The bedrock of western Norway

Hordaland's bedrock - like "a bottom-heavy layer cake"

Geologically, Hordaland is located near the margin of an ancient continent that is called Fennoscandia or the Baltic Shield." For the most part it is a large and stable shield area that has grown along its margins through collisions with other shields over geological time. We therefore find some of the world's oldest bedrock toward the centre of this shield (Finland and Russia), with younger bedrock out toward the margins (southern Norway). Most of the bedrock in Hordaland is nonetheless quite old, from over one and a half billion years to 155 million years old for the youngest.

The structure of Hordaland's bedrock reminds one of a "layer cake". The Precambrian Basement forms a very thick and solid base at the bottom. Above this is a layer of phyllite, somewhat resembling a layer of vanilla. Then comes another covering of Precambrian basement rocks followed by Cambro-Silurian bedrock that was formed in an ocean far to the west of modern-day Hordaland (so-called "exotic", meaning "from afar", rock types).  These consist of among others greenstone, gabbro and granite together with phyllite. These bedrock types have at one time been overlain by Mesozoic layers and possible also deposits of Tertiary age.

Only crumbs left of the youngest layers of the ''cake''

The whole "cake" has now been sliced by faults and was penetrated by veins of magma during the Permian and Triassic Periods. The natural forces of nature have also "helped themselves" to the cake for a very long time, so that the different layers have been exposed at the surface and some have been partially eroded away. The layers from the Jurassic Period and any younger layers that had been deposited have now been totally "eaten away". Only a few crumbs are left under Vatlestraumen iin the Fjell municipality - otherwise one must travel westward beyond the coastal islands to find any remnants of these layers. 

At the very top of Hordaland's bedrock "layer cake" lies the decoration: deposits of gravel, sand and clay from the Quaternary glacial melt and the subsequent interval of time. Most of this is around 10,000 years old, which in a geological time-frame is very young. So these layers have not yet been transformed into stone - they are still "just" sand and gravel.

The base of the layer cake

The Precambrian basement rocks, which comprise the foundation or "base of the layer cake" in Hordaland, were formed during the Earth's Precambrian Era. They arose when magma solidified as granite and gabbro in between somewhat older more superficial bedrocks, such as basalts, rhyolites and quartzites.  In large parts of Hordaland these bedrocks were gradually pressed together and became transformed into different kinds of gneisses.  This occurred both in the Precambrian Era (during the sveko-Norwegian mountain-building event) and in connection with the subsequent collision with Greenland (The Caledonian mountain-building event) In areas where there has not been gneiss metamorphism one can still find the original bedrock types. Deep intrusive igneous ¬rock types, and especially granitic plutons, are prominent in southeastern Hordaland (Hardangerfjord-Finse-Hardangervidda¬ area). Several of these rock types can be beautiful to look at, such as the Finse- and Eidfjord granites. The Eidfjord granite has been used both as a building stone and as a paving stone for a long time. (Quarry)

The surface bedrock is well preserved in the northeastern part of the Folgefonna Peninsula and further toward Sveio. The rock types present include quartzite, rhyolite and metamorphosed basaltic lavas squeezed in between large bodies of granite and gabbro. Ripple marks can be found some places and cross-bedding from when the quartzite was deposited as loose sand. There are also layers of conglomerate that once were gravel and stones in a Precambrian river system or coastal zone. These surface bedrocks recur several places southward toward Telemark, where they have been given the name "Telemark supracrustals".

Quartzite that might possibly be of the same type as in Telemark is also found in the bedrock farther north, in western Stølsheimen and Norhordland east of Fensfjorden. The Precambrian basement in this area is mostly dominated by gneisses and migmatites without the massive bodies of granite and other intrusive rock types that are so common in the Hardanger region. Smaller bodies of eclogite are found in the northwestern part of these gneisses, formed during the deep burial in Silurian times. A third area with Precambrian basement is found in Sund, Fjell, Fedje and the western part of the Bergen municipality. This area is also dominated by gneisses with occasional bodies of granite and gabbro. Reddish pegmatite veins are common both here and in other parts of the Precambrian basement, and they break up the other rock deposits from Precambrian times.  Geological dating suggests that these Precambrian bedrocks were formed roughly 800-900 million years ago.

Vanilla layer of phyllite

A layer of phyllite is spread out over the Precambrian basement and is metamorphosed remnants of clay-rich sediments that were deposited on top of the basement in the beginning of Cambro-Silurian time. The layer once extended continuously from eastern Norway over the Hardanger plateau, out toward the west coast of Hordaland. Much of the phyllite layer is now gone after millions of years of erosion. The black flecks in the geological map of Hardanger plateau area are places where the phyllite is preserved, in some places with remnants of the so-called thrust sheet overlaying it. In the western parts (Voss and westward), where the temperature was highest during the Caledonian metamorphism, the bedrock has been transformed to mica schists with large flakes of mica and the mineral garnet.

