Geology of British Columbia. Sydney Cannings

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СКАЧАТЬ the Rainbow Range in Tweedsmuir Provincial Park, the product of a hot spot in the earth’s mantle, are a testament to the dynamic nature of British Columbia’s geology.

Acknowledgements

      THIS BOOK WAS inspired by many years of conversations along forest trails, around campfires and in university coffee rooms and government offices. The naturalist tradition in British Columbia is largely an oral tradition, and many knowledgeable people have freely shared their stories with us. We could not have begun to write this book without them.

      Jim Monger, father of terrane theory in the interpretation of the Cordillera, has been the well of understanding and inspiration to us all. Maurice Colpron of the Yukon Geological Survey freely shared his knowledge and ideas of terrane evolution and created all the terrane maps, keeping them simple without sacrificing understanding and precision. Eric Leinberger expertly composed the other maps. Others that gave freely of their knowledge were Dennis Demarchi, Carlo Giovanella, Richard Hebda, Catherine Hickson, Rick Kool, Don McPhail, June Ryder, Howard Tipper and Doug van Dine. Their comments and suggestions greatly improved the depth and accuracy of the book. Any remaining errors, however, are ours alone. Trevor Goward, Margaret Holm, Leah Ramsay, Douglas Leighton, Nancy Baron and David Stirling all gave valuable suggestions on the content and style of the original chapters.

      Donald Gunn crafted most of the line drawings; other illustrations were provided by Bob Carveth and David Budgen. A number of excellent photographers contributed their images—we would especially like to thank Chris Harris, Jared Hobbs, Ken Wright and Mark Hobson in that regard. Rolf Ludvigsen and ubc Press kindly gave permission to use the photographs of fossils from their fine book Life in Stone; the photographs were provided originally by the authors of the chapters of that book: Andrew Neuman, Elisabeth McIver, James Basinger, Mark Wilson, Ruth Stockey and Wesley Wehr. Other images were provided by Bob Turner, Jeff Mottershead, Jim Baichtal, Jim Pojar, Richard Hebda, Robert Cannings and Scott Webster.

Introduction

      AS BOYS WE poked into a lot of natural history—we looked at birds, we caught trout in small creeks, we helped our older brother make a butterfly collection, and—for a short time at least—we kept a leaf collection. But we poked a bit into geology, too. As small boys we looked wide-eyed at the perfect twigs of a dawn redwood from the White Lake fossil beds, marvelling when we were told they were millions of years old. And the cliffs nearby were made of lava, meaning that there were volcanoes here once, too. Our father had read Hugh Nasmith’s late glacial history of the Okanagan, so we learned that the Big Hollow of our toboggan runs was a glacial kettle, that the silt cliffs we clambered over were bottom sediments of Glacial Lake Penticton and that an ice dam at McIntyre Bluff south of Vaseux Lake had dammed the Okanagan drainage, sending the water north instead of south. One of our favourite spots was Coyote Rock, a giant boulder perched atop a tower of unconsolidated rock and silt. How did it get there? Geology is history, and every landscape is full of fascinating geological stories.

      The Okanagan has a complex geological history, and in that respect it is a characteristic corner of British Columbia. The original gentle shores of this province have been squeezed, melted, broken, piled and sheared by collisions with offshore terranes, or pieces of the earth’s crust. The foreign rocks of the terranes added to the confusion of local rocks, and then glaciers came and piled immense quantities of rubble on top of the whole tableau.

      But while all this was going on, animals and plants lived here—on land, in fresh water and in the Pacific. What did it look like back then? And how did the flora and fauna of British Columbia move, adapt and evolve with the changing landscape?

      Our landscape is still changing. Vancouver Island is being squeezed narrower and higher as you read this. Many earthquakes shake the land and sea each year. The climate is warming and glaciers are receding further, for the moment. And trees, flowers, birds, bugs, bears and fish continue to move across the landscape, testing and altering ecosystems as they go. These are the stories told in the following pages. The link among them all is change, for they are stories of transformation.

      The deciduous dawn redwood was a dominant tree of swampy areas in the B.C. Interior 45 million years ago. These fossil twiglets are from beds in the White Lake area near Penticton.

Part One Origins

      IN BRITISH COLUMBIA, it is hard to ignore geology. Most of us may not understand the rocks around us as fully as we would like, but we are well acquainted with them—they stare at us from mountain cliffs and rugged shorelines every day.

      If you were to take a flight across the province—say, from Jasper, Alberta, to Port Hardy on Vancouver Island—you would see a wide variety of geological features. First come the sedimentary rocks of the glistening Rockies, rising abruptly above the forested plains. Next, the snowy peaks of the Cariboo Mountains rise up across the Rocky Mountain Trench, their crystalline rocks tortured by the heat and pressure of unimaginable forces beneath the surface of the earth. Now the broad Interior plateau comes into view, with its flat surface of poured lava surrounding the eroded valleys and canyons of the Fraser and Chilcotin Rivers. And finally, the shining white ice of Mount Waddington rises ahead, towering over the Coast Mountains’ choppy sea of granite. The face of the province that you have seen—its two great montane belts, separated by the Intermontane plateau and fringed to the west by the Insular belt with its islands and passages—is the cover of a book in which we may read a long and fascinating story.

      In more ways than one, geology is the foundation of natural history. Geological formations not only form the physical base of terrestrial life and control the climate around it but also tell the temporal history of nature. Geology tells us how things came to be the way they are. Moving continents, rising and falling mountain barriers, vast volcanic eruptions and continental ice sheets all played an essential role in creating the diversity of life in British Columbia today.

      In the field of geology, an oft-quoted maxim is “The present is the key to the past,” meaning that in order to understand old rocks, we have to look at geological processes that are at work today. The reverse is also true: to understand the diversity of landscape and life as we see it now, we need to delve into the deep past to see how the land of British Columbia came to be.

       The Building of British Columbia: Plate Tectonics

      British Columbia is part of the North American Cordillera—the mighty set of mountain ranges that stretch from northern Alaska to southern Mexico (Map 1, overleaf). This mountainous landscape arose through plate tectonic processes. Plate tectonics is how the earth works. Its crust and underlying relatively stiff upper mantle form a carapace of plates like the bones of a baby’s skull before they suture and lock together. The plates are constantly moving—some growing, some shrinking—at about the speed a fingernail grows. The key to the Cordillera is a long history of interactions between the western edge of the continent, the plates that make up the floor of the Pacific Ocean, and the small, mobile pieces of crust in between that have been created, that have evolved and that have shifted between ocean and land.

      Our planet is unique in having plate tectonics. The constant swirling and recycling of the ocean’s rocky floor requires that the planet’s interior—like Goldilocks’s bowl of porridge—must be just right, not too hot and not too cold. Plate tectonics results from a balance between subduction—the sinking of oceanic plates at trenches like the modern Cascadia subduction zone off British Columbia’s west coast—and spreading at ocean ridges, where new crust is created by the rise of hot material СКАЧАТЬ