Cut off from the mainland

For hours yesterday, two tourists were trapped on a rock in the roaring sea. One of the two arches of the rock sculpture “London Bridge” on Australia’s famous “Great Ocean Road” suddenly collapsed. This cut off the visitors’ way back. They had to be rescued by a helicopter.

The young couple had walked to the end of the second arch to enjoy the fantastic view of the sea and coast. Once there, they heard an ominous crunching sound. When they looked around, the arch had already collapsed, cutting the connection to the shore. Fortunately, no one had just stayed on the first arch, there were no other victims. After five hours of waiting, the couple was happily brought back to shore by a helicopter.

The double arch of “London Bridge” was one of the most famous rock formations on Australia’s south coast. Wind and waves are increasingly eroding this coastline, causing part of the tourist attraction to now collapse. After the collapse, the “London Bridge” was renamed without further ado: It is now called “London Arch”.

On the Great Ocean Road

The surf is wild on Australia’s south coast, along which the famous Great Ocean Road runs. The stormy sea has already claimed many victims here: Over a hundred ships have already been wrecked on the rocky coast. Wind and waves grind up everything in their path here. And that is above all the relatively soft limestone with its bizarre rock colossi: London Bridge was only one of them, the “Twelve Apostles” or the “Island Archway” are also world-famous. The collapse of “London Bridge” shows how fragile the coast is: the rock disintegrates in the raging sea almost like sugar in hot tea. Without pause, the forces of nature gnaw at the coast and reshape it. So if you still want to see the twelve apostles in all their glory, you should hurry.

From rock to grain of sand – weathering

Today, northern Canada is a gently undulating landscape. Many millions of years ago, however, a mountain range stood here. In fact, over a very long time, even high mountains can turn into small hills.

The reason for this transformation: the rock on the earth’s surface is constantly exposed to wind and weather. If, for example, water penetrates into cracks in the rock and freezes, it blasts the stone apart. This process is called frost blasting. Changes in temperature between day and night and the force of water and wind also cause the rock to become friable. In other words, it weathers. This process can also be observed on buildings or stone figures. During weathering, the rock breaks down into smaller and smaller components down to fine grains of sand and dust. Different rocks weather at different rates: granite, for example, is much more resistant than the comparatively loose sandstone.

Some types of rock even dissolve completely when they come into contact with water, for example rock salt and lime. Rock salt is chemically the same as table salt – and that already dissolves in ordinary water. Lime is somewhat more resistant, but limestone also dissolves in acidic water. Acid is formed, for example, when rainwater in the air reacts with the gas carbon dioxide. This “acid rain” attacks the limestone and dissolves it over time. On the earth’s surface, weathering leaves behind fissured limestone landscapes, while caves form underground.

But it is not only solution weathering, but also heat and pressure that wear down and crumble rock beneath the earth’s surface. Where plants grow, roots dig in, blast the rock apart piece by piece and also ensure that it is eroded millimetre by millimetre.

In this way, weathering not only works on individual rocks, it gnaws away at entire mountain ranges. But it will take a few million years before the Black Forest is as flat as northern Canada.

The sea shapes coasts

Wherever seawater meets dry land, we speak of a coast. Because the coast is exposed to the force of the sea without interruption, it is constantly changing. How strongly the water gnaws at the mainland depends on the strength of the rock, the height of the waves, the ocean currents and the tides.

Gentle ocean waves that wash sand and gravel onto the flat land heap up sandbanks and create a beach. The water further crushes the debris, reshaping the beach again and again. If waves and wind shift the sand sideways, a hook of sand grows into the sea. When this hook reaches the opposite end of a bay, the hook becomes a spit. Enclosed by the spit, a lake remains from the seawater: the lagoon.

But the surf does not only work the fine sand. It can even erode hard rock when it thunders with force against the cliffs of a steep coast. If the water drags broken-off pieces of rock with it, it grinds the rock further at the height of the waves: Cavities form. If the overlying rock collapses, receding bays and capes remain, reaching into the sea like arms of land. Sometimes only individual towers of rock remain in the sea, which are further worked by the water and eventually also collapse. The power of the sea is particularly strong during storm surges. They can extremely change the shape and course of the coast.

An eternal to and fro of fine sand and clay prevails on shallow tidal coasts. The alternation of low and high tide ensures that the material is washed up and away again and again. The result is a mudflat coast. The mudflats were washed up and deposited by the water and are covered by the sea at high tide. At low tide, channels appear in the mudflats – the tideways. The seawater flows through them, similar to a river bed, at low tide and back towards the land at high tide.

