Forested ridges and bare rock surfaces at Siljan! Siljan is dominated by forested areas with knobby hills around a central valley, the Siljan valley.
The valley was carved by glaciers and the glacial deposits have provided good agricultural areas. On a tour trough the Siljan region one can see many traces of the last Ice Age, for example, polished grooves, glacial erratic's of various sizes, glacial flutings and potholes.//par
The two rock types, syenite and larvikite, dominate the exposures in Siljan. These rock types have consolidated from lavas far below the earth crust. 300 million years ago the area today is Siljan was full of volcanic activity and large fissures were formed. This period lasted for about 50 million years and resulted in what geologist today call the " Oslo rift".
Syenites and larvikites are both so-called "magmatic deposits", but they consist of unlike minerals and have different characteristics. These characteristics determine he appearance of the landscape. When larvikite breaks down, it gives a significant amount of nutrients to soil. This is obvious from the vegetation. Auenkollen consists of larvikitee low an rounded knoll with fertile vegetation.//par
Syenite, on the contrary, erodes quite slowly, and therefor contributes little nutrient to the soil. Skriua in Siljan is a typical knoll consisting of syenite. It has bare, steep rock side and sparse vegetation such as fir, juniper and heather.