Land uplift and the sea
Our World Heritage still rises from the sea because of the latest Ice Age. When the three kilometres thick inland ice of the Ice Age covered the World Heritage Site, it pressed the Earth’s crust down about one kilometre. When the ice melted away, it released the pressure, and the land started rising towards its original position. This phenomenon is called postglacial land uplift, and it occurs everywhere in the area, also beneath the water surface. The land uplift was extremely fast at first and the Earth’s crust had already risen 500 metres in the High Coast when the ice sheet had melted.
Speed of the land uplift
When the ice sheet melted away from the World Heritage Area 10 500 years ago, the speed of the land uplift was much greater than today, over 10 centimetres per year.
Nowadays the land is rising with a speed of about 8-9 millimetres per year in the World Heritage area. This number varies, according to what kind of land uplift measured. 9 mm per year is the levelled land uplift, which is measured from the zero level in our height systems. But the sea level is also rising. That makes it so that the noticeable land rise, the apparent land uplift is about 6 mm per year. One can also measure the land uplift from the centre of the earth with the help of satellites. That result is a little bit larger than the levelled land uplift.
In the future the speed will decrease even more. Climate change will also make the sea levels to rise faster and faster, making the land uplift less and less visible. But still, there are thousands of years of land uplift still left. The crust of the earth needs to rise another 100 metres before it is back to the original position.
The world’s highest coastline
When the inland ice melted 10,500 years ago, there were only small islands emerging above sea level in the High Coast. The first shoreline of these islands is nowadays the highest coastline, which the land uplift raised as much as 286 metres above sea level. The islands are now also the tops of the till-capped hills in the High Coast.
The hills have a cap of forest on their tops. The forests grow in the till transported by the inland ice, and that is why the hills are called till-capped hills. Only a few pines and shrubs can grow on the steep hillsides because there is not enough suitable soil to grow on. Waves washed away the finer materials when the hillsides were under the sea. The finer soil ended up lower down on the seabed.
The border between the forest at the top and bare hillsides marks the highest coastline, and is still easy to see in nature today, 10 500 years later. The highest scientifically described such border is found on Skuleberget, 286 metres above the sea level of today.
The low-lying Kvarken Archipelago
The Kvarken archipelago is also affected by the land uplift phenomenon, but this is noticeable in other ways compared to the High Coast. Since Kvarken Archipelago is level and shallow it is very noticeable how much new land that rises from the sea in only a few decades. Scientists have calculated that the land uplift gives Kvarken Archipelago one square kilometre each ear. In addition to this the vegetation in the shallow bays captures sediment adding even more new land to the beaches.
Cobble fields in the World Heritage Area
The inland ice, waves and sea ice have created cobble fields, which are beaches made up of rounded stones. Cobble fields are created of the unsorted soil material called till, which the inland ice ground and transported over the area. As the land uplift elevated the till to sea surface, waves washed away finer materials. Remaining stones ended up on shallow shores. Storms and sea ice rolled the stones against each other, which made them rounder. The result is a cobble field with round and almost same sized stones. Cobble fields can be found in both the High Coast and Kvarken Archipelago. New cobble fields are still formed on some beaches in the World Heritage.
Due to land uplift, these ancient beaches have ended up high up on mountain slopes. Several newly discovered cobble fields are right outside the World Heritage on elevations up to about 280 meters above sea level. For example, Hamningsberget, outside of Ullånger in the High Coast. New cobble fields can form to exposed beaches. In Finland these cobble fields are called ‘devil’s fields’, since it used to be believed that they were created by the devil.
Beach ridges
Beach ridges are formed by large waves during storms or by sea ice that pushes material on the beach to ridges. Then land uplift then lifts the ridges beyond the reach of the sea. As time passes, more ridges are created, each showing where the shoreline was when they were created. This gives the landscape a wavy look. Beach ridges like this isn’t just found on cobble beaches, but also on gravel or sand beaches. Beach ridges are found in both the High Coast and Kvarken Archipelago.
Onion-shaped tunnel caves
Tunnel caves are a special feature in the High Coast scenery, and they have been formed by the forces of the sea. Tunnel caves start as a crack in a steep cliff which is below water surface. Waves erode the crack so that it becomes wider. Then the land uplift gradually raises the cliff so that new parts of the crack are eroded. Waves move stones and gravel back and forth in the crack, and eventually carve out a cave with smooth walls. Tunnel caves are narrowest at the top and widest at the bottom, just like onions. That is why tunnel caves are also called onion caves. There are only about 60 known tunnel caves in the World, and half of them are in the High Coast.
Where did the water go?
That the land is rising out of the sea hasn’t been known that long. People living at the coast have of course noticed that the sea level changed. But all the way to the 19th century, the prevailing theory on both sides of the Bothnian Sea was the disappearance of the water, wattuminskningen, in the beginning connected to theories that water was still disappearing after the biblical deluge. There were many explanations for the disappearance of the water, for example a hole in the bottom of the sea or the erosion of the straits in the Baltic sea. Land uplift as an explanation was suggested already in the 18th century, but it wasn’t until the end of the 19th century that theory could be proven. Researchers then discovered the Ice Age and could then connect the traces left by glaciers with the traces of the Ice sheet we have in northern Europe and North America.
Next chapter: similar but still so different
Why is the High Coast so high and the Kvarken Archipelago so flat?