Please respect the following points when you visit the site:
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do not hammer, break off or excavate rock, nor interfere with the exposure
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do not take away samples from the bedrock or from loose material
The sedimentary bedrock which is visible in the Skålberget Nature Preserve was deposited during the Ordovician Period, some 460 to 440 million years ago. There are two series of Baltoscandian Ordovician rock here, the Viruan (Middle Ordovician) and the Harjuan (Upper Ordovician). These correspond roughly to the British Caradocian and Ashgillian Series. The older sections are to the northwest, the younger ones to the southeast in the direction of Kullsberg. During the period when these rocks were being formed, this region, which was part of the Baltica palaeocontinent, drifted northwards from approximately 30 degrees south of the equator to the tropics.
Two separate levels of a remarkable, very pure limestone, mound limestone, were formed in some regions of the Baltoscandian Platform during the Ordovician Period. The principal component of this rock is inorganic calcium carbonate. These formations have been found in the Siljan area, in Estonia, and in smaller deposits in Östergötland and subsurface on Gotland.
These two levels of mound limestone were originally described and named in studies of outcrops in the Siljan area. The lower level, from the later Middle Ordovician Period, is known as Kullsberg Limestone, reference locality approximately 1 km. southeast of here; the upper level, from the Upper Ordovician Period, is called Boda Limestone, reference locality approximately 10 km. to the northeast. In a few places the Boda Limestone lies directly on top of the Kullsberg Limestone, in spite of the fact that many millions of years separated the last deposits of the Kullsberg Limestone from the first deposits of the Boda Limestone. One such location is here at Skålberget. These layers, together with the surrounding rocks, were tilted to an almost vertical position by a meteorite impact 360 million years ago.
A largely endemic invertebrate fauna lived in the Kullsberg and Boda mound limestone. Trilobites, cephalopods, brachiopods, bivales, echinoderms, corals and bryozoans are the main constituents of the very rich fossil assemblages in the limestones. Locally, monospecific lenses of thousands of trilobite carapaces are found, and more rarely brachiopods occur in similar concentrations.
Two factors make the nature preserve Skålberget a geological preserve of international importance. The contact between the two layers of mound limestone is well exposed, as are the layers of ordinary limestone wedged between and around them. In addition, the preserve includes a section of the northern part of the Kullsberg Limestone formation which has been studied extensively. The results of this research form a basis of our understanding of how mound limestone was formed.