On a strangely windless December morning, we visit the dune fields just south of Dillon Beach, a small coastal village at the mouth of Tomales Bay, on the north coast of Marin County. After paying a seven-dollar entrance fee—the site is privately owned and has been operated as a campground since 1957—we park behind the shoreward dunes. I’d visited before, usually walking along the beach in winter counting waders gathered in large flocks—willets, godwits, dunlin, and curlews—or in spring, searching for western snowy plovers.
On this visit we veer inland, across some hardpan salt-grass flats where a few head of cattle graze, then up into the open dunes, home to flora and fauna that have evolved to survive the changeable conditions of a mobile landscape. We stoop to identify the sparse vegetation on these back dunes—beach bursage (Ambrosia chamissonis), dune tansy (Tanacetum camphoratum), and salt rush (Juncus leseuerii). Each has adapted its own ingenious and complex physiology to cope with growing amid such constant drying by salt and wind on a substrate that’s exceedingly thrifty with nutrients.
This landscape, both hearty and fragile, once proliferated along the coast of California, including the westernmost portion of San Francisco. But humans have tended to take over in these places, placing fixed structures—roads, residences, and resorts—on the transient homeland of the dune tansy and salt rush. The Tomales Dunes at Lawson’s Landing is one of the few places where it is still possible to experience a remnant of this evanescent habitat.
The graceful geometries of an active dunescape are captivating—parabolas, advancing slip-face surfaces, transverse ridges, and curvilinear slopes. In time-lapse video, they would appear as a fluid ballet of shapes morphing through an infinite series of soft-edged polygons—arbelos, crescents, ovular and elliptical curves—all sculpted by the wind. The shape-shifting is fairly gradual most of the time, slow motion, but an energy burst from an intense storm can create entirely new forms and volumes, as if the wind sculptor had a sudden epiphany of creative inspiration. Indeed, a few heavy winter storms can completely rearrange the topography and hydrology of a dune system.
Such a place is something of a landscape laboratory. Here at Tomales Dunes, you’ll see not only the rapid changes natural to a wind-driven dunescape, but also the impact of a long history of human use, from the subtle traces of Native Americans who left behind arrowheads and middens to the more obvious and recent impacts of camping, sand mining, and an influx of exotic beachgrass and other plants that are starving the dunes by depriving them of the thing they need most: new sand.
And yet, says ecologist and dune expert Peter Baye, the Tomales dune field is one of the few remaining coastal dune habitats in California that retains some natural dynamism. It’s also the one that most closely resembles the long-gone dunes of western San Francisco.
We trudge through loose white sand farther up the northwest slope, across the crest of an avalanching slip face of a mobile dune lobe that encircles a tranquil seasonal pond, and up to the top of a hill the locals call Sugarloaf, the tallest landform at Tomales Dunes. Here a blanket of sand nearly envelops an ancient sea stack—erosion-resistant rock that once sat just offshore but has since risen above sea level due to tectonic uplift.
From the top of Sugarloaf we are afforded a panoramic view of the dune fields in all their complexity and singular aeolian geometry. The dunes are framed by a dramatic backdrop: the constricted mouth of the narrow, fjord-like estuary, a bottleneck through which tides roil in and out across a shallow sandbar. The granitic headlands of Tomales Point, the northernmost extent of the Point Reyes Peninsula, are poised across the mouth of the bay, only a mile away.
We scan the Tomales Dunes to take in the unique geomorphology. With Baye’s guidance, we recognize two dune deposits, or “sheets,” of distinct ages. The younger sheet to the north, extending from Dillon Beach to Brazil Beach, overlays older shallow bay sediments; the top layer is “active,” constantly being reconfigured by wind and water. The older dune sheet, surfacing at Brazil Beach and stretching southeast to Tom’s Point, is a more ancient landform, relatively stable, inactive.
Different plant communities occupy these distinct substrates. The younger dunes host semi-open dune scrub, bare sand, and dense clumps of exotic gray-green beachgrass (Ammophila arenaria). The older sheet is covered by coastal scrub or maritime chaparral, mature eucalyptus groves, and open rangeland. In fact, the older sheet is barely recognizable as a dune field; it looks much like other California coastal prairie. The older, weathered dune sands are darker, yellowish- to blackish-brown, rich in organic nutrients; the younger dune sand is whitish or gray-white, though stained in places by slow weathering of iron-bearing sand grains or accumulations of organic matter. The older sheet consists of stabilized parabolic or “U-dunes.” The younger sheet is a festival of improvised formations.
None of Central California’s habitats is as mutable or dynamic as a pristine coastal dune system, nor has any natural habitat, including tidal marshland, been as altered since European settlement of the Pacific coast. In a natural state, shifting beach sands, in concert with an indomitable wind, replenish coastal dune fields.
