Tvären is a circular bay close to Studsvik, Sweden. Within the bay is an ancient buried

impact crater An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...

about 2 km in diameter. It was first identified using

reflection seismology Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seis ...

. The crater is estimated to be about 455 million years old (

Late Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

).Tom Flodén, Pål Tunander & Frans E. Wickman (1986) The Tvären Bay structure, an astrobleme in southeastern Sweden, Geologiska Föreningen i Stockholm Förhandlingar, 108:3, 225-234, DOI: 10.1080/11035898609454687 Asteroid 7771 Tvären is named after the bay.

References

Impact craters of Sweden Ordovician impact craters Ordovician Sweden Sandbian Landforms of Södermanland County {{Södermanland-geo-stub