Geology of the Cape Breton Plateau
The dominant feature of northern Cape Breton is the Cape Breton Plateau, averaging 1100 feet at its edges but rising to more than 1600 feet at its center. The Cape Breton Plateau is part of the worn down Appalachian mountain chain which stretches from Georgia to Newfoundland. Extending over 70% of the park, the plateau appears flat-topped but actually consists of broad, gently rolling hills, deeply cut by steep-walled river canyons and broad valleys.
On the west coast the plateau meets the Gulf of St. Lawrence in steep cliffs. On the east, the highlands border the Atlantic Ocean in a more gently sloping coastal plain with low headlands and a few long sandy beaches.
The formation of Cape Breton Island
To understand the geology of Cape Breton Island, we must first look at how the geological landscapes (terranes) that form the island today came together. Plate tectonics – the movement and collision of continents – has played a major role in forming the island of Cape Breton.
Three terranes make up Cape Breton Island. The Blair River inlier on the northwest tip is composed of the oldest rocks known in the Maritime Provinces. The Bras d’Or terrane which makes up most of the northern half of Cape Breton Island is a series of sedimentary and volcanic rocks that began forming off the northwest coast of the continent we now call South America. The Avalon terrane which makes up the southern half of Cape Breton Island is volcanic rock that first began forming on the northwest coast of what is now Africa.
In the final stage (450 to 360 million years ago), Cape Breton Island was assembled as continents collided again. Baltica bumped into Laurentia to form a new continent called Euramerica. The Bras d’Or and Avalon terranes were dragged along by Baltica to become part of Euramerica too. The Blair River inlier, formed a billion years earlier and lying on the edge of the Laurentian Plate, was finally sandwiched together with these other two terranes to form the core of what we now know as Cape Breton Island.
The formation of the Cape Breton Plateau
The plateau as we see it today is the product of millions of years of geological change, including mountain building, erosion, and glaciation. Plate tectonics – the movement and collision of continents – has played a major role in forming the highlands through uplift, folding and faulting.
Starting about 360 million years ago, the ocean separating Euramerica (EA) from Gondwana slowly closed, bringing these two continents together near the equator. The collision pushed up and folded the seabed along what is now the east coast of North America, creating the Appalachian mountain chain.
From 340 to 325 million years ago sea level rose and fell, flooding this basin as the ocean closed and giving rise to alternating layers of sedimentary rocks formed from the salts left behind by evaporating water, such as gypsum, and red sandstone, conglomerate and shale.
By 325 million years ago, Euramerica and Gondwana were united as the supercontinent Pangea, with the Appalachians running down the seam which joined them. At this time, the Appalachian Mountains were probably higher than the Himalayas are today! Most of the northern Maritimes except for northern Cape Breton Island was lowland covered by a tropical forest swamp which later became coal fields such as those found throughout Cape Breton Island.
About 290 million years ago the climate changed dramatically, putting an end to the tropical forests and replacing them with a brick red desert. There are no rocks of this age in present-day northern Cape Breton Island because the desert didn’t extend up into the Appalachian Mountains, but the famous red soil of Prince Edward Island comes from rocks of this period.
By around 250 million years ago the Maritime provinces were joined to what are now Morocco and Spain, at the center of the Pangean supercontinent. As Pangea started to pull apart, the Maritimes were found on the west side of the slowly spreading Atlantic Ocean, on the edge of a new continent: North America.
While dinosaurs probably did walk the hills of northern Cape Breton Island, there is no trace of them because the hills have long since eroded, wearing away any trace of dinosaur bones and footprints. This erosion on the edge of the North American continent cut the whole of Atlantic Canada down to a broad, flat valley. Part of this ancient valley plain is now the flat top of the Cape Breton Plateau.
The last two million years.
The Atlantic Ocean continued widening and the North American continent drifted to its present location. Repeated glacial advances – ice ages – through the last two million years and ending about 10,000 years ago caused modifications to the already eroded landscape, carving out U-shaped valleys, shallow lakes, deep scratches called striations, and leaving behind extensive deposits of mixed mud, silt, sand and gravel called glacial till.
The Bras d’Or Lakes were carved out of the bedrock by glaciers during the ice ages. Many other geological and topographical features of Cape Breton Island also owe their form to the numerous glacial advances that occurred over tens of thousands of years.
The glaciers of northern Cape Breton Island were not always part of the continental ice sheet. At times there was only a small ice cap on the highlands, with the rest of northern Cape Breton ice-free.