At a few places conglomerate can be found at the base of the phyllite layer, in contact with the Precambrian basement  (p. R463 ). This is petrified beach sand from the beginning of the Paleozoic Era (the Camrbian Period), when the basement rocks of southern Norway were flooded by an ancient sea. The phyllite is metamorphosed clay that had been deposited on the bottom of the sea. Layers of quartzite and limestone testify to the deposition of sand layers and marine life. In easternmost Hordaland fossils can be found in the phyllite belt. At Hårteigen, Finse, Holberget and Dvergmednuten on the Hardanger Plateau fossil remains of 500-600 million year old organisms have been found - signs that the Paleozoic seas of the Cambrian Period were teeming with life. The Cambro-Silurian calcium-rich phyllite disintegrates easily. This bedrock is often called "decaying stone" in many parts of Hordaland.  But this "decaying stone" gives a good growing soil both in the high mountains and in the lowlands. The high calcium content prevents the mountain lakes from becoming acidic and losing their fish. It can therefore be a good idea to check the geological maps if you are planning on fishing on the Hardanger Plateau. Proximity to phyllite in the water's catchment area gives good hope for finding fish. An example of a settlement that has benefited from the lush phyllite belt is the Voss-Bulken area, where the phyllite layer is several hundreds of metres thick.

The top of the “cake” - pushed in from the side

Remains of a thick cover of Precambrian basement bedrock rests on the phyllite. During the Caledonian Mountain-building event enormous flakes were torn completely off from the bedrock and thrust up onto the Cambro-Silurian deposits. Such detached bedrock units are called thrust sheets. We find remnants of such thrust sheets a number of places in Hordaland, among other places on the Hardanger Plateau, around Bergen and around Oppheim. Some of the lowermost bedrock units (Bergsdal nappes) are surrounded by phyllite both above and below. This disruption of the otherwise systematic "cake structure" is the result of complex movements in the earth's crust during the Caledonian Mountain-building event. Some of the displaced thrust sheets consist of rock types that we recognise from the Precambrian basement underlying the phyllite, albeit a more flattened, or perhaps on their way to becoming gneisses.
 
Other thrust sheets consist of quite distinctive rock types.  These include first and foremost the displaced Precembrian basement bedrock that stretches from the southern part of the Bergen municipality via the Osterøy, Lindås, Austrheim, Radøy and Meland municipalities (the so-called Lindås nappe), in addition to the area around Stallheim-Mjølfjell (the Jotun nappe).  We find large volumes of anorthosite and anorthosite-gabbro in these two units. They are light, nearly white bedrocks and are relatively uncommon. The name "Mjølfjell" (="flour rock") comes from the light, flour-white colour.

The anorthosite rock types represent magma that intruded into the earth's crust roughly one billion years ago and then solidified. Since then it has been altered and obtained its light colour. The light colour, chemical composition and other characteristics have made it interesting for a number of uses, from an abrasive in toothpaste to road gravel and garden stone. It contains a lot of aluminium and it is hoped that advances in technology will make it possible to make use of this sometime in the future. In addition, titanium iron and nickel deposits associated with the anorthosite in the Bergen area have been mined in earlier times. A variety of the anorthosite-gabbro known as Coronite is also found here. It consists of olivine surrounded by black pyroxene and a reddish garnet in a white matrix and is Hordaland's municipal stone. Anorthosite bedrock is quarried in considerable amounts both in the Bergen area (R334 ) and in Nærøydalen. In the Meland municipality some of the anorthosite includes some eclogite. This is important for understanding the Caledonian development of western Norway. Deposits like the titan-rich mineral rutile in the eclogite can potentially be of economic interest in the future.

Both the Jotun nappe and the Lindås nappe contain another type of unusual intrusive rock - the so-called mangerite-syenites. Mangerite is the best known of these and is in the intrusive rock type that lies between granite and gabbro in composition, formed deep below the earth's surface. This rock type got its name from Manger in the Radøy municipality.

Ancient sea floor - came from America?

Remnants of the so-called "exotic rock types" (meaning that they came from another place) of Cambro-Silurian age are only found near the coast and overlie both the phyllite belt and the detached sheets of Precambrian basement.  The rock types are quite complicated and consist mainly of metamorphosed intrusives such as greenstone, rhyolite, serpentine, granite and gabbro, together with phyllite, quartzite, marble and conglomerates.  These are exotic rock types since they were not originally formed in Hordaland, but were transported long distances by violent forces during the Caledonian mountain-building event. They were originally formed on ancient oceanic crust far to the west of modern day Hordaland, roughly 4-500 million years ago. There is much to suggest that they originated nearer to the North American continent than the Norwegian. However, some of the rock types have more in common with the more indigenous bedrock. Phyllite and mica schist are examples of this. The exotic phyllite is nonetheless different; it is more basic and in some places greener than the indigenous rocks in the phyllite belt. Together with the layer of marble they make very fertile soil and this is a sharp contrast to the granitic bedrock in the same unit. One can see this especially well in Sunnhordland where there is a visible contrast between the phyllite- and the mice-rich shale in the southerly part of Stord and Tysnes and the more barren granite-dominated area in the north toward Austevoll and Reksteren. Parts of the northwest side of Hardanger Fjord (Øystese-Nordheimsund-Tørvikbygd) are also enriched by these exotic Cambro-Silurian shales. Nearer Bergen there are elements of Cambro-Silurian phyllite and mica schist both in what are known as the "Bergen Arcs"  (Os-Samnanger area) and in the "Little Bergen Arc" (from Grimstad via Lake Nordåsvatnet and centre to the east side of Askøy). 

