Ice-age Bergslagen



Ice-age Bergslagen


Sweden’s topography has been moulded and formed by earthquakes and volcanoes. The heating of the Earth’s crust, the presence of enormous pressure, and a variety of chemical processes—all these combined to convert old rock forms to new ones. Iron ore was formed by chemical compounds that were made during volcanic eruptions that occurred more than 1.8 billion years ago, when the planet was in its infancy. Copper, zinc and lead were also created at that time. So the rock formations in Sweden are the result of a long, natural process in which geological changes occurred frequently.


When we talk about the ice age, we usually mean the most recent glacial period, which started some 70,000 years ago and ended about 8,500 years ago. But even during a glacial period, changes in climate can still occur. In the last glacial period, “short” periods when the climate was warmer occurred when parts of the country were ice free. The greatest spread of the latest glaciers occurred 20,000 years ago, reaching as far south as north Germany, and Poland. After that, the ice started to melt. The south of Sweden became ice free about 12,000 years ago and, 3,000 years later, the ice in the Bergslagen region had also receded.


The glacier carried away the old surface and sheared off boulders from the crust beneath. Everything became frozen fast in the glacier and was crushed into pieces of all different sizes, from large boulders down to particles of clay only thousandths of a millimetre in size. This mixture or mass of debris is called moraine. After the ice had receded, the moraine was left in a largely continuous layer over the crust of the Earth.


The meltwater collected in icy rivers that rushed through tunnels below and through the ice. Any moraine that was in the path of the water was picked up and carried along to the snout of the glacier. During this journey, the particles became smooth and rounded and were naturally sorted by size by the flow of the river.


The heaviest deposits, boulders, rock, gravel and sand, were left on the bottom of the ice tunnels, close to the snout of the glacier, whilst lighter particles were carried further away by the flow of water known as glacier milk. As the ice receded further north, boulder ridges, gravel hills, deltas and other features sprang up. The gravel and sand contained in the glacial boulder ridges are of great economic importance, but the ridges also serve as important historical monuments.


When the glacier milk or river flowed out into the sea, the heavier particles sank in the estuary. Particles of clay, on the other hand, remained in suspension, swirling around in the water for a long time, and then being spread and deposited over a wide area. These deposits now constitute our most fertile arable land.


As we have already seen, glaciers melt from south to north and, as they did, icebergs would break away and be carried into the Baltic Sea and up as far as Riddarhyttan. This is where the sea ended: but the glacier continued to melt on land. As it did, it gradually changed its shape: from exhibiting a steep, almost vertical form it changed into a low, sloping promontory. This process of change, of course, took time (geological time). The edge of the glacier thus remained at Riddarhyttan for a long period and it is this geological event that has left such a wealth of natural features to be seen around what is now lake Lien.


Where the glacier milk—the river—flowed out into shallow water, it created a bay in the ice. Since the glacier remained in place here for a long time, large quantities of gravel and sand were deposited in the river estuary. Eventually, the accumulated deposits in the estuary broke through the surface of the bay, leaving the hilly delta known to Swedish geologists as the Riddarhytte field.


Large icebergs became grounded on the sea bottom and embedded in the sand and gravel. After the glacier had completely melted away, and the land had risen out of the sea, a shifting vista of hills and valleys could be seen. Great hollows and depressions appeared where the icebergs had been left stranded. Where sand and gravel had settled, the topography consisted of ridges, hills and plateaux. It can all be seen clearly from the ground and the vista is a sharp reminder of the natural history of the land.


It was the melting of the glacier, and the rising of the land out of the sea, that created lake Lien.


Once the ice had fully receded, the pressure it had exerted was relieved, and the land started to rise. Those areas that rose out of the sea exhibit fairly clear signs of the swell, the rise and fall of the sea, and its influence on the shore. The highest shorelines, together with the height of some of the glacial deltas, constitute what is called the “highest coastline” (HK), which is an important geological limit and an important landmark in natural history. Land above this level largely consists of moraine (or till), while the land below the HK is made up mainly of fine-grained soils.


And the land is still rising, even today. In central Sweden, the height of the land increases by 4–5 mm a year along the coast of Ångermansland, and by 10 mm a year at Höga kusten. Movement of the land is probably also caused by movements inside the Earth’s crust.