NATURAL HISTORY

OF EARTH

INTH

THE PAGE CALLED PRINCE EDWARD ISLAND

S

ince I was a child I have been a reader. In our summer cottage were old children's books and comics that had been read by a generation of my forebears, some of whom had been less careful than others when it came to looking after their things. As often as not I would get partway through a

By John R. DeGrace

book to find that text was missing (usually the best parts). Sometimes whole chapters had disappeared; but the days were long and it was fun imagining the story that wouldfillthe blanks. Like those stories, the history of the earth as written in the rocks is a book with missing pages. Parts of the narrative cannot be seen, because they are covered by soil or water. Whole chapters have been removed by erosion. For those who love puzzles, though,

and who have patience, the story of the earth that unfolds is a fascinating one. The broad sweep of history is clear enough, even though many details may have been lost. The fun of geology is that, wherever we are, we all walk the same earth; and in so doing we can all read that part of the story that lies beneath our feet. The Prince Edward Island page in the Earth's story is a good read by itself, and it forms a bridge that helps to LEGEND

approximate i assumed [ Geo Iog i ca1 c o n t a c t

tentative

Pictou Group

)

Approximate line of transition from grey to red beds North Point

General Geology of Prince Edward Island (van de Poll, 1983).

#•*"'

Vector direction of sediment transport Dominant litnology: mainly siItstone , , mainly sandstone [ f inIng-upward megacycHc seq coarse sandstone and conglomerate

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 billion 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 have been built by the collision of continents at the age of 42, with the presentday Atlantic ocean beginning to open not long afterwards. Filling a low area along the margins of the new ocean, sediments — including those underlying Prince Edward Island — were deposited at about the age of 43. Dinosaurs dominated the land and sea at about the age of 45. And humankind? Our earliest direct ancestors appeared on the scene only 29 days ago, and the 6,000 years of written history span only the last 40 minutes. It is the vast span of geologic time that allows mountains and continents to be built and worn down, species to evolve and become extinct, and Islands to form. Far from an inert lump rolling through space, our planet is dynamic and ever-changing. Prince Edward Island, geologically, is part of the "Maritimes Basin," a geographically low area that was filled, hundreds of millions of years ago, by sandy sediments eroded from the newly-formed Appalachian mountains to the south and west. These mountains, in turn, had been formed by the collision of huge crustal plates rafted about upon the Earth's mantle like froth on the surface of soup slowly heating on the kitchen stove. In that context, our Island might be thought of as a sort of geologic afterthought. Nature has no afterthoughts, however, and there is much of interest to be found in the cliffs that mark our shores, and in our beaches and rolling hills. Let's explore for a bit, and see what might be the

Cliff exposure of the PEI Redbeds, North Cape. A large "cross-bed" of conglomerate occurs within a horizontally-bedded section of sandstone. The direction of stream flow was from left to right. answers to the questions we might ask of the earth as it speaks to us in Prince Edward Island. What Are We Made Of? Prince Edward Island is underlain by a thick pile of sedimentary rock — conglomerate (made mostly of pebbles), sandstone and siltstone. In most areas, if one were to drill deeply enough (more than three kilometers!) salt would be encountered. This deeply-buried salt is an extension of the same salt unit that is mined near Windsor, Nova Scotia, and is evidence of a restricted ocean basin in a hot, arid climate at that time. With further drilling, eventually one would come to the "basement" rocks of the Canadian Appalachians — an extension of the complex rock formations exposed at the surface in Nova Scotia and New Brunswick. These rocks are present at depth under Prince Edward Island because, regionally, all of the rock units are tilted gently to the north at an angle of perhaps two or three degrees. That northward tilt defines our part of the "Maritimes Basin." We may stand above sea level, but we are geologically low nevertheless. For most of the more than 100 years that geologists have been examining the Island rocks, it has been known that the "PEI Redbeds" (more properly

