The eskers, or ridges, that cut through the Rokua Geopark area are the most important land formation in the park. When they melted, masses of ice, kilometres thick released huge amounts of water, which had the power to carry and pile up till into sandy ridges tens of metres high, known as eskers. They were formed around 10,700 – 10,400 years ago.
The most extensive esker formations occurred on the island of Manamansalo and in Ruokuanvaara, where the edge of the glacier stopped in one place for a longer period and the meltwater had time to layer the till into eskers. Teeriniementie, the road leading to the Manamansalo hiking area, is built on top of a sharply winding esker. Säräisniemi, Kuostonsaari, Laajankangas in Vaala, the sandy areas in Ahmas and the extensive sandy areas in Suokylä in Muhos are all esker formations.
The largest esker formation in Rokua Geopark is Rokuanvaara, the hill in the heart of the Geopark itself. The esker is about 20 kilometres long, an average of five kilometres wide and 100 metres thick. It is the most prominent part of the Rokua esker, which is massive as a whole. The Rokua esker valley starts in Ilomantsi on Finland’s eastern border and runs as a coherent sandy ridge for more than 400 km before reaching the island of Hailuoto on the shore of the Gulf of Bothnia. Along it are several other prominent sites besides Rokua, such as Paltaniemi in Kajaani, the island of Ärjänsaari in Lake Oulujärvi, Oulunsalo and Hailuoto itself.