In the symbolic landscape that is the body, one of the features that all shamans explore is the Underworld.  One of the ways in which the Underworld can be accessed is via a symbolic cave, however, an alternative is the crack or crevice.  Thus a crack or crevice is yet another type of portal, but one leading figuratively speaking ‘down’

Portals can be cracks or crevices in a rock face and it is not unknown for these cracks and crevices to be exact copies of real material physical cracks and crevices. 

In other words, the shaman on his spiritual journey has a vision that starts with an entry – eyes closed – into a crevice in the rock.  Once the trance has completed he finds that the identical crack or crevice exists in the place where he had the vision or a place known to him.

At this point, the physical place is often marked as a ‘sacred site’ because it is clearly a figurative entry point to the spiritual world – perhaps conducive to shamanic travel.

We might think that these markings would be few and far between, but they occur in vast numbers in North America.  A major concentration of petroglyphs, for example,  occurs in the Coso mountains and the Great Basin region across Nevada into Oregon and Utah.  Shoshone, Paiute, and Kawaiisu peoples practised in this area, as did the Chumash shamans in California.  There are over 100,000 rock art depictions in the Cosmo mountains alone.  The Great Basin rock art is extremely old.  Dating of the desert varnish on which the rock art was made put it in the paleolithic era (over 19,000 years ago) the time that Alfred Watkins placed ley lines.