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 eastern 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. On the north shore, sand eroded from bedrock cliffs is buff in colour rather than brick-red, because the energy of the Gulf of St. Lawrence waves is sufficient to remove the hematite. In parts of the south shore, however, the energy contained in the waves lapping the Northumberland Strait is insufficient to do the job, and the sand remains deep red in colour. At the time the Redbeds were deposited, the present-day Atlantic Ocean was just beginning to open. The mid- Atlantic ridge — an important but now- distant earthquake zone, was located close to Cape Breton. Moreover, there is evidence that, before opening in its "Rotated block/' Point Prim. This blockof bedded sandstone was rotated out of present location, the ocean "tried" to position by the violent passage through thesedimentary pile of an unknown volume rift apart down what is now the Gulf of of wet silt and mud. St. Lawrence. This, also, would have years in age. This is helpful, but not tail" (but standing very precise. The estimate of the age up to two or three in the case of the Prince Edward Island metres in height), rocks is "fine-tuned" by examining the and Walchia, one remains of ancient life, fossils, con- of the earliest coni- tained in them. Throughout the world fers, are wide- the sequence of rock deposition and spread. Trackways emplacement can be worked out preserved in sev- from relationships "in eral locations indi- the field," and it has cate that animals, been shown that small and large, successive parts of roamed widely. We the global sequence find little in the way are defined by of fossil remains different assem- of these animals, blages of fossil life. though. Conditions Radiometric dating were not particu- of rocks worldwide larly favourable for has enabled these the preservation of fossil assemblages animal remains, to be dated accu- Dimetrodon, the name now given to Bathygnathus. The sail on the animal's back because the fast- rately. In Prince is thought to have been a means of regulating body temperaturean • advantage for moving streams Edward Island, the a carnivore. tended to disaggre- fossil remains of gate skeletal mate- plants, pollens and rials and deposit animals — by comparison with assem- convection-like currents in the hot, them as fragmented "bone beds" rather blages of accurately known age else- semi-plastic mantle beneath. In addi- than as complete (or even partial) skel- where in the world — allow us to define tion, the geographic poles of the earth etons. the age of our rock much more accu- — the axis of the planet's rotation, have Nevertheless, the remains that we rately than would be the case if we did wandered, and the record in the rocks do have hint, as do our trackways, at a not have a global context within which of the movement of the magnetic north fauna similar to that found elsewhere in to work. and south poles helps us trace their the region. The Island's most famous path through time. When we roll the fossil is of the left side of the face of tape backward, as is were, we find that, a large animal — clearly a carnivore What Was Our Geography like, when the PEI Redbeds were depos- from its fierce teeth — discovered in Then? ited, what is now the Island was located the 1840's and described by Joseph within about five degrees of the equa- Leidy in 1853. Leidy called the fossil Throughout most of geologic time, the tor. We can take cold comfort, then, in Bathygnathus borealis and was continents have drifted slowly across knowing that this was once a tropical the earth's surface, rafted on "tectonic paradise. plates" whose motion is driven by huge Plant life was established on land then, but not nearly as ubiquitously as now. Plants and animals lived along the banks of ever-shifting stream beds, the streams fed by water cascading down the barren flanks of the high Appalachian Mountains. Away from the streams, the presence of dune deposits in the Island bedrock, in places, sug- gests that all was not lush and damp.

What Was life like?