referred-to as the Pictou Group) are, mostly, derived from stream sediments laid down above sea level at the base of high mountains to the south and west. The arrows defining the direction of stream transport of sediment, as shown above, were derived from many hundreds of measurements taken from "fossil" streams exposed in cliffs around the Island shores. It was difficult to say more than that, however. Rock exposure is scanty in most of the Island, and the streams that carried the sediments were small and complexly interwoven. Research by Dr. H.W. van de Poll in the early 1980's shed much light on the general characteristics of this sedimentary pile. He showed that, examined statistically, the redbeds resolve into four "fining upwards" sequences, in each of which conglomerates predominate near the base, sandstones in the middle and siltstones at the top. Because these are stream deposits, and because faster-flowing streams carry coarser material, eachfining-upwardssequence is taken as recording a period during which stream activity decreased and water flowed more slowly on average. In turn, this is thought to indicate four successive deepenings and fillings of the Maritimes Basin as the Appalachian mountain belt developed. These "megacyclic sequences," as van de Poll termed 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 PermoCarboniferous 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 absolute terms, by the rate of radioactive decay of elements contained in the minComplex injection feature in a cliff near Charlottetown. While still a pile of wet sediment, silt and mud (outlined, by a pale, reduced-iron zone) was fluidized and erals that comprise them. Radiometric age-dating is useful, mostly, for dating injected into sand leaving this complex pattern. rocks that have crystallized from a melt, or for dating the last episode of deformation of a rock that has been reheated been an area of earthquake activity. to the south and the youngest to the and folded. In the case, of the PEI redEvidence of repeated earthquakes is north, thanks to the gentle northward beds, the rocks are "sandwiched" in widespread in the Redbeds. Just as, in tilt of the rocks, roughly paralleling the time between young rocks that intrude modern settings, an earthquake may arcuate shape of the Island. them (on Hog Island, in Malpeque Bay) cause saturated clayey sediment to fluWhat is perhaps most striking about and have been dated at about 100 milidize and collapse under fields and our rocks is their distinctive brick-red lion years in age, and the "basement" buildings, so the Prince Edward Island colour. Our sandstones are red because rocks of the Appalachian belt that range siltstones show abundant evidence of each individual quartz sand grain is in age, mostly from 450 to 350 million remobilization, after the clays were coated with a fine dust of hematite — iron oxide, rust, the same chemical that produces the distinctive brick-red colour of our older automobiles. The rocks are not particularly well-cemented, 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 midAtlantic ridge — an important but nowdistant 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 sedimentary pile of an unknown volume position by the violent passage through the rift apart down what is now the Gulf of of wet silt and mud. St. Lawrence. This, also, would have

tail" (but standing years in age. This is helpful, but not up to two or three very precise. The estimate of the age metres in height), in the case of the Prince Edward Island and Walchia, one rocks is "fine-tuned" by examining the of the earliest coniremains of ancient life, fossils, confers, are widetained in them. Throughout the world spread. Trackways the sequence of rock deposition and preserved in sevemplacement can be worked out eral locations indifrom relationships "in cate that animals, the field," and it has small and large, been shown that roamed widely. We successive parts of find little in the way the global sequence of fossil remains are defined by of these animals, different assemthough. Conditions blages of fossil life. were not particuRadiometric dating larly favourable for of rocks worldwide the preservation of has enabled these animal remains, fossil assemblages to be dated accu- Dimetrodon, the name now given to Bathygnathus. The sail on the animal's back because the fastan• advantage for moving streams rately. In Prince is thought to have been a means of regulating body temperature tended to disaggreEdward Island, the a carnivore. gate skeletal matefossil remains of rials and deposit plants, pollens and them as fragmented "bone beds" rather convection-like currents in the hot, animals — by comparison with assemthan as complete (or even partial) skelsemi-plastic mantle beneath. In addiblages of accurately known age elseetons. tion, the geographic poles of the earth where in the world — allow us to define — the axis of the planet's rotation, have Nevertheless, the remains that we the age of our rock much more accuwandered, and the record in the rocks do have hint, as do our trackways, at a rately than would be the case if we did of the movement of the magnetic north fauna similar to that found elsewhere in not have a global context within which and south poles helps us trace their the region. The Island's most famous to work. 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 when the PEI Redbeds were deposfrom its fierce teeth — discovered in What Was Our Geography l i k e , ited, what is now the Island was located the 1840's and described by Joseph Then? within about five degrees of the equaLeidy in 1853. Leidy called the fossil tor. We can take cold comfort, then, in Bathygnathus borealis and was Throughout most of geologic time, the knowing that this was once a tropical continents have drifted slowly across paradise. the earth's surface, rafted on "tectonic Plant life was established on land plates" whose motion is driven by huge 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, suggests that all was not lush and damp. What Was l i f e l i k e? 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 leaves and stems. Tree ferns were ubiqBathygnathus borealis leidy, a fossil repuitous, and the remains of Catamites, tile discovered near New London in 1845. a plant resembling the modern "horse-

Fossil imprint 0/Walchia, an early conifer, from western Prince Edward Island.

