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.
~
" " " "
-
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