The monumental Pirunpesä Gorge is a cliff fissure cutting through the quartzite rocks of Tiirismaa and has been a well-known visitor attraction since the 1800s. The walls of this impressive rock formation are up to 20 metres tall. Pirunpesä, which translates literally as Devil’s Nest, is one of the most important geosites of the Salpausselkä Aspiring UNESCO Global Geopark. A marked hiking trail of 4,5 km passes by Pirunpesä. The trail can be reached from the parking area at Arvi Hauvosen tie.
Pirunpesä is a gorge that was formed along a weak zone in the Tiirismaa bedrock, where over the course of time the rock has fractured and broken up. At the end of the Ice Age, about 12,000 years ago, the southern margin of the continental ice sheet enclosed an ice lake, which was formed from glacial meltwaters, and located to the north of Tiirismaa cliffs. Pirunpesä was formed when the water of the ice lake burst via the weak zone in the bedrock through to the south, carrying loose rock debris. The finer material was transported further with the current, but the larger boulders piled up to form a boulder deposit at the southern break-through point of the channel. The place may have got its name from this “nest” made of boulders.
The Tiirismaa bedrock is quartzite, which is a type of rock almost entirely composed of quartz. The quartz has its origins about 1,850 million years ago as layered, petrified quartz sand in shallow coastal waters. Ripples can still be seen in the boulders and cliff faces of Pirunpesä - these are patterns left by ancient waves in the seabed sand. Tiirismaa cliff area is nationally recognised for its value in terms of nature and landscape conservation. The originally high mountain has been eroded in the course of time, but is still the highest point of southern Finland, 223 m.
Quartzite is the hardest stone type in Finland, but it too weathers, for example, as a result of variations in temperature. Also the cliff faces of Pirunpesä have weathered over the course of time and the amount of loose boulders has increased. Even though the stones may be attractive, it is not permitted to remove them from the area or to move them. Pirunpesä has been a sight of geological research for over a hundred years, and it is hoped that the location will remain as unchanged as possible in the future. The Pirunpesä nature reserve was established in the year 1970 and now belongs to the Natura 2000 Network.