Skanska w Polsce

some history

some history

During construction works on the "Nowy Rynek" complex in Poznan at Przemysłowa 3, five large erratic boulders of Scandinavian origin were excavated from a depth of about 5 to 9 meters. Erratics are rock fragments that have been transported by glaciers or ice sheets and deposited far from their original location.

At the time when Scandinavian granitoids and gneisses were forming, i.e. over a billion years ago, there was no Carpathian, Sudeten or Greater Poland region yet.

The land was a barren desert, torn by strong winds, heat, and frost – a little like the surface of Mars. The area of ​​today's Masuria and Podlasie along with Scandinavia was part of an ancient large continental bloc – Baltica. At that time, Baltica was near the South Pole. It was there, at a depth of several kilometers below the surface, that the granitoids and gneisses were formed. Then, along with the whole of Baltica, they moved gradually north to finally settle in their current place. Ice shields over two kilometers thick had formed in the Quaternary on the Scandinavian Peninsula. Ice sheets tore up fragments of rock from their substrate and transported them frozen in the ice throughout the Central European Lowlands.

This is how erratics finally reached Poznan where they were released as a result of the melting of the ice sheet in the Leszno phase of the main stage of the Northern Polish (Baltic) glaciation, i.e. about 20,000-24,000 years ago.

 

1. The largest boulder measuring approximately 1.7 m x 1.3 m x 1.5 m is a gray-reddish coarse gneiss. It consists of red potassium feldspar, gray translucent quartz, white plagioclase and black biotite, which clearly emphasizes the streaky texture of the rock. This metamorphic rock comes from Scandinavia and is over a billion years old.

2. A dark gray granitoid from central Sweden (Uppsala region) measuring approx. 1.4 m x 1.1 m x 0.5 m is a plutonic igneous rock. It is composed of gray quartz crystals, white, and sometimes also slightly gray-greenish plagioclases and a few yellowish-pink potassium feldspars. Black biotite and other dark minerals are found in large numbers. Granitoids from the Uppsala area are among the oldest rocks transported from the north to the Central European Lowlands by ice sheets - they are about 1.8 to 1.9 billion years old.

3. One boulder measuring approximately 0.7 m x 1.0 m x 1.2 m, found in Nowy Rynek, is a metamorphic rock. It is a medium-blastic, in places unevenly blastic pinkish gneiss with large "eyes" of pink potassium feldspars. The gneiss is characterized by quite high variability visible on the surface of the boulder. It comes from Scandinavia and is over 1 billion years old.

4. The erratic has a two-part structure. It is possible to distinguish a gray section, clearly streaky with a gneiss-like structure with traces of dark primary rock, and a much lighter and coarse-crystalline pegmatite with a pink-reddish tint. In the pegmatite, in addition to reddish potassium feldspars, white plagioclases and gray quartz, there are isometric reddish grenades. The boulder measures approximately 1.1m x 1.1m x 0.8m. The rock comes from Scandinavia and is over 1 billion years old.

5. The smallest (approx. 0.7 m x 0.6 m x 0.4 m) of the excavated boulders is a gray-brown medium-crystalline, and in places turning into fine-crystalline granitoid. It is a deep-sea igneous (plutonic) rock, comes from Scandinavia and is over 1 billion years old.