Parks Canada
Symbol of the Government of Canada

A Guide to Geology
for Visitors in Canada's National Parks


Many thousands of generations of men have passed since the first of them found that an island makes a fine place to build a protected camp, away from wild beasts and tribes with unfriendly ideas. Islands have long intrigued people from a scenic point of view. For us they are interesting not only for their scenery but also for their origins. Some are made when molten rocks are exuded onto the sea floor and gradually pile up to thrust at last above the sea. Some are found as masses of fine mud and silt in river deltas. Some are found as remnants of cliffs cut off from the mainland by erosion. Still others are made of the stony skeletons of minute organisms. But let us examine the kind of islands one at a time so that we may appreciate a little better how they are formed.

Islands of Submergence

You may recall from an earlier section that oceans seem to lie in distinct basins with the continents standing up as sharply marked blocks, and that the ocean basins are brim-full. We mentioned also that very slight adjustments of the relative levels of oceans and continental masses result in very large changes in shapes of shorelines as the oceans lap up on the shallowly dipping edges of the continental blocks. This all leads us to the idea that some shorelines are parts of the land mass which have been submerged in the sea. Hills on the old landmass would be surrounded by water in some cases and become islands. Long ridges would become long peninsulas sticking out to sea and the adjacent valleys might become deep indentations.

This is the way that most islands are formed. The myriads of islands off the coast of Newfoundland including those in Terra Nova Park, Nova Scotia, and New England came about this way as did the islands of the west coast of Canada and the Alaska Panhandle.

You may remember that glaciers commonly blocked off the old courses of rivers with deposits of till and outwash so that lakes were formed. This effectively submerged parts of the landscape so most of the islands in lakes in Canada were formed by submergence. The scenery in St. Lawrence Islands National Park was formed in this way. Now what kinds of islands would be produced by submergence would depend entirely on what kinds of topography were submerged. A gentle rolling, river-worn country would produce islands of gentle contours while a rugged, steep topography would produce a rugged shoreline with rugged and irregular islands. In some places glacial deposits have been drowned so that islands are entirely made of glacial debris.

Thus when we sit on the shore and admire islands of submergence it is interesting not only to speculate on their origin but also on what the country must have looked like prior to the flooding of the land.

Volcanic Islands

Volcanic islands are not seen in Canada but are very important in some parts of the world. The Hawaiian Islands are really the tops of great volcanic piles which lie in some 15,000 feet of Pacific Ocean water. The Azores and islands of the mid-Atlantic like St. Helena and Ascension are the same. Bermuda is a mass of coral which rims the very top of another great volcanic island in the Atlantic. Many of the islands of the South Seas are of this kind, too, with older histories of violent volcanic activity which contrast sharply with the peace and quiet of the present.

Islands Made by Rivers

When large rivers carry sedimentary materials down to the great deltas are sometimes made. These usually consist of many islands separated by channels of the river as it breaks up into what are called distributaries. The Mississippi, the Nile, the Amazon—all the great rivers of the world have these. The deltas of the Mackenzie, Fraser, and Athabasca Rivers are studded with islands.

One does not have to go all the way to deltas to find islands of river deposition, for islands of sand, gravel and mud are often formed in times of high water in the river channels themselves and left high and dry during normal water. In some rivers the channels are so completely choked with sediments that the rivers are forced to wander in and out among the islands of their own making and look a little like braided plaits. On that account they are called braided streams. Some of the streams which issue from modern western Canadian glaciers are like this because of the great quantities of glacial debris fed to them by the melting ice. Rivers occasionally make another kind of island as they cut into the rocks which underlie their valleys. If a river encounters a zone of rock which is tougher to erode than the average it sometimes cuts around both sides of it making an island in mid-channel.

Islands Made by Waves

When shallow, shelving ocean bottoms are attacked by waves it often happens that long islands are formed a short distance from and parallel to the shoreline. As waves sweep in they eventually stir up bottom sands and muds and may carry them forward towards the shore. If the waves break at some distance out from the shoreline their load of sand and mud may be deposited at the line of breakers or just inside it. Storm waves may pile such materials high enough so that islands appear at times of normal water level. Long offshore bars and islands formed in this manner parallel the coast from southern New Jersey southward to Mexico and many resort cities are built on them.

Islands of Erosion

When a shoreline is attacked by wave erosion it rarely is cut away evenly for always some parts of it are less resistant than others and wear away more quickly. If waves thus eat into the softer parts the harder parts may be left sticking out as points. These may become cut off completely to form islands if the waves can cut in around behind them. Almost all exposed coastlines have islands of this kind on them.

Coral Islands

Corals are tiny sedentary animals which secrete heavy stony skeletons around themselves. They live in vast colonies in warm tropical waters and, over long periods of time, their hard parts may make great masses of limestone rock. Many of the volcanic islands in the warmer parts of the world have fringes of coral around their shores. Still others have reefs of coral which ring them just beyond a lagoon. A third type is the coral atoll, which is a ring of low coral islands with a sunken volcanic core completely covered with corals and coral debris. Again we do not have these in Canada but we add them to complete our list.