Geologists mean by "breccia" a rock that is mostIy made up of sharpedged splinters. In the Gardnos- breccia the pieces of gneis are cemented together by finely crushed stone and a little carbon. This unusual type of rock has, until recently, been regarded as having been formed in the craters of volcanic explosions. It is now shown that the Gardnos-breccia was fommed by a meteorite impact about 650 million years ago.
Around 1963 a hydro-electric tunnel was driven through the breccia for about 3,5 km. The rock that was taken out forms now a large tip a little further up the valley towards the Garnås-farms. Here it is easy to collect rock samples of the breccia.