Sandstone and shale
About 1,900 million years ago a high chain of mountains was formed on the prehistoric landmass when the earth’s plates collided. The mountain chain was worn away by the power of ice, wind and water, and approximately 1,300 million years ago it had been smoothed until it was almost flat. As a result, the rock formed in the depths of the mountains such as granite, was exposed on the surface.
As the mountains levelled out, western Finland was covered by a shallow sea. At that time part of the bedrock in the northwest part of Rokua Geopark started to sink. A deep bay formed at this “Muhos formation” as it is known, which started to collect layers of gravel, sand and clay, carried by the rivers. Over hundreds of millions of years the strata hardened into gravel, sandstone and shale, in places more than a kilometre thick before the granite bedrock. Kieksi in Muhos on the Lemmenpolku trail, has exposed solidified gravel from the Muhos formation on the bank of the Oulujoki River. This is the only place where the Muhos formation is exposed on the surface and the rock on the river bank is protected for this reason. The edge of the Muhos formation clearly stands out in the landscape: From Montta towards Muhosperä there is a clear step in the ground where the granite bedrock plunges deep below the surface.
The Muhos formation is an important element in the birth of Rokua Geopark. The masses of ice polished the soft sandstone during the Ice Age and the torrents of meltwater carried huge amounts of sand towards Rokua. The large amount of sand on the Rokua esker and the fact that this sand is extremely fine is explained by the Muhos formation being in the way of the route of the meltwater.