EROSION AND SCULPTURING OF THE LAND

The actual erosion or carrying away of materials produced by the breakdown of rocks is carried on by many different agencies in nature. Rivers carry vast amounts of sand, mud, and silt to the sea, beginning in the tiniest rivulets, and gaining force as the volume increases. Glaciers carry rock rubble and waste to the sea, or into lower valleys. Wind picks up dust and sand and blows it somewhere else. Water trickling underground dissolves limestone, salt, or gypsum and carries it off towards the rivers of the area. Waves and currents in the sea cut into cliffs and carry off rock waste to dump it along the beaches or into deep water offshore. Landslides and mudflows carry very large amounts of materials quickly in what sometimes are spectacular displays of nature's power. It is the last of these that we will think of in some detail now—that is, those forms of erosion or transportation of materials on the surface of the earth where gravity affects directly large masses of material. By some this is called mass-wasting.

When we look at kinds of mass-wasting we find that they can be divided at once into two principal types—those of rapid movement and those of slow movement. Landslides and rock-falls are obviously accomplished by rapid motion, while those we call creep and some types of mudflows are accomplished by slow, long-continued movement. Let us examine the rapid ones first.

Landslides and Rock-falls

On a quiet evening in a mountain valley it is not at all uncommon to hear an occasional rock-fall from the cliffs. Sometimes these falls are almost continuous, particularly in the spring when thawing loosens rocks and everything is wet and well lubricated. If you stand near the cliffs along the shore of the Bay of Fundy, or in the Arctic islands, or steep places in the mountains, and watch carefully you can see rock fragments continuously trickling down. Occasionally, after a rain, great masses of rock are let loose and fall downward to a stopping point. The sloping piles of such debris, which lie under the cliffs from which they come, may be called talus or scree. In some places the blocks of material may be as large as a house, in others we find very finely divided particles the size of sand or even dust.

A spectacular fall of rock occurred at Frank, Alberta, in 1903. Here a mass of limestone, in Turtle Mountain above the town, was criss-crossed by joints or small breaks. Softer rocks supporting the mass were apparently gradually giving way, weakening the whole structure. Heavy rains undoubtedly supplied a little extra lubrication along breaks in the limestone. Finally all these led to a tremendous rock-fall, wherein a mass of stone half a mile long and 150 feet thick, fell from the upper part of the mountain, crashed on down the slope, overrode part of the town, and slid up the other side of the valley some 400 feet. Such was the fury of the internal banging about in the mass of rubble that the rock was completely shattered so that the largest fragments remaining were scarcely ten feet in diameter. Man's feeble structures which happened to be in the way of the colossal mass of moving rock were crushed completely with the loss of many lives.

In 1958 a rain-saturated slope near Prince Rupert, British Columbia, suddenly let go, and an enormous slide went down the hillside. Along the banks of certain rivers in the St. Lawrence lowlands great slides of water-soaked clays and sand have taken place, off and on, as long as the area has been settled. One at Nicolet, Quebec, did considerable damage in November, 1955.

In Saint John, New Brunswick, a few years ago a more gentle type of landslide took place. It happened just back of a residential area, quite close to the famous Reversing Falls. Steep banks of glacial debris were weakened by extra heavy rains. A new roadway had just been built across the lower part of the slope into the Saint John River, probably cutting off the natural drainage lines under the gravels and clays and, certainly, adding to the load on the slope. When the whole mass had been lubricated with extra water it began to give way. Residents noticed small cracks in the fresh but thin blanket of snow one morning. Within a few hours the back gardens and garages of some of the houses were carried tens of feet straight down an almost vertical wall, as a great mass of material sheared away from the bank. Near the water's edge, below all this, a bulging mass marked the classically-shaped lower end or toe of the landslide. This much slower movement over several days contrasts with the catastrophic swiftness of the Frank and Prince Rupert landslides.

A still slower variety of mass movement of surface materials is found in many parts of the world where large masses of clay and rock become more and more heavily saturated with water until they begin to behave like large, slow-moving rivers. In fact, there is a complete gradation, from the clear waters of northern rivers, through silt-laden streams, through thick muddy streams, to watery muds and eventually to solid-appearing muds and clays with no tendency to flow at all.

