Littoral Sinkholes
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Littoral Pseudokarst Vertical Sea Caving Dave Bunnell1 Our March, 2007 issue featured a world, I’ve encountered some very impres- caver rapelling over a sea cave arch. While sive littoral sinkholes, some of them akin to many sea caves are probably much more the open-air pits of TAG, albeit not nearly so easily accessed by rapelling down a cliff deep. Indeed, the very first cave we surveyed to reach them, in only a few instances in California (Caverns of Mystery in Shell can one actually rappel IN to them. Beach, California) had such an entrance, Vertical entrances to see caves are a rare with a 45-foot sheer drop to the floor phenomenon. But with over 20+ years possible. This is my first attempt, perhaps of surveying hundreds of sea caves, I’ve anybody’s, to compile a list of known sea encountered and surveyed some fairly caves with these features. For each entry spectacular ones. in the table on page 12 I’ve determined diameter of the collapse, the rough area of Littoral pseudokarst, which is ultimately the opening, the depth of the drop, and the a product of marine erosion, is most typi- cave’s overall length. cally characterized by the littoral sinkhole. This is simply an opening to the surface MENDOCINO COUNTY, CALIFORNIA formed by collapse or gradual erosion into Bill Halliday, in his description of the a sea cave chamber or passage beneath. caves of Mendocino County in Caves of Descending into a sea cave pit at the rear of Unlike its kin in soluble rocks, the littoral California, described the county as being a cove in Shell Beach, CA sink almost always forms by erosion and “the most extensive locality of littoral pseu- collapse rather than dissolution. I’ve never dokarst on the Pacific coast.” As near as I’ve surveyed littoral sink we have (see page 15) noted shallow, doline-like features above sea determined, he is quite right in that assess- since a surface survey circumnavigates it. It is caves, though I’ve not seen many sea caves ment, though Santa Rosa Island, as noted 10 x 10 feet and 70 feet deep. This one in soluble rocks. In most cases the author has below, comes close. has a lot of slope to it, as much of it is walled observed, the cave chamber in question has By my reckoning there are six, possibly only in soil. Indeed, only the side leading into a seaward entrance and the collapse tends to seven major littoral sinks associated with the cave shows much bedrock, and it extends occur at the most distal region of the cave. caves in Mendocino. Two are quite acces- only about halfway up the side. This is a Sinkholes have formed in both sedimentary sible. There is a hundred-foot-diameter, very popular attraction with the locals, who and volcanic rocks. circular collapse pit, 70-feet deep, into a sea sometimes use a knotted clothesline to reach Two other names commonly applied cave at Russian Gulch State Park. We haven’t the bottom. A group of them who arrived to littoral sinkholes are “punchbowls” and sought permission to rappel into this for a during our survey was certainly amused by “blowholes.” The latter term more properly survey, especially since the sea tends to be our overly-cautious descent with helmets and applies to caves that eject spray with incom- so rough here. But the cave passage lead- full vertical gear on proper rope, but disap- ing waves, a phenomenon dependent on ing into it covers 250 feet, based on surface pointed that we had no beer to share… both tide level and swell conditions. It may survey by Bruce Rogers (5), who noted that The view from the back is very impres- occur through either a passage constric- the predominent rock type in this area is a sive, with two large tunnels inersecting to tion in a single-entrance cave (such as the “Cretaceous-age Franciscan terrane gray- form the main cave. A sunbeam enters from famous “La Bufadora” near Ensenada in wacke and the cave is formed along a series the righthand tunnel and lights up the interior Baja, Mexico) or through small openings in of prominent near vertical joints.” From the of the cave nicely. Entering a sea cave from sea cave roofs. The latter process will obvi- photo on page 16 it is clear that the bedrock its back end isn’t something you can do in ously tend to enlarge such an opening over does not extend all the way to the surface, many places, and in fact, the view afforded time. I recently described such a feature in which probably helps account for its depth. by this sink access would be unlikely on any Samoa, where a woman had fallen into a Also seen by many tourists, the punchbowl but a few especially calm days at this spot. sink opening into the middle of a cave () . In at Mendocino Headlands State Park is an It’s one of the more scenic sea caves I’ve this cave the sink filled with water but didn’t impressive feature with a 40+ foot freefall visited and one of the best “sport vertical” eject, geyser-like, into the sky (at least, under drop to the water below. Three of us donned sea cave trips around. normal sea conditions). Most news accounts wetsuits one Fall day, climbed down the cliffs There are three more sites in Mendocino referred to it as a “blowhole,” however, no on one end of the headlands, and surveyed where I’ve observed large littoral sinks, but doubt to emphasize the severe conditions the cave to a length of 233 feet. This one is access for survey may be problematic. One the woman had to contend with. Not having somewhat unusual in that it is in the middle of these is visible on the tip of Point Arena seen it, I also can’t say whether this was a of a passage that opens at both ends. We had from the lighthouse. We haven’t approached skylight collapse into a lava tube or a collapse fun during the survey while waving at tourists the coast guard about letting us rappel into into a wave-cut cave, which is my focus here. looking down at us from the hole above. that one, which is a cave perhaps more pit But there are apparently a number of such Another monster sink, similar in size and than cave. I also located what may be a features on Samoa. depth to the one at Russian Gulch, is found possible double-sink cave just south of here, In the course of surveying sea caves in on the south side of Little River. A group of on Google Earth, while preparing this article! California, Hawaii, and other parts of the us from the Mother Lode Grotto along with One of these may be an intermediate stage local caver Derek Hoyle surveyed this over between partial and full collapse, something 1 Photos were taken by the author unless the course of three trips (it was too rough to I haven’t seen elsewhere. The third site is a otherwise credited. finish the first visit). This is perhaps the best sink on the peninsula at Fort Bragg, but on NSS NEWS, October 008 11 private property. We hope to survey some we had rappelled into another cave indented Gone. The seacliffs here are only some 50 of these in the future. into a cove. As a testament to erosion rates feet high, so the cave had developed ceil- There are numerous circular coves along here, much of this cave and the cove itself ings high enough to intersect the surface. the Mendocino coast which may well have seem to have vanished in storms in the Unfortunately, in the days of Coast Guard resulted from the collapse of other littoral intervening years. At least, they don’t show operation this sink was seen as a convenient sinks over time. on either Google Earth or the California trash dump. Aside from two on Kauai, Coastal Project’s detailed aerial images of the discussed below, this is the only littoral sink ELSEWHERE IN CALIFORNIA coast. These caves are in colorful Pleistocene seen in igneous rocks. Caves with littoral sinks are compara- sandstones. Santa Rosa Island, which has caves tively rare in the rest of California. One such A cave in Santa Cruz near Davenport in a variety of sedimentary marine rocks, cave is found in each of 3 counties, San (Basketball, see map in ref. #)) has what hosts four caves with littoral sinks, three of Diego, San Luis Obispo, and Santa Cruz. might be early-stage littoral sinkholes with them quite sizable. These three are on the San Diego’s Sunset Cliffs region is pock- two separate but small vertical entrances north shore, the two biggest on the most marked with a dense series of caves. On the in addition to its seaward entrance. In this wave-battered portions of the island. One bluffs above is a strikingly circular collapse pit case it is probably more a case of the hillside is on the south shore, and subject to the surrounded by a fence, just a stone’s throw above the cave having eroded into the cave, southeast swell that sometimes develops. from Sunset Cliffs Boulevard. It drops about rather than a collapse driven by removal of We’ve managed to survey all of these caves: 0 feet into a short sea cave with a single material below.