In the geological past, a collision with Greenland pushed long slices of the seabed on top of the existing bedrock, today forming the bedrock from Dovrefjell and Trollheimen south of Trondheim stretching north in Trøndelag and through Nordland to justh north of Tromsø. This Cambrian—Silurian bedrock, much of it mica schist, is by far the largest area in Norway with soft bedrock rich in nutritions good for plant growth. It forms the bedrock in the fjord areas, while the islands off the coast and some of the easternmost areas along the border with Sweden are made up of hard bedrock (usually granite). In some areas, as in Tysfjord and Sørfold, the bedrock is a mix of soft bedrock and hard granite. Much of the Lofoten mountains are of precambrian eruptive origin and 3.5 billion years old, among the oldest on earth. The youngest rock in Norway is on Andøya, also known for its fossils of dinosaurs and other life forms. As the land was depressed by the ice sheet in the ice age, substantial areas in the lowest altitudes was beneath the surface of the sea for thousands of years acquiring marine deposits. Due to post-glacial rebound, this is now dry land, reaching 120 metres above sea level today in Saltdal, 100 m in Narvik and Brønnøysund, and 30-50 m in Lofoten and Vesterålen. Limestone is very common in Nordland, with many caves throughout the county, such as Grønligrotta in Rana. There are more caves in Rana than any other area in northern Europe. In August 2006 the Tjoarvekrajgge cave in Sørfold was explored and verified as the longest cave in Scandinavia (22 km long); Raggejavreraige in Tysfjord is the deepest in Scandinavia and Svarthamarhola in Fauske has the largest cavity. There are more than 900 caves in Nordland, one of these have a 70-metre-high waterfall (DN.no). Marble is found in several locations. Fauske is sometimes referred to as the marble capital, and has exported marble worldwide (one customer being the UN building in New York City).