To get back to the watery muds on slopes, these begin to flow very slowly downhill, perhaps at the rate of a few inches or even a few feet a year. They are rarely spectacular, as are quick-moving landslides or rock-falls, yet in some places enormous quantities of surface materials are moved from place to place. Once in a while these slow-moving mud and rock streams will suddenly let loose in a great messy gush of mud, clay, rocks, and slurp, in a gigantic sort of way, down and into a valley. In the mountain areas of the National Parks spring thaws and sudden rainstorms occasionally loosen slides onto some of the roadways.

This transfer of large masses of materials sometimes has startling effects on the scenery. Half of a mountain suddenly is no more, while in the valley below a great chaos of broken rock, fractured forests, and perhaps a dammed-up stream appears suddenly. One large lake in Labrador near the village of Hebron, far north of the timberline, can be seen on aerial photographs to be the result of a landslide-created dam at one end, with the scarred hillside clearly visible. Sometimes rivers are pushed out of their courses by these masses of material shoving out into them from one side.

In 1957, Grand Lake, near Goose airport in Labrador, suddenly turned from its usual blue to muddy brown over 30 miles of its length. The cause was not hard to see from the air, for there on its shore was a giant slide of mud and clay which had exposed a great gash of raw earth and had pushed a large bulge out into the western shore of the lake. In the western Canadian mountains this kind of thing is commonplace, particularly in the banks of large rivers.

Slow Slides and Creep

The really slow mass-motion of surface materials can be called creep. It usually affects only a few feet next to the surface and may be due to a variety of causes. In northern and cold regions the ground may be permanently frozen. In the short summers, however, a few inches or even a few feet at the very surface will thaw. This upper few feet is quite likely to be pretty wet. If it should be on a slope, even though slight, it will probably begin to ease very gently downhill over the solid, frozen mass underneath. Along comes the winter again and things come to a halt as everything is frozen tight. Then in the warmer season, thawing and a gentle downhill motion may once again set in. This variety of movement is given the name solifluction.

In other places creep may be of a different character. Masses of rocks in talus slopes, or even rocks standing by themselves, may gradually creep downhill by an alternation of freezing and thawing of the water between rocks or between boulders and the ground. If a thin film of moisture or water lies between a boulder and a rock surface, freezing will lift the boulder outward from the surface. This is not straight up in the air but outward from the sloping surface. When the ice melts the boulder is let down again—not in the opposite direction, but in a vertically downward direction. This means that every time the boulder is frozen under, it is pushed outward and away from the surface it is lying on, and then, on thawing, is let down again but at a slight distance downhill. Each cycle may move the boulder only a fraction of an inch, but if the cycle of freezing and thawing is repeated thousands of times, the boulder will gradually creep downhill. If all the boulders in a talus slope or a rocky surface are doing the same, the whole surface sheet will be in slow motion. This is, however, only one of the causes of motion. On other slopes we find that a very slow creeping is the same flowage of wet muds that we would call a mudflow if it were moving faster.

Unusual erosional forms like these "Hoodoos" near Leanchoil, in Yoho National Park, result from the wasting away of loosely consolidated masses of boulder-filled sands and clays. The boulder "caps" protect the underlying material from erosion and as the surroundings gradually wear away they get taller. © D. M. Baird, 1963

Some interesting practical results come from creep. Since it is likely to be faster at the surface than down below, in a given time the surface part will have moved a little farther than the substrate or lower part. A tree which has its roots extending downward through the faster-moving top layers into the slower-moving or stationary substrate will find itself gradually pushed over. Most trees try to straighten up again and this of course results in curved trees. In Newfoundland, Nova Scotia, and some other coastal regions where boat-building demands curved pieces of wood, it is common for men to go out into the woods and carefully pick out the trees with just the right natural curve for the bow, ribs or other parts of the boat. These sometimes come from badly rooted trees in swamps but they also come from bent trees in areas where creep is constantly pushing the trees over. Man-made affairs like telephone poles, fence posts, and foundation piers are sometimes affected in the same way. Engineers are wary indeed of building in areas of creep, but on the other hand are sometimes called in to try to halt creep and stop damage to various structures. Thus we see that gravity directly affects large masses of material and may produce sudden movements like landslides or slow movements like creep. The former leave gaping scars on the steeper parts of the scenery and piles of debris below. The latter sometimes do the same but are more apt to be integrated into the generally changing scenery without obvious discontinuity.