Part of Baja’s ancient granite core is visible in the boulder field region around Cataviña. Erosion has carried away soils that were deposited in this region 100 million years ago. Subsequently the exposed granite surface has weathered and decomposed to produce the odd looking landscape of the boulder fields. In some instances entire hills of solid granite have been reduced to boulders, which look as though they were placed in a pile. Some nearby mesas and hills are capped with erosion resistant volcanic material, which protected the underlying material from erosion. In these areas the underlying granite is not exposed and the flat tops of the Mesas and smooth surfaces of the hills bear witness to volcanic ash deposits that occurred after the formation of the underlying granite.