The fossil record as preserved in Prince Edward Island is scanty, but there are enough plant and animal traces preserved to give us a good idea of life at the time. Plant fossils are widespread, mostly as the low-relief impressions of Bathygnathus borealis leidy, a fossil rep- leaves and stems. Tree ferns were ubiq- Fossil imprint 0/Walchia, an early coni- tile discovered near New London in 1845. uitous, and the remains of Catamites, fer, from western Prince Edward Island. a plant resembling the modern "horse- uncertain as to its affinity. Discoveries What About "Just Yesterday?" that snake across the countryside in elsewhere in the world confirm that the western part of the province. it is a specimen of the animal now Many pages are missing from the geo- This most recent ice cap began melt- known as Dimetrodon, a large carnivo- logic book of eastern North America. ing about 15,000 years ago, and in rous reptile distinguished by a "sail" The early history of erosion of the tower- Prince Edward Island the ice was gone on its back, supported by bony spines ing Appalachian Mountains of 350 million by 13,000 years ago — leaving behind and believed to have been a means of years ago is preserved in the sedimen- a thick blanket of loose material that temperature regulation.* Dimetrodon tary rocks filling the Maritimes basin, had been carried in the ice, including must have been a formidable predator but for most of the intervening 300 mil- boulders ("erratics") that sometimes in its day. The sail would have enabled lion years or so the dominant geological have been mistaken for meteorites. The it to warm its body core by the morn- process has been erosion — pages torn Island had been depressed by the weight ing sun, so as to hunt while smaller out, so to speak. Overlying the PEI Red of the ice, and "glacial rebound" was not reptiles and amphibians were still slug- Beds are glacial deposits only a few tens instantaneous with its removal. Rising gish in the cool air of daybreak; and to of thousands of years old. seas encroached, so that for a time the radiate heat so as to continue hunting Like much of North America, Prince Island was divided into three. Rebound while its prey sheltered in the shade. Edward Island is thought to have been overtook rising sea levels, however, so Of Dimetrodon's prey the fossil record subject to four major episodes of con- that by 7,000 years ago the Island was tells us little. One nearly-complete skel- tinental glaciation, but only the most relatively high and was not, in fact, an eton of a small reptile or amphibian recent is recorded in the surficial depos- Island at all — being tied to the mainland has been found, numerous bone frag- its on the Island. The evidence of a by a natural bridge (in about the same ments, and small animal tracks. thick — perhaps kilometers-thick — location as the Confederation Bridge), ice cap covering Prince Edward Island until finally the sea won out and the Northumberland strait was formed only *See John DeGrace's "Bathygnathus Comes is preserved in the presence of a dense Home," in The Island Magazine, No. 32 (fall- "glacial till" occurring next to bedrock about 5,000 years ago. winter), 1992. in places, and in the widespread evi-

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Years Before Era Period Major Events Today, in eastern Canada, we are experi- Present encing a relative rise in sea level of perhaps two or three millimeters per year. Cenozoic Tertiary Ascendency of mammals This sea level increase, and the lack of and Flowering plants resistance to erosion of the Redbeds, 65,000,000 Cretaceous Extinction of dinosaurs; makes for widespread beautiful beaches Flowering plants appear — and an average retreat of the coast- line of about 1/2 metre each year. If sea 141,000,000 Mesozoic Jurassic Dinosaurs abundant; mammals and birds appear levels were to rise more rapidly, perhaps as a result of global warming, this 195,000,000 Triassic Dinosaurs and flying one Island might once again become reptiles appear; three and eventually might best be called First modern corals appear "Prince Edward Spit." This could hap- 230,000,000 Permian Rise of reptiles and pen rapidly in geological terms but, amphibians; fortunately, not in human terms. Our Conifers and beetles appear beaches our cliffs, our rolling hills will endure, ever-changing but still beautiful, 280,000,000 Carboniferous Reptiles and winged insects appear for many years to come. 345,000,000 Devonian Amphibians, spiders and trees appear; rise of fishes Sources 395,000,000 Paleozoic Silurian Earliest-known coral reefs; spore- bearing land The major sources used in this article plants appear were: K. Kranck's "Geomorphological 435,000,000 Ordovician Trilobites abundant; first Development and Post-Pleistocene Sea fish-like vertebrates appear Level Changes, Northumberland Strait, Maritime Provinces'9 (Canadian Journal 500,000,000 Cambrian First appearance of of Earth Sciences, v.9., 1972); H.W. van abundant fossils de Poll's Geology of Prince Edward Island 570,000,000 Proterozoic Scanty remains of (Prince Edward Island Department primitive organisms of Energy and Forestry Report, 1982); and V.K. Prest's map Surficial 2,600,000,000 Archeozoic First life-forms appear Deposits of Prince Edward Island (Geo- 4,600,000,000 (?) Planet earth forms logical Survey of Canada, Map 1366A, 1973). I8I