Period reworked this ancient sandscape into horizontal sandstone beneath Philipsville and Kanata, layers we call Nepean sandstone. South of the St. Lawrence, our neighbours call it Potsdam sandstone. It is so pure that it is mined near Elgin for making synthetic building stone. It has been prospected in South Burgess for making computer chips. As the Paleozoic sea persisted during the Ordovician Period, it left massive layers of Beekmantown dolomite on top of the Nepean sandstone, patchy at Portland, continuous beneath Smiths Falls and Kemptville. Eastward ho! Beneath Chesterville and Maxville, the Paleozoic sea left strata called Trenton limestone; sea beds with the same recipe and fossils as you can find on the western side of the Frontenac Axis. One guess; beneath Trenton.
What I find truly fascinating about our "Keystone of the Rideau" is that we live on this bridge between the Paleozoic Era and the pre-Paleozoic. We can stand on strata of Nepean sandstone that were formed long before Blue Whale or T-Rex. But drill down a few metres or drive a few kilometers and you enter the more ancient landscape of the pre-Cambrian, academically fascinating, aesthetically awesome and practically essential. We bore wells into the gravelly basal conglomerate between the Cambrian Period and the pre-Cambrian. Folks in Portland pump fresh water from this aquifer. We take for granted this drinkable water. Folks across much of our world live and die for clean water.
Whether Elgin's angular unconformity fits an exact definition, you still get the picture, a picture that has been seen around the world. It tells an incredible narrative within the history album of our planet.
Zumberge, James H. and Nelson, Clemens A., 1972, Elements of Geology, 3rd Edition,
John Wiley and Sons, Inc., Toronto Geological Highway Map Southern Ontario, Map 2441, 1979,
Ontario Geological Survey.
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.