NATURAL HISTORY

INTH THE PAGE CALLED PRINCE EDWARD ISLAND

OF EARTH

Since 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 (usu-ally the best parts). Sometimes whole chapters had disappeared; but the days were long and it was fun imagining the story that would fill the blanks.

Like those stories, the history of the earth as written in the rocks is a book with missing pages. Parts of the narra-tive cannot be seen, because they are covered by soil or water. Whole chap-ters 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

North Point

LEGEND

approximate i

assumed [ Geo Iog i ca1 con tac t

tentative )

Pictou Group

Approximate line of transition from grey to red beds

#•*"' 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 have been built by the collision of conti-nents at the age of 42, with the present-day Atlantic ocean beginning to open not long afterwards. Filling a low area along the margins of the new ocean, sed-iments — including those underlying Prince Edward Island — were depos-ited 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 geo-graphically low area that was filled, hundreds of millions of years ago, by sandy sediments eroded from the new-ly-formed Appalachian mountains to the south and west. These mountains, in turn, had been formed by the colli-sion 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 — con-glomerate (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 exten-sion 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 direc-tion 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 dif-ficult 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 sedimen-tary pile. He showed that, examined sta-tistically, the redbeds resolve into four "fining upwards" sequences, in each of which conglomerates predominate near the base, sandstones in the mid-dle and siltstones at the top. Because these are stream deposits, and because faster-flowing streams carry coarser material, each fining-upwards sequence 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 "mega-cyclic sequences," as van de Poll termed them, are exposed with the oldest beds

Complex 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 injected into sand leaving this complex pattern.

to the south and the youngest to the north, thanks to the gentle northward tilt of the rocks, roughly paralleling the arcuate shape of the Island.

What is perhaps most striking about our rocks is their distinctive brick-red colour. Our sandstones are red because each individual quartz sand grain is 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-cement-ed, and wave erosion easily makes sand-stone 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 hema-tite. In parts of the south shore, how-ever, 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 depos-ited, 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 present location, the ocean "tried" to rift apart down what is now the Gulf of St. Lawrence. This, also, would have

been an area of earthquake activity. Evidence of repeated earthquakes is widespread in the Redbeds. Just as, in modern settings, an earthquake may cause saturated clayey sediment to flu-idize and collapse under fields and buildings, so the Prince Edward Island siltstones show abundant evidence of remobilization, after the clays were

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 sand-stone within bedrock cliff exposures attest to the suddenness and violence of this process.

How Old Are We?

In technical language, the Rocks under-lying Prince Edward Island are Permo-Carboniferous in age — just a little younger than coal-bearing sedimen-tary 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 decay of elements contained in the min-erals that comprise them. Radiometric age-dating is useful, mostly, for dating rocks that have crystallized from a melt, or for dating the last episode of defor-mation of a rock that has been reheated and folded. In the case, of the PEI red-beds, the rocks are "sandwiched" in time between young rocks that intrude them (on Hog Island, in Malpeque Bay) and have been dated at about 100 mil-lion years in age, and the "basement" rocks of the Appalachian belt that range in age, mostly from 450 to 350 million

"Rotated block/' Point Prim. This block position by the violent passage through the of wet silt and mud.

of bedded sandstone was rotated out of sedimentary pile of an unknown volume

years in age. This is helpful, but not very precise. The estimate of the age in the case of the Prince Edward Island rocks is "fine-tuned" by examining the remains of ancient life, fossils, con-tained in them. Throughout the world the sequence of rock deposition and emplacement can be worked out from relationships "in the field," and it has been shown that successive parts of the global sequence are defined by different assem-blages of fossil life. Radiometric dating of rocks worldwide has enabled these fossil assemblages to be dated accu-rately. In Prince Edward Island, the fossil remains of plants, pollens and animals — by comparison with assem-blages of accurately known age else-where in the world — allow us to define the age of our rock much more accu-rately than would be the case if we did not have a global context within which to work.

What Was Our Geography l ike , Then?

