Rocks roll downhill

Deep weathering of granite and associated rocks in China’s wet tropical and subtropical southern provinces produces corestones.  Corestones are largely unaltered, spherical rock bodies of various sizes surrounded by saprolite, a clay-rich, soft weathered material.

















Figure 1:  Schematic drawing of corestones forming from rock weathering (Credit: M. K. Hubbert).  The soft, clay-rich weathering product surrounding the corestones is called saprolite (see Figure 2).

Figure 2:  Exposed corestones encased in saprolite.  Site is a zeolite mine near Heping, Guangdong (Photo by W. Parham).












Once the vegetative cover is removed from such sites, rainfall easily erodes the enclosing saprolite and exposes the hard corestones (Figure 3).

Figure 3:  Eroded saprolite exposes corestones of various sizes on a Hong Kong hillside (Photo by W. Parham).

Additional erosion of saprolite leaves the rounded corestones in unstable positions on the land.  A corestone’s general spherical shape facilitates its down hill movement caused by heavy rains or sometimes during mild earthquakes.  Once a corestone starts rolling it is a clear danger to humans, animals, and man-made structures below, particularly where the terrain is steep.  Urban sprawl places humans closer to deeply weathered steep hills and thus increases the danger from rolling corestones.

The transition from corestone to saprolite is abrupt (See Figures 3, 4, and 5).

Figure 4:  Contact between a corestone above and its supporting soft saprolite is marked by the geologic hammer (Photo by W. Parham).













To prevent corestones from suddenly rolling down hill, geological engineers have stabilized some of the stones especially at sites above urban environments (Figure 5).  Sometimes, however, whole hillsides slide and release numerous corestones all at once.

















Figure 5.  An exposed corestone on a steep Hong Kong slope with an engineered support beneath (Photo by W. Parham).

Vegetation destruction became extensive in southern China in the Sung Dynasty, some 1000 years ago.  This was probably the time when many core stones were released and began movement downhill.  Some corestones even show up in Chinese art.  For instance, many paintings of Hong Kong island during the 1700s and 1800s clearly show corestones on steep hillsides.  Figure 6 is an example of Chinese wall paper produced in a southern province during the early 1800s showing corestones on the hills.







Figure 6:  Chinese wall paper manufactured in southern China during the early 1800s shows corestones on hills near the houses (Photo by W. Parham).

Over time, corestones make their way down hill and in some cases they reach the sea.  Many southern China coasts

Figure 7:  Corestones on a beach near Sanya, Hainan.  Immediately behind the beach is a tall, weathered hill, the likely source of these particular corestones (Photo by W. Parham).

have collections of corestones lining the beaches.  Here, they are subject to erosion from wave action and sand abrasion and in time are reduced in size.  Also, some stones show the  effects of Chinese rock carvers who chip names or sayings into the smoothed rock surface and then paint the written characters in red.  “The end of the earth” (Figure 7).



%d bloggers like this: