As recently as the Miocene period, twelve million years ago, the entire watershed of Piner Creek was on the floor of the Pacific Ocean. After significant volcanic uplift and tectonic movement, the headwaters region of the Mayacmas Mountains emerged above the ocean. Piner Creek was one of the drainages that began to drain the new landmass, in formation of the eventual Santa Rosa Plain, through which the middle and lower reaches of Piner Creek flow. Occasional basaltic outcrops are seen in the upper reach of Piner Creek, betraying the volcanic origin of the Mayacmas Mountains.