Honeycomb weathering in sandstone of the Chuckanut Formation

Beautiful rock features: Honeycomb weathering along the Chuckanut Coast

Posted January 11, 2010, by Dave Tucker.

Golden sunlight and honeycomb weathering at Larrabee State Park, Washington. The photo is about 1 meter across. Click to enlarge all photos.

Honeycomb weathering is found in many rock types, including granite and rhyolite, as well as sandstone. It occurs in humid and desert environments, cold and hot. Honeycomb weathering produces intricate and highly photogenic structures in seaside exposures of Chuckanut Formation sandstone. George Mustoe, at Western Washington University’s geology department, did his masters thesis research on the development of honeycomb weathering, which he published in 1982. This was the first serious attempt to explain how honeycomb weathering forms. George used chemical analyses to find that in the Chuckanut, and probably by extension in similar coastal locales, honeycomb results from disaggregation of mineral grains by evaporation of salt water splashing on the rock; algae living within the rock surface protect the walls separating the cavities (septa) from further evaporative erosion. You can read all about George’s work in his GSA Bulletin paper, available in full online here.

The honeycomb features that result from this weathering style are also known as ‘tafoni’, though I didn’t find that word in my copy of Dictionary of Geological Terms (American Geological Institute, 1984 Doubleday). It is in my copy of A Dictionary of Earth Sciences (1999 Oxford Press). For those interested in matters etymological,  the origin of  ‘tafoni’ (singular is ‘tafone’) is perhaps from the Greek word taphos, ‘tomb’, but I’ll put my money on the Corsican word taffoni, meaning ‘windows’, or tafonare meaning ‘to perforate’. Tafoni also means ‘windows’ in Sicilian. Tafoni were originally described by De Prado (1864) from central Spain, where they occur in granitic rocks, and later by Reuch (1883) in Corsica. See Twidale and Vidal Romani, 2005, for the complete references.

A web search for ‘honeycomb weathering’ and ‘tafoni’ will turn up numerous articles and examples, and quite a bit of wondering about origins. There is a web site all about honeycomb weathering. A wonderful worldwide gallery of  photos, including some by George Mustoe and also WWU geologist Scott Babcock, is available there.  There is also a time sequence of photos which show how fast these structures develop.

You can easily see examples of honeycombs anywhere along the Chuckanut shore, including

These tafoni penetrate through bedding planes. Dark algae coat the rock in places and inhibit further salt-water erosion of the Chuckanut Formation arkosic sandstone. Larrabee State Park.

seaside rocks near Bellingham’s Boulevard Park and the South Bay Trail pedestrian causeway, the slabs at Post Point (along the railroad tracks south of Marine Park), and Larrabee State Park. The shapes formed by this weathering style lend themselves to photography, especially in angled lighting. Note that the honeycomb pockets tend to align along bedding planes in the crossbedded sandstone of the Chuckanut. Climbers (including yours truly, in my glory days) have used these pockets as handholds. However, the thin septa between the honeycombs are fragile and this use should be discouraged.

Really large honeycombs are visible along the Rock Trail in the Chuckanut. These are big enough to crawl inside. There are some other really big ones along North Shore Drive (Lake Whatcom) west of Agate Bay Lane.

Getting to the Governor Lister Cliffs: Park at the wide spot on the sea side of the road, cross, and find the steep, rough trail behind the big boulder with the bronze monument. The route gains about 400 feet in less than 1/2 mile, with some handlines, mossy slabs and slick logs across a creek near the bottom. The climbing cliff is unmistakable. Scramble along the base toward the south. With luck, you’ll crane your neck to see the honeycombs shown in the photo.

References:

Mustoe, G. E., 1982, The Origin of Honeycomb Weathering, Geology Society of America Bulletin, v. 93, p. 108-115. This paper is available in full online.

Mustoe, G. E., 2010, Biogenic origin of coastal honeycomb weathering, Earth Surface Processes and  Landforms. DOI: 10.1002/esp.1931

Twidale, C.R., and Vidal Romani, J.R., 2005, Landforms and Geology of Granite Terrains; A.A. Balkema Publishers, Leiden, The Netherlands

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4 Responses

  1. I googled Larrabee sandstone and got here – thanks for your very interesting post – understandable for the curious non-scientist like myself, but with good information, too.

  2. I became interested in this topic while visiting Sand Canyon near Mesa Verde. At the lower entrance of the hiking area there are highly honeycombed rocks with small scale hollows.. Then on the sandstone wall of the canyon are hollows large enough for a person to stand inside where the Indians stored their grain.inside smaller stone-walled cribs. Anyway, having my interest stirred, I have done a fair bit of reading about it. I later saw more honeycombed rock in “Negro Bill Canyon which had larger scale hollows with smaller scale ones forming inside it including on the ceiling of the larger hollow. In my mind this odd arrangement may provide some argument away from micro-climate arguments. Don’t know if you are interested but I can provide a photo which i could upload to my company website.

    • George Mustoe responds:
      These large cavities (tafoni) occur over a wide range of latitudes, from Antarctica to equatorial regions, from elevations that range from coastal to mountainous, and in climates that range from arid to humid. Tafoni can be found in many different rock types. For example, they are common in sandstone, but also common in crystalline igneous rocks (granite, dolerite, etc.). These variations suggest that there is not a single mechanism to explain tafoni genesis. However, once cavities have begun to develop, their further development probably involves microclimate effects. Cavity interiors are likely to receive less exposure to sunlight which influences temperature. They may be subject to less air movement (being wind-protected), and combined with the cooler temperature, the interior humidity may by greater than at the surface. Conversely, cavities are protected from direct precipitation, and that may cause interior surfaces to be drier than exterior surfaces during storm events, and perhaps for an entire season. In permeable rocks, moisture can be delivered via pore fluids, or water migrating along fractures. I published a paper back in 1983 describing tafoni at Capitol Reef, Utah, a desert environment (