Читать книгу Prince Edward Island National Park: The Living Sands - David M. Baird - Страница 3
ОглавлениеTHE SURFACE
The low-lying land mass of Prince Edward Island is an extension of a lowland that covers a large area in New Brunswick and Nova Scotia. This extensive flat area has been developed on rocks about 250 million years old. In fairly recent times streams have cut shallow valleys into the old flat surface, and this gives the country its rolling, undulating appearance. It would seem from a study of the coastal areas that the streams were at one time able to cut a little more deeply into the country than they can now, for the valleys continue out under the salt water for some distance. This is another way of saying that the land seems to be flooded by the sea so that what were once river valleys are now estuaries.
The shoreline
Waves and currents along the shores have made profound changes in the shape of the flooded shoreline of Prince Edward Island. Promontories and points have been worn off by wave-erosion. The water offshore is shallow everywhere, the bottom has been scoured by the waves, and sediments have been picked up and moved forward to the line of breakers. This material, combined with the products of erosion of the headlands and points, has been piled along the shore in long lines to form bars and spits. Because most of the materials available to the waves in the region of what is now the Prince Edward Island National Park are of sand size, the resulting deposits have formed magnificent beaches.
How bars and spits are built along shallow shorelines
As you stand on any of the beaches of the Park and look out to sea, the seemingly limitless water extends off to the horizon. From where you stand there seems to be no way of telling whether the water at the horizon is a mile deep or only a few feet deep. Yet the man who knows shorelines can tell you immediately that very likely the water is shallow for a long way out from the shore. A marine chart of the area would show that all along this section of the coast the water is still only 50 feet deep at a distance of 10 miles from shore. This is only a drop of one foot in a thousand—in other words, the bottom and the top of the sea are nearly perfectly parallel. Two very important results follow from this relationship between the sea and the land. The first has to do with what happens if the sea-level changes, and the second is concerned with what goes on at the shoreline itself.
In a coastal region with a very gentle slope, very small changes in the level of the sea would make very great changes in the shape and position of the shoreline. By contrast, changes of sea-level at a place where the land meets the sea in a vertical cliff would not cause the position of the shoreline to move in or out, but only up or down.
You can easily see the difference between what happens when the sea-level changes on a shallow, shelving shoreline and what happens when it changes on a steeper shore area by going to different parts of the Bay of Fundy; there the daily tides reproduce the effect exactly. At Fundy National Park, for example, [8] you may notice that where the bottom is flat, such as around the village of Alma, the water at low tide is nearly half a mile away from the dock area, but along the steep coast beyond Herring Cove, the shoreline at low tide is displaced only a few tens of feet, and the seaweed is left dripping on the steep cliffs above.
Now what has this to do with Prince Edward Island National Park? If you look at any map of Prince Edward Island you will see that everywhere along its coast there are estuaries that look exactly like what they are—flooded river valleys. Because the country was very flat and the river valleys were very gently rounded, the flooding went back into them a long way so that most of the estuaries penetrate deeply into the land. Indeed in one place near Summerside, the Island is nearly cut in two where two of these flooded valleys happen to be opposite one another. Here on the north side of the Island lies Malpeque Bay, famous for its oysters. It is odd to think that this marine fishery is now on a site that was once a wooded lowland with a network of small brooks and rivers.
In the region of the National Park itself these drowned river valleys are very well developed though not very large. Rustico Harbour, Rustico Bay, and Brackley Bay are examples. One can see in each of them the outlines of the main river valley flooded back for several miles with the river still entering the inland end of the valley. Tributaries are all flooded, with the heads of the brooks still flowing down the valleys and into the ends of the branch estuaries.
Figure 1.
Sandy red siltstone and sandstone form the coastline west of North Rustico. The thin layering parallels the ancient surface on which these sedimentary materials were laid down some 250 million years ago. The rocks are broken by vertical fractures or joints which can be seen trending diagonally across the picture.
The second effect of the shallow shelving shorelines has to do with the behaviour of waves and wave-erosion. Everyone who has been to the lakeshore or seaside has seen the waves breaking along the shore and stirring up the mud and sand. Waves affect the bottom farther out too. Great storm-waves are known to agitate the bottom to depths as great as 400 feet, strongly enough to move fine sand and mud particles. In water as shallow as 20 or 30 feet their force is strong enough to actively erode the bottom materials and move them about. Part of the stirred-up sedimentary material is moved seaward toward the deeper water and part of it is moved forward to the line of breakers by the forward motion of the waves themselves. During a storm, when great winds pile up the waves against the shore, sand and silt picked up from the ocean bottom may be built into long ridges parallel to the shore. These become bars and spits and often are built up several feet above the ordinary level of the ocean. They are commonly strung out along the coast parallel to the general outlines rather than following all the intricate details of bays and headlands. This tends to smooth out coastlines. And this is what has happened on Prince Edward Island’s north shore where the National Park is situated.
Where did all the sand come from?
Marine charts show that for 20 miles off the shore of the Park the sea bottom is sandy. It is from the scouring and dredging of these sands by the waves, and their shoreward movement, that much of the present sand along the beaches has come. Two other sources, however, [11] have added their shares. One is the mantle of glacial deposits that covers the surface of Prince Edward Island and the other is the bedrock that juts out into the sea in a few places.
