As a physical geographer I perpetually strive to know the landscape deeply. From the rapidly uplifting marine terraces, vast dune sheets, and offshore landforms that comprise Mendocino’s unmistakable coastline; to the oak-studded, wildflower-strewn tectonic valleys of Laytonville, Hopland, Covelo, Boonville, Comptche, and Branscomb; to the thousands of savannahs, wetlands, riparian corridors, coastal prairies, scrublands, riverine systems, pygmy forests and evergreen forests in between, the dynamic physical geography of Mendocino never fails to amaze and inspire.
It used to feel impossible and overwhelming, but still I yearned to truly know the why’s of the architecture of the landscape, and the what’s and how’s of the geologic ingredients of the subterranean building blocks beneath our feet. When we know this, or at least pursue the knowing, we can find satisfaction and even comfort in understanding what underlies and supports the veneer of our terrestrial environments.
The living landscape that is Mendocino County wholeheartedly delivers these confounding questions, and reveals magnificent and monumental answers. The history and scope of Mendocino’s geologic setting, geomorphic evolution, and the diverse ecosystems that arise from it can be examined and revealed bit by bit as wondrous natural elements of our complex and diverse landscape puzzle.
Zooming out, Mendocino County is an embodiment of the northern Coast Range and is defined by the tectonics of the San Andreas fault system. Mendocino County is vertically divided in two by the deep tectonic boundary of the Maacama fracture zone, which runs the length of the county following the tectonic valleys along the 101 corridor from Hopland to Laytonville and on into central Humboldt county.
The Maacama fracture zone separates two broad geologic units of the northern Coast Range in Mendocino County: the steep ranges and deep forested valleys of the Coastal Belt to the west of the Maacama; and the volcanically inclined Central Belt extending to the east of the Maacama fracture zone and on into the rugged and towering peaks of the Mendocino National Forest.
Mendocino County is being shaped by the staggering multi-component mechanics of the Mendocino Triple Junction system where three tectonic plates (the Pacific, North American, and Gorda plates) conjoin at the the infamous triple junction at Cape Mendocino. This highly active tectonic zone is responsible for continual seismic activity, and is dragging, uplifting, and pulling apart the landscape of Mendocino County. Earthquakes, geothermal, and volcanic activity are manifestations of these processes, but so are the geologic units of Mendocino County, which are as complex and varied as the convoluted geologic environment that is California.
These complex geologic assemblages feed and form the terrestrial environment. Serpentinite, and other metamorphic, sedimentary, and volcanic rocks, soils, and landforms of the Mesozoic era subduction complex support serpentine grasslands, wetlands, mixed oak woodlands, and other rare ecosystems of inland Mendocino.
Ancient sedimentary deposits and geologic units of the coast are uplifting so rapidly by the oblique grinding pressures of the San Andreas fault system that stair-stepped flights of marine terraces, once submerged and carved flat by wave action, retain their nearshore morphology and even their beach sands for hundreds of thousands of years as they ascend upwards and eastward. These stair-stepped marine terraces are so flat that rainwater leaches most nutrients from the soils over time, leaving a mineral hardpan that Mendocino’s ultra-rare Pygmy Forest ecosystem has evolved to inhabit.
Rowena Forest is a physical geographer at Cal Geographic and resides on the Mendocino Coast at Wave Cut Ranch.
I, too, am fascinated by this landscape. Thank you for the well written article. Judith in Piercy