uncertain as to its affinity. Discoveries elsewhere in the world confirm that it is a specimen of the animal now known as Dimetrodon, a large carnivorous reptile distinguished by a "sail" on its back, supported by bony spines and believed to have been a means of temperature regulation.* Dimetrodon must have been a formidable predator in its day. The sail would have enabled it to warm its body core by the morning sun, so as to hunt while smaller reptiles and amphibians were still sluggish in the cool air of daybreak; and to radiate heat so as to continue hunting while its prey sheltered in the shade. Of Dimetrodon's prey the fossil record tells us little. One nearly-complete skeleton of a small reptile or amphibian has been found, numerous bone fragments, and small animal tracks. *See John DeGrace's "Bathygnathus Comes Home," in The Island Magazine, No. 32 (fallwinter), 1992. ~

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What About "Just Yesterday?" Many pages are missing from the geologic book of eastern North America. The early history of erosion of the towering Appalachian Mountains of 350 million years ago is preserved in the sedimentary rocks filling the Maritimes basin, but for most of the intervening 300 million years or so the dominant geological process has been erosion — pages torn out, so to speak. Overlying the PEI Red Beds are glacial deposits only a few tens of thousands of years old. Like much of North America, Prince Edward Island is thought to have been subject to four major episodes of continental glaciation, but only the most recent is recorded in the surficial deposits on the Island. The evidence of a thick — perhaps kilometers-thick — ice cap covering Prince Edward Island is preserved in the presence of a dense "glacial till" occurring next to bedrock in places, and in the widespread evi%

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that snake across the countryside in the western part of the province. This most recent ice cap began melting about 15,000 years ago, and in Prince Edward Island the ice was gone by 13,000 years ago — leaving behind a thick blanket of loose material that had been carried in the ice, including boulders ("erratics") that sometimes have been mistaken for meteorites. The Island had been depressed by the weight of the ice, and "glacial rebound" was not instantaneous with its removal. Rising seas encroached, so that for a time the Island was divided into three. Rebound overtook rising sea levels, however, so that by 7,000 years ago the Island was relatively high and was not, in fact, an Island at all — being tied to the mainland by a natural bridge (in about the same location as the Confederation Bridge), until finally the sea won out and the Northumberland strait was formed only about 5,000 years ago.

,

...And Of The Future? Era

Period

Major Events

Cenozoic

Tertiary

Ascendency of mammals and Flowering plants

Cretaceous

Extinction of dinosaurs; Flowering plants appear

Jurassic

Dinosaurs abundant; mammals and birds appear

195,000,000

Triassic

Dinosaurs and flying reptiles appear; First modern corals appear

230,000,000

Permian

Rise of reptiles and amphibians; Conifers and beetles appear

280,000,000

Carboniferous

Reptiles and winged insects appear

Today, in eastern Canada, we are experiencing a relative rise in sea level of perhaps two or three millimeters per year. This sea level increase, and the lack of resistance to erosion of the Redbeds, makes for widespread beautiful beaches — and an average retreat of the coastline of about 1/2 metre each year. If sea levels were to rise more rapidly, perhaps as a result of global warming, this one Island might once again become three and eventually might best be called "Prince Edward Spit." This could happen rapidly in geological terms but, fortunately, not in human terms. Our beaches our cliffs, our rolling hills will endure, ever-changing but still beautiful, for many years to come.

345,000,000

Devonian

Amphibians, spiders and trees appear; rise of fishes

Sources

Silurian

Earliest-known coral reefs; spore- bearing land plants appear

435,000,000

Ordovician

Trilobites abundant; first fish-like vertebrates appear

500,000,000

Cambrian

First appearance of abundant fossils

Years Before Present

65,000,000 141,000,000

395,000,000

Mesozoic

Paleozoic

570,000,000

Proterozoic

Scanty remains of primitive organisms

2,600,000,000

Archeozoic

First life-forms appear

4,600,000,000 (?)

Planet earth forms

The major sources used in this article were: K. Kranck's "Geomorphological Development and Post-Pleistocene Sea Level Changes, Northumberland Strait, Maritime Provinces'9 (Canadian Journal of Earth Sciences, v.9., 1972); H.W. van de Poll's Geology of Prince Edward Island (Prince Edward Island Department of Energy and Forestry Report, 1982); and V.K. Prest's map Surficial Deposits of Prince Edward Island (Geological Survey of Canada, Map 1366A, 1973). I8I