Constant dripping wears the stone

Deep gorges in the mountains, wide sandy beaches by the sea and broad rivers meandering through meadows and fields – these are all landscapes we know well. Because they are so varied, we find them impressive and beautiful.

The sculptor of all these landscapes is the cycle of water. More than any other force, water sooner or later shapes the surface of the earth. It washes away soil after a downpour. It burrows into the subsoil and loosens parts of the rock. It carries soil and weathered rock debris down into the valley. Where the water flows away more slowly, it releases its load of silt, sand and debris. At high tide, it floods the shallow areas of a valley, the river floodplains. Here, too, it deposits fine silt. When the water finally flows into the sea, it works the coasts and forms very different landscapes, for example cliffs or long sandy beaches.

Water also shapes the landscape in the form of ice. When water freezes in rock cracks, it blasts the stone. As a glacier, it planes notch-shaped river valleys into round trough valleys. And the moraine landscape in the foothills of the Alps with its scree hills and boulders is also the result of glaciers that shaped the subsoil long ago.

Sedimentary rocks

Some rocks look as if they are striped. In the Dolomites, for example, such transverse bands are often clearly visible. Sandstone or limestone quarries also sometimes have similarly pretty patterns.

The “stripe design” is already created during the formation of the rock. The starting material is weathering debris that is carried away by water or wind. Rivers, glaciers and dust storms lose their power at some point: river courses slow down towards their mouths and eventually flow into the sea or a lake. Glaciers advance into warmer regions and melt. Dust storms also weaken at some point. Then they can no longer carry dust, sand and debris. The crushed rock that is dragged along settles. Over time, the deposited material forms an ever higher layer – the sediment. Especially on the seabed and on the bottom of lakes, where rivers wash up a lot of material, such sediments accumulate, including remains of dead animals or calcareous shells.

Gradually, different sediments are layered on top of each other. One layer may consist of sandstone, for example: In dry times, the wind has blown desert sand onto it. When the sea level rises again, this layer is covered by water: calcareous shells of marine animals sink to the seabed and deposit another layer on top of the sand. Over millions of years, the climate changed again and again, causing the sea level to fluctuate. This allowed different layers to be deposited.

Over time, the sediment cover becomes thicker and thicker. Under the weight of their own weight, the initially loose sediments are compressed more and more, small cavities disappear, the mass thickens. Further layers are deposited on top, the sediment becomes increasingly solid and finally, under pressure, sedimentary rock. In geology, this process is also called diagenesis. If, for example, the shells of tiny marine animals are pressed into stone, limestone is formed. Fine grains of quartz sand cement together under high pressure to form sandstone.

In addition to debris, dead animals, for example fish, also settle on the seabed. Sealed airtight, their bones and scales were preserved and fossilised. Such fossils have immortalised themselves in the stone. Even after millions of years, they reveal a lot about the time when the sediment was formed. This is why geologists can read the rock layers like a history book.

Normally, only the top layer is visible to us. However, when a river digs through the sedimentary rock, it is uplifted during mountain building or blasted free in a quarry, we get a view of the cross-section. The individual sediment layers are then clearly visible as “stripes” or bands in the rock

Carved in stone – landscapes made of sedimentary rock

Like the layers of a cake, different sedimentary rocks can lie on top of each other. If the subsoil beneath the layers rises, they are tilted. If earth plates collide, they are compressed and unfolded, as in the Alps. Weathering and erosion gnaw away at these sedimentary layers over millions of years. Depending on the hardness of the sediment, the forces of water, cold and wind leave their mark and carve impressive landscapes into the rock.

A famous example of this is the Grand Canyon in Arizona: here the Colorado River has carved a channel through different layers of rock. It is easy to see how soft and harder rock alternate: The soft rock gives way quickly, creating sloping hillsides, while the harder rock remains standing and forms steep, almost vertical walls. Like a staircase, these sedimentary steps lead down to today’s river course and offer the visitor a spectacular sight.

Sedimentary strata also formed a well-known large landscape in Germany: the Southwest German stratified plain, which stretches from Baden-Württemberg through Hesse and Bavaria to Thuringia. After the Upper Rhine Graben collapsed, the sedimentary layers here became inclined. Depending on the hardness of the rock, the individual layers were eroded to varying degrees. Hard limestones formed steep steps, while soft, clayey layers were more eroded and today make up the landscape as gentle slopes and wide stepped areas. On the left side of the Rhine, this landscape is – almost as if mirror-inverted – opposite the North French Stepped Lands.

What causes erosion?