Throughout most of geologic time, the continents have drifted slowly across the earth's surface, rafted on "tectonic plates" whose motion is driven by huge

Dimetrodon, the name now given to Bathygnathus. The sail on is thought to have been a means of regulating body temperature • a carnivore.

Bathygnathus borealis leidy, a fossil rep-tile discovered near New London in 1845.

convection-like currents in the hot, semi-plastic mantle beneath. In addi-tion, the geographic poles of the earth — the axis of the planet's rotation, have wandered, and the record in the rocks of the movement of the magnetic north and south poles helps us trace their path through time. When we roll the tape backward, as is were, we find that, when the PEI Redbeds were depos-ited, what is now the Island was located within about five degrees of the equa-tor. We can take cold comfort, then, in knowing that this was once a tropical paradise.

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 l i fe l ike?

The fossil record as preserved in Prince Edward Island is scanty, but there are enough plant and animal traces pre-served 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 ubiq-uitous, and the remains of Catamites, a plant resembling the modern "horse-

tail" (but standing up to two or three metres in height), and Walchia, one of the earliest coni-fers, are wide-spread. Trackways preserved in sev-eral locations indi-cate that animals, small and large, roamed widely. We find little in the way of fossil remains of these animals, though. Conditions were not particu-larly favourable for the preservation of animal remains, because the fast-moving streams tended to disaggre-gate skeletal mate-rials and deposit

them as fragmented "bone beds" rather than as complete (or even partial) skel-etons.

Nevertheless, the remains that we do have hint, as do our trackways, at a fauna similar to that found elsewhere in the region. The Island's most famous fossil is of the left side of the face of a large animal — clearly a carnivore from its fierce teeth — discovered in the 1840's and described by Joseph Leidy in 1853. Leidy called the fossil Bathygnathus borealis and was

the animal's back an advantage for

Fossil imprint 0/Walchia, an early coni-fer, 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 carnivo-rous 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 morn-ing sun, so as to hunt while smaller reptiles and amphibians were still slug-gish 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 skel-eton of a small reptile or amphibian has been found, numerous bone frag-ments, and small animal tracks.

*See John DeGrace's "Bathygnathus Comes Home," in The Island Magazine, No. 32 (fall-winter), 1992.

What About "Just Yesterday?"

Many pages are missing from the geo-logic book of eastern North America. The early history of erosion of the tower-ing Appalachian Mountains of 350 million years ago is preserved in the sedimen-tary rocks filling the Maritimes basin, but for most of the intervening 300 mil-lion 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 con-tinental glaciation, but only the most recent is recorded in the surficial depos-its 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-

~ " " " " - % * ~ * - * ~ ~ V*. WV*~ * V ^ . ^ T V ^ V ^ , ^V***^^. ~*^.».V^. ~ ,

Years Before Present

65,000,000

141,000,000

195,000,000

230,000,000

280,000,000

345,000,000

395,000,000

435,000,000

500,000,000

570,000,000

2,600,000,000 4,600,000,000 (?)

Era

Cenozoic

Mesozoic

Paleozoic

Proterozoic

Archeozoic

Period

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

Major Events

Ascendency of mammals and Flowering plants

Extinction of dinosaurs; Flowering plants appear Dinosaurs abundant; mammals and birds appear Dinosaurs and flying reptiles appear; First modern corals appear Rise of reptiles and amphibians; Conifers and beetles appear Reptiles and winged insects appear Amphibians, spiders and trees appear; rise of fishes Earliest-known coral reefs; spore- bearing land plants appear Trilobites abundant; first fish-like vertebrates appear First appearance of abundant fossils Scanty remains of primitive organisms First life-forms appear Planet earth forms

that snake across the countryside in the western part of the province.

This most recent ice cap began melt-ing 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?

Today, in eastern Canada, we are experi-encing a relative rise in sea level of per-haps 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 coast-line of about 1/2 metre each year. If sea levels were to rise more rapidly, per-haps 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 hap-pen 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.

Sources

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 (Geo-logical Survey of Canada, Map 1366A, 1973). I8I