Corestones

The first excursion locality is located in the northern part of Plzeň Region and is adjacent to town Žihle. 

The most distinctive structure is well developed 10 km long fault in N-S direction that forms steep rocky slope with eastern orientation. This fault divides the western elevated plateau formed by Čistecko-Jesenický granitic massif (550-600 m a.s.l.) and the eastern wide flat depression at the elevation of ca. 450 m a.s.l. The eastern depression is filled with sediments of the Carboniferous age, the western granitic massif is of the Cadomian age. 

During the Cadomian orogeny, the large body of granites was formed deep below the Earth surface and subsequently cooled down. During the cooling process, the granitic body shrinked and the typical perpendicular joints were formed (typical granit cube). The plate tectonics caused the very slow movement of this granitic body from the southern hemisphere across the equator to the present place. This movement took millions of years. Granites are very rigid rocks in the temperate climate. However, in the tropical climate the can weather much faster. They can weather not only on the surface but also in the underground conditions. The abundant precipitation of the tropics caused seeping of the water that continued into the depth along the perpendicular joint planes. This underground water caused weathering of the adjacent rocks; the weathering was more intensive at the intersections of the joints. This underground weathering thus caused rounding of granitic cubes into the spherical shape. Subsequently, during and after the movement of the granitic body into the present place, the overlying soil and rocks where eroded and washed out and the residual rounded corestones were left lying on the flat surface where we can see in today.