When rock weathers, it rarely stays in its original place. Often, rock debris rolls down the slope, is washed away by water or pushed away by ice masses. Fine rock dust or sand can also be carried by the wind. Whether the rock is carried away by water, ice, wind or gravity, all these processes are called erosion.

The erosion caused by flowing water is particularly drastic. Streams and rivers dig a bed in the ground, rock slides down and a valley forms. When a glacier rolls down the valley, it planes the valley wider by dragging debris with it. Such trough valleys show that there was a glacier here long after the ice has melted. The surf of the sea, on the other hand, attacks the coast. Steep cliffs are eroded and collapse, sandy beaches are washed away by the waves. In deserts, the wind sweeps away large areas of sand. The harder it blows, the more sand it can carry away. A sandstorm gradually grinds away obstacles made of solid rock like a sandblaster.

When rain and wind wash or blow away the soil cover on large areas, this is called soil erosion. Landslides on slopes are also referred to as soil erosion. The problem is that the fertile upper layer of the soil disappears. In the worst case, it can no longer be used for agriculture.

If the soil is overgrown with plants, this slows down erosion. The roots of the plants hold the soil in place and prevent wind and water from carrying it away. However, if the plant cover is destroyed, for example by deforestation, the soil lacks this hold and is eroded away.

Stone corrosion on Cologne Cathedral

It is crumbling, splintering and weathering: the ravages of time are gnawing away at Cologne Cathedral. Acid rain has already eaten away at the famous building. Laws on air pollution control have been reducing the pollution levels for some years now. But pigeon droppings, exhaust fumes and the weather continue to attack the old walls and never leave the craftsmen of the cathedral’s construction hut out of work.

The foundation stone of Cologne Cathedral was laid on 15 August 1248. Since then, about fifty different types of stone have been set into the Rhenish sand here. Many of them were only used by the builders on a trial basis; not every rock withstood the weather. In addition, the stone had to come from nearby, because transport was fabulously expensive in the Middle Ages. As a result, the cathedral consists mainly of trachyte, shell limestone, sandstone and basalt. The calcareous sandstones and shell limestone are particularly susceptible to weathering and environmental influences. These are already heavily pitted. To save the sensitive shell limestone from weathering, various protective coatings have been tried. This should at least slow down the crumbling. The trachyte from Drachenfels, on the other hand, has held up well. The basalt rocks are also weather-resistant and are still in good condition today.

Despite all efforts, components have to be replaced again and again. Every year, 15 to 20 cubic metres of natural stone are used to maintain the famous church building. Even though Cologne Cathedral was already finished in 1880: the stonemasons of the Dombauhütte still have their hands full today!

Stone incompatibility

Cologne Cathedral apparently weathers more than other comparable buildings. Preservationists now have a guess as to why this is so: the cathedral is built of many different types of stone. And not all stones are compatible with each other. For example, the damage is particularly severe where trachyte from Drachenfels meets sandstone from Obernkirchen. A group of researchers is now trying to find out whether and why some rocks actually damage each other and which of them can work well together.

Landslide disaster

Terrible devastation was caused by a landslide in the Schwyz district on 2 September. Late in the afternoon, after heavy rainfall, a rocky peak of the Rossberg broke off. The earth masses underneath started to slide and buried the villages of Goldau and Röthen as well as parts of Lauerz and Buosingen within a few minutes. 457 people died in the natural disaster.

In the weeks before, it had rained almost continuously, softening the layers of earth. Around 5 p.m., the rock masses broke loose and thundered violently down into the valley. Blocks of rock weighing several centimetres were hurled through the air, taking houses, people and livestock with them. The destructive earth masses buried the neighbouring villages under a ten to fifty metre high layer of rubble and even rolled up the Rigiberg opposite. Some of the rock masses thundered into Lake Lauerz. This triggered a tidal wave that also claimed several lives.

A total of 457 people were killed in the Goldau landslide. Among them were seven people from a group of travellers from Bern, who were arriving in Goldau at the time of the landslide. 111 dwellings, 2 churches and 2 chapels were buried, 220 barns and stables destroyed and 323 head of cattle killed. Next to the Basel earthquake of 1356, the Goldau landslide is the largest natural disaster in Switzerland to date.

Appeal for donations

The Swiss Confederation has already responded to the Goldau disaster: the neighbouring cantons of Zug and Lucerne sent relief workers to the affected region as early as yesterday. Delegates from Zurich and Bern are also expected to arrive soon to support the recovery work and the reconstruction of the villages. Rapid help is now needed for the survivors. Switzerland is therefore appealing for donations throughout the country. You too can help the victims of the Goldau landslide! Addresses and information can be obtained directly from the editorial office.