But all is not pristine here. Either side of Sugarloaf is bordered by excavation zones where the landowners quarried sand commercially for decades. The quarry pits have abrupt scarps and inelegant angles, none of the grace of the mobile dunes. Old roadbeds lead into the pits, incongruously contouring the upland edge; remnant strips of pavement peek out from overlying drifts. This loss of sand from within has compounded the troubles caused by the planting of European beachgrass in the foredunes in the late 1800s, for the express purpose of arresting the relentless movement of sand and thereby converting a windswept environment into a calmer one more suitable for people.
Ammophila is particularly adept at capturing moving sand particles as they shift along the surface, inland from the upper beach. These dense plantings did their job, building stable berms parallel to the shoreline and perpendicular to the prevailing winds. This rapidly growing grass has created a landform new to California: a continuous sand bunker with a steep-faced windward ridge and a longer, gentler leeward slope. Prior to the introduction of Ammophila, foredunes were low hummocks that rose gradually and were dominated by native plants. The higher topography built by the Ammophila dunes has interrupted the sand squall and starved the dunes. In areas of the Central California coast that were once dynamic dune fields, we now have golf courses, campgrounds, roads, parking lots, homes, grazing land, and Golden Gate Park—as well as the campsites and trailer encampment at Lawson’s Landing.
European beachgrass is so aggressive that it tends to dominate the dunes and exclude other species—biodiversity declines dramatically and an ecological paralysis sets in. Native dune plants like American dunegrass (Leymus mollis), seashore bluegrass (Poa douglasii), and yellow-sand verbena (Abronia latifolia)—and the “dune mat” community they create—are displaced by Ammophila. Endemic insects are important but often overlooked members of native dune communities, especially burrowing beetles and arthropods. Several studies show that insect diversity diminishes as the density of beachgrass increases, but actually increases in dunes stabilized by native plants.
- Exotic beachgrass has blocked the flow of new sand to the opendunes. In the background is Sugarloaf, which is not a dune but rather aformer sea stack now covered in sand. Photo by Gary Brand.
On a summer day, the leeward flats of the Dillon Beach dunes are a veritable bazaar of RVs, campers, trailers, tents, and lawn chairs. These colorful encampments belong largely to refugees from the Sacramento Valley, people who come here to escape the oppressive valley heat and enjoy the temperate marine air. A slew of permanent house trailers, more than 200 of them, are lined up along the shoreline at the south end of the Ammophila dunes. The visitors—many of whom have been coming here for decades—have found a coastal refuge in an environment that was virtually uninhabitable before the dunes were stabilized. They bring with them what most any crowd of a thousand or so vacationing Americans might carry—fishing gear, surfboards, boats and jet skis, scooters, golf carts, pets, and every variety of ball game. They dig clams, surf cast, hang glide, and carouse, like vacationers at beaches and lakeshores all across the continent. In this place, the question is whether the infrastructure and the crowds it attracts are compatible with the restoration of a disappearing landscape.
As we descend Sugarloaf, we approach a deflation plain below, where flats eroded by wind down to the water table form wetland “slacks”—seasonally flooded marshes and flats near sea level. These ephemeral ponds undergo their own unique evolution, migrating downwind ahead of sands that accumulate at their windward edge, a process known as transgression. Baye points out yet another unique landform, a low ridge that outlines the former edge of the downwind migrating slack—what he calls a “gegenwall.” These low dune ridges are formed along the edge of the wetland slacks, trapped in vegetation—salt rush, salt grass, beach-bur, beach strawberry, and silverweed. Sequences of gegenwalls trace the path of the migrating dune head and slack over time, leaving footprints of the transgression.
Compared to other coastal dunescapes, the Tomales Dunes support a diverse complement of dune slacks and swales and associated parabolic dune ridges in various stages of transformation.
Like vernal pools, dune slacks support unique communities of plants and animals, providing foraging opportunities for migrant shorebirds and breeding habitat for amphibians and aquatic insects. Ironically, the now-stable shoreward dunes play a role in protecting these hind-dune wetlands, and this anomalous relationship creates an ecological dilemma: If the beach grass were suddenly eradicated, and the dunes restored to a more natural, mobile state, the wetlands might be covered by migrating sand.
Twenty years ago these ephemeral wetlands were a dependable place to find some rarer shorebird species, especially in fall and winter. It was one of only a few locations in North America where Pacific golden plovers, beautiful shorebirds that breed in westernmost Alaska and northern Siberia, spent the winter, moving between these dune slacks and damp pastures on outer Point Reyes.
The slacks still attract an array of animals unexpected in a dune system, offering foraging opportunities for migrant phalaropes and other visiting wading birds—dowitchers, dunlin, yellowlegs, and “peeps.” Pacific chorus frogs come to lay their eggs, followed by red-sided garter snakes that feed on their tadpoles. Red-legged frogs, a federally threatened species, also use the slacks. Damselflies alight on sprigs of western glasswort (Lileaopsis occidentalis) or thin-ribbed arrowgrass (Triglochin striata) that grow along the gegenwall.
