Marine Terrace Geology and the Pygmy Forest

From Mendocino County south to San Diego, an irregular series of marine terraces, or wave-cut benches, occur between the shoreline and the coastal foothills. Carved by waves and wind-driven water at sea level, these terraces feature a vertical, or steeply rising, cliff face at their seaward edge (bluffs) backed by a level or gently sloping bench. Where harder rocks occur within a forming terrace, these erosion-resistant rocks eventually rise above the surrounding terrace as offshore rocks or sea stacks. Where the coastline is being pushed upward over time by the collision of two tectonic plates, a series of terraces exists, each one backed by another terrace around 100,000 years older and 100 feet higher than the one closer to the coast. Well developed marine terraces along the Mendocino coast dramatically demonstrate the process of coastal uplifting and terrace formation. Stand on the bluff’s edge at the Jughandle or Mendocino Headlands and look seaward over the new terrace forming at sea level. It’s most apparent at low tide when you can see the smoothed rocky bottom that will become the next terrace. Look eastward from the coast to see the level or gently sloping first terrace that was the ocean floor at sea level roughly 100 millennia ago. Look farther east beyond that first terrace and you may see the distinct rise of the seaward edge of the next, or second terrace, about 100 feet above your blufftop elevation. You can typically locate five succeeding wave-cut terraces in a two- to three-mile strip along the Mendocino coast.

Marine terraces occur in many places along the world’s coasts. In California you can also see them above Bodega Bay in Sonoma County, at Duxbury Reef in Marin County, along the Santa Cruz coast, and at Dana Point in Orange County. In the Palos Verdes Hills near Los Angeles, a series of 13 terraces rises to 1300 feet above sea level.

Marine terraces generally feature distinct plant populations corresponding to the climate since the terrace rose above sea level, and to the terrace’s rocks and soils. Only on the Mendocino and Sonoma coasts, however, do the upper terraces feature the unique botanical habitat called Pygmy Forest. These dwarfed forests occur in pockets on the upper ends of the third, fourth and fifth terraces, where a combination of flat terrain and sandy soils undergoes a complex transformation that creates impoverished acidic soils with poor drainage caused by underlying hardpan, leading to mature pockets of stunted vegetation. In the Pygmy Forest habitat, mature Mendocino cypress, Bolander and Bishop pines, and even redwood may grow only ten, five, or even three feet tall over 100 years. Other plants thriving in this harsh habitat include Fort Bragg manzanita, Labrador tea, rhododendron, huckleberry, reindeer moss, and the tiny sundew, an insect-eating plant.