In the Story of Earth, the Page Called Prince Edward Island

NATURAL HISTORY INTH OF EARTH THE PAGE CALLED PRINCE EDWARD ISLAND ince I was a child I have been book to find that text was missing (usu- and who have patience, the story of the Sa reader. In our summer cottage ally the best parts). Sometimes whole earth that unfolds is a fascinating one. were old children's books and comics chapters had disappeared; but the days The broad sweep of history is clear that had been read by a generation of were long and it was fun imagining the enough, even though many details may my forebears, some of whom had been story that would fill the blanks. have been lost. less careful than others when it came Like those stories, the history of the The fun of geology is that, wherever to looking after their things. As often earth as written in the rocks is a book we are, we all walk the same earth; as not I would get partway through a with missing pages. Parts of the narra- and in so doing we can all read that tive cannot be seen, because they are part of the story that lies beneath our covered by soil or water. Whole chap- feet. The Prince Edward Island page ters have been removed by erosion. in the Earth's story is a good read by By John R. DeGrace For those who love puzzles, though, itself, and it forms a bridge that helps to LEGEND approximate i assumed [ Geo Iog i ca1 contact Pictou Group tentative ) Approximate line of transition from grey to red beds North Point #•*"' Vector direction of sediment transport Dominant litnology: mainly siItstone , , mainly sandstone [ f inIng-upward megacycHc seq coarse sandstone and conglomerate General Geology of Prince Edward Island (van de Poll, 1983). link together the chapters of the global book. To understand the rocks underlying this Island, it is necessary to place them in context — not only in their relation to other rocks, but also in the context of geologic time. The sweep of time extends so very far beyond our lives, or the lives of our civilization, or even of our species, that we have to use models to get a feel for it. Imagine the 4.6 bil- lion years of earth history modeled as the life span of a 46 year-old person, whose birthday is on the day you read this article. Each year of that person's life would be 100 million years of earth history. Life would have arisen at about the age of ten or twelve; but the first clear record of that life, in the form of abundant fossils, would not appear until the age of 40. The ancient Appalachian mountain belt, extending along the east- ern margin of North America, would Cliff exposure of the PEI Redbeds, North Cape. A large "cross-bed" of conglomerate have been built by the collision of conti- occurs within a horizontally-bedded section of sandstone. The direction of stream nents at the age of 42, with the present- flow was from left to right. day Atlantic ocean beginning to open not long afterwards. Filling a low area along the margins of the new ocean, sed- answers to the questions we might ask referred-to as the Pictou Group) are, iments — including those underlying of the earth as it speaks to us in Prince mostly, derived from stream sediments Prince Edward Island — were depos- Edward Island. laid down above sea level at the base ited at about the age of 43. Dinosaurs of high mountains to the south and dominated the land and sea at about west. The arrows defining the direc- the age of 45. And humankind? Our What Are We Made Of? tion of stream transport of sediment, earliest direct ancestors appeared on as shown above, were derived from the scene only 29 days ago, and the Prince Edward Island is underlain by a many hundreds of measurements taken 6,000 years of written history span only thick pile of sedimentary rock — con- from "fossil" streams exposed in cliffs the last 40 minutes. glomerate (made mostly of pebbles), around the Island shores. It was dif- It is the vast span of geologic time sandstone and siltstone. In most areas, ficult to say more than that, however. that allows mountains and continents if one were to drill deeply enough (more Rock exposure is scanty in most of the to be built and worn down, species to than three kilometers!) salt would be Island, and the streams that carried the evolve and become extinct, and Islands encountered. This deeply-buried salt is sediments were small and complexly to form. Far from an inert lump rolling an extension of the same salt unit that interwoven. through space, our planet is dynamic is mined near Windsor, Nova Scotia, Research by Dr. H.W. van de Poll in and ever-changing. and is evidence of a restricted ocean the early 1980's shed much light on the Prince Edward Island, geologically, basin in a hot, arid climate at that time. general characteristics of this sedimen- is part of the "Maritimes Basin," a geo- With further drilling, eventually one tary pile. He showed that, examined sta- graphically low area that was filled, would come to the "basement" rocks of tistically, the redbeds resolve into four hundreds of millions of years ago, by the Canadian Appalachians — an exten- "fining upwards" sequences, in each sandy sediments eroded from the new- sion of the complex rock formations of which conglomerates predominate ly-formed Appalachian mountains to exposed at the surface in Nova Scotia near the base, sandstones in the mid- the south and west. These mountains, and New Brunswick. These rocks are dle and siltstones at the top. Because in turn, had been formed by the colli- present at depth under Prince Edward these are stream deposits, and because sion of huge crustal plates rafted about Island because, regionally, all of the faster-flowing streams carry coarser upon the Earth's mantle like froth on rock units are tilted gently to the north material, each fining-upwards sequence the surface of soup slowly heating on at an angle of perhaps two or three is taken as recording a period during the kitchen stove. In that context, our degrees. That northward tilt defines which stream activity decreased and Island might be thought of as a sort our part of the "Maritimes Basin." We water flowed more slowly on average. of geologic afterthought. Nature has may stand above sea level, but we are In turn, this is thought to indicate four no afterthoughts, however, and there geologically low nevertheless. successive deepenings and fillings of is much of interest to be found in the For most of the more than 100 years the Maritimes Basin as the Appalachian cliffs that mark our shores, and in our that geologists have been examining mountain belt developed. These "mega- beaches and rolling hills. Let's explore the Island rocks, it has been known cyclic sequences," as van de Poll termed for a bit, and see what might be the that the "PEI Redbeds" (more properly them, are exposed with the oldest beds deposited but before they turned into solid rock. Injections of sedimentary rock, crisply outlined by grey-green zones in which the iron oxide is in a reduced state, are widespread. In a few places, large rotated blocks of sandstone within bedrock cliff exposures attest to the suddenness and violence of this process. How Old Are We? In technical language, the Rocks underlying Prince Edward Island are Permo- Carboniferous in age — just a little younger than coal-bearing sedimentary rocks of Cape Breton and New Brunswick. They were deposited about 285 million years ago. The age of rocks is determined, ultimately and in abso- lute terms, by the rate of radioactive Complex injection feature in a cliff near Charlottetown. While still a pile of wet decay of elements contained in the min- sediment, silt and mud (outlined, by a pale, reduced-iron zone) was fluidized and erals that comprise them. Radiometric injected into sand leaving this complex pattern. age-dating is useful, mostly, for dating rocks that have crystallized from a melt, or for dating the last episode of defor- to the south and the youngest to the been an area of earthquake activity. mation of a rock that has been reheated north, thanks to the gentle northward Evidence of repeated earthquakes is and folded. In the case, of the PEI red- tilt of the rocks, roughly paralleling the widespread in the Redbeds. Just as, in beds, the rocks are "sandwiched" in arcuate shape of the Island. modern settings, an earthquake may time between young rocks that intrude What is perhaps most striking about cause saturated clayey sediment to flu- them (on Hog Island, in Malpeque Bay) our rocks is their distinctive brick-red idize and collapse under fields and and have been dated at about 100 mil- colour. Our sandstones are red because buildings, so the Prince Edward Island lion years in age, and the "basement" each individual quartz sand grain is siltstones show abundant evidence of rocks of the Appalachian belt that range coated with a fine dust of hematite — remobilization, after the clays were in age, mostly from 450 to 350 million iron oxide, rust, the same chemical that produces the distinctive brick-red colour of our older automobiles. The rocks are not particularly well-cement- ed, and wave erosion easily makes sandstone into beach. In the process, this rusty coating is knocked loose from the sand grains.

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