We continue walking westward, toward the encampment of trailers. As anyone who has worked through the maze of Marin County’s building permit process knows, there are strict laws regulating septic treatment and setbacks of structures from sensitive wetland habitats. Somehow, the trailer park at Lawson’s Landing has operated outside these regulations for many years, passively allowed by county and state agencies. This is perplexing since the encampment is perched on the shore of one of the most pristine embayments on the Pacific coast; Tomales Bay is included in the Gulf of the Farallones National Marine Sanctuary and has been designated a wetland of global importance by the international Ramsar Convention on Wetlands.
This contradictory situation has come to the fore in recent years. The owners, under pressure from various regulatory agencies, are attempting to upgrade the septic system as a prerequisite for getting an overdue Coastal Development Permit. The state Coastal Commission is exerting regulatory pressure to bring the site into compliance with the Coastal Act. State and local environmental groups, spearheaded by the Environmental Action Committee of West Marin, are lobbying the Coastal Commission and the county to include dune restoration as a requirement for any land use permit, and the public is gradually becoming aware of the dunes’ unique habitat value.
The dunes are within the area considered potential habitat in the federal recovery plans of three special-status species—the western snowy plover, California red-legged frog, and Myrtle’s silverspot butterfly. As populations of these and other threatened species continue to fall, conservationists and government agencies are focusing on those areas where restoration might forestall or reverse population declines.
Today, most of the state’s 21 remaining coastal dune fields are ecologically compromised. The nearby Bodega dunes to the north were immobilized in the 1950s by plantings of European beachgrass and bush lupine (Lupinus arboreus). The Point Reyes dunes to the southwest cover more area but lack the large, mobile transverse ridges and the extensive dune wetlands of the dunes at Lawsons’ Landing.
The historic San Francisco dune complex was a similar, if more extensive, mosaic of mobile transverse dune ridges, dune wetlands, and ponds, over-riding older raised marine terraces and Franciscan bedrock. But it was long ago obliterated by Golden Gate Park and the row houses of the Sunset and Richmond districts, so the Tomales dunescape is about as close as we can come to understanding this aboriginal landscape.
Is restoration possible here at Tomales Dunes? The experience gained at the Lanphere-Christensen Dunes Preserve on Humboldt Bay gives some hope. Restoration began in 1992 with an intensive effort to remove Ammophila. Apparently, seed banks of native dune vegetation lay dormant in the sand: By 1997 the native dune mat vegetation had reclaimed nearly 50 percent of the 473 acres. In Central California, only Point Reyes and Tomales Dunes retain enough of their natural integrity to make restoration an attainable goal, and the proximity of each to the other would add further value and diversity. Restoration of dunes at Abbott’s Lagoon on Point Reyes, initiated in 2001 with the removal of Ammophila and ice plant, has proved more successful than expected: The dunes have regained their mobility and dune mat vegetation is recolonizing the site and increasing habitat diversity.
But the time for restoration of Tomales Dunes is running out. Like the restoration of salt ponds and tidal marsh, dune restoration requires a well-planned and nuanced effort. If the beachgrass were instantly removed, the sudden influx of sand might obliterate the sensitive dune wetlands. On the other hand, if restoration is not initiated soon, the beachgrass will continue to usurp the open dunes, perpetuating the current cycle of diminishing diversity and ecological paralysis. What’s needed now are an engaged public, a cooperative owner, and regulatory agencies with a commitment to protect the ecological integrity of the coastal zone.
As our group returns to the gate, we follow the paved road that runs parallel to the foredunes and the shoreline. The wind begins to pick up a bit. We pause to look back at the dunes, the multicolored trailer park, and the glimmering bay in the distance. A large raft of grebes and loons rests on the ocean just offshore. It is a peaceful, serene scene, more so because this is a winter weekday with few other people about. It’s hard to imagine Labor Day or Memorial Day weekend, when hundreds of vehicles descend on this singular landscape, creating an instant tent and RV city. What part do the dunes play in that busy scene?
We come to know a place by listening to the voices of the geology, of the indigenous plants and animals, of the wind and weather. If we are inattentive to these narratives, we cannot really know our homeland—its past, present, or future. By watching the improvised dance of the dunes, we can still discover the story of this place—the true nature of the Central California coast—in all its complexity, in all its potential, in all its eloquence.
Most recent in Geology
A visit to Kehoe Beach takes you on a journey to one of the Bay Area’s most dramatic geologic sites, where you can see rocks that have traveled far through time and space to pause temporarily in the Bay Area.
In a newly published paper, scientists link groundwater depletion in the Central Valley to geologic uplift and maybe even earthquakes.