THE ENLARGEMENT OF 19TH CENTURY DRILL HOLES IN ARKOSIC SANDSTONE BLOCKS IN THE SPRAY ZONE AT SALT POINT STATE PARK, NORTHERN CALIFORNIA COAST

The arkosic Cretaceous sandstone of Salt Point State Park are famous with photographers for their picturesque tafoni, which are weathering pits developed in uniform rock. Many different theories have been proposed to explain tafoni, but all of them depend on the idea that once a pit forms in salty places like sea coasts and deserts, the hole will weather faster than the other rock around it. However, we have the first proof that this conjecture is true. Blocks of rocks were quarried and abandoned in the surf and spray zone at Salt Point, California, in 1869. The blocks have drill holes in them which have now expanded up to 4 times the width of the original cylindrical diameter. The blocks were taken from adjacent beds which have well developed tafoni. Some of the artificial surfaces have developed tafoni, others have not. Where new tafoni has developed since 1869, the drill holes have enlarged much faster than the new naturally made holes. It is clear the tafoni making process has enlarged the artificial drill holes, just as the same process enlarges the natural holes faster than it makes new holes. This suggests that tafoni making is a positive feedback loop by which the rock in a hole weathers faster, so the hole grows bigger, so the rock in the hole grows even faster.