http://mineraalamos.com/news/?id=418

http://www.defiancesilver.com/news/defiance-continues-to-intercept-wide-zones-of-mineralization-at-san-acacio

 Selective erosion of softer materials is not always natural, as mankind activities also tend to favor the removal of the less competent rocks. Those 4 meters tall spikes in the picture are in a “cantera verde” (building stone) quarry near Oaxaca City in Mexico. These were spared mining due to silicification and minor quartz veining along a structure.