Phyllite, mica schist, greenstone and greenschists are rock types that disintegrate relatively easily. Therefore they commonly form lowlands or valleys in the terrain. In contrast, intrusive rock types like granites and gabbros tend to form hillsides and mountainsides. Therefore there is a sharp contrast between the lower phyllite- and greenstone area on the southern side of Stord and the mountainous central and northern parts of Stord where intrusive rocks dominate.

The limestone that is found in this exotic bedrock was long ago transformed into marble, and the phyllite is for the most part compressed and folded. Nonetheless, it is still possible some places to find fossils of organisms that lived in the ocean during Ordovician and Silurian times.

Conglomerate from the Devonian - a geological splash of colour

Devonian conglomerates are mainly found along the coast of Sogn and Fjordane, especially in the areas of Solund, Kvamshesten and Hornelen. They are also found in Hordaland at Holmengrå in the Fedje municipality (p. 403 This little island consists entirely of Devonian conglomerates. The conglomerates were formed when up to metre-sized stones including quartzite, granite, gneiss and shale were deposited on top of the exotic Cambro-Silurian bedrocks. The Devonian deposits are remnants of the Caledonian mountain chain that was eroded down about 390-360 million years ago. After the deposits we can see today were deposited, several kilometres of new sediments were laid down over these, so that the conglomerates were pressed together to form solid bedrock.

Volcanoes during the Dinosaur Age?

Along the coast of Hordaland there are hundreds of places where one finds intrusive rock types that are younger than both the Cambro-Silurian and Devonian bedrock. These igneous bedrocks occur in a North - South to North/Northwest -South/Southwest oriented depositional break. They were formed when magma from the mantle flowed up toward the surface of the continental crust along these breakages. The veins have a basaltic composition and are seldom more than a half of a metre wide. They can, however, be up to several hundred metres long. They appear very dark and cut through all the other bedrocks and bedrock structures. Using modern dating methods it has been possible to determine that these were intruded during the Permian and Triassic Periods. It appears that there were a series of such intrusions around 260 million years ago and also a newer pulse roughly 220 million years ago. This is very far back in time, but the bedrocks are still quite a bit younger than the Devonian and older bedrocks of Hordaland.

There is much to suggest that these relatively young igneous bedrocks have come up from great depths. It is not impossible that some of them reached the surface at that time and caused local volcanic eruptions in the area around Sunnhordland and Sotra. If the lava flows did reach the surface, they were in any case protected from wind and weather since.

Jurassic Rock under the Vatle Waterfall

Until just a short time ago, it was believed that the Permian and Triassic veins were the youngest bedrocks in Hordaland.  That was until sedimentary rocks of Jurassic age, roughly 150-160 million years old, were discovered in connection with building a tunnel under the Vatlestraumen passage between Alvæen and Bjorøy near Bergen (page 364). We know these layers well from the North Sea Basin, where they can be followed eastwards toward land. They disappear roughly 15 km from land, but now it is known that they are preserved locally under the Vatlestraumen passage. Previously they covered parts of the Hordaland coast, but were since eroded away.  The little remains we know of from the tunnel under Vatlestraumen passage were protected from erosion where they lie preserved in a fault zone. Similar bedrocks are only known for certain in Beitstadfjorden (in the Trøndelag municipality) and on Andøya, and probably also in the Karmsundet Sound at Haugesund.

This is our view of how Hordaland's bedrock was formed, with its history stretching from 1700 to 150 million years back in time. Hordaland has undergone changing landscapes and climate throughout this enormous interval of time. There were mountains as high as the Himalaya here, but also landscapes consisting of low plateaus. Hordaland was flooded by the ocean during some periods, and shaken by earthquakes or covered by volcanos in others. The landscape has been liften up, and eroded down, several times. Again the bedrocks lie like a thick-bottomed cake of Precambrian basement, phyllite and thrust sheets. We find them everywhere, like silent witnesses to Hordaland's long and dramatic history: Gneiss, granite, gabbro, limestone, phyllite, greenstone and anorthosite - and many others. Quiet and lifeless, but testimony to a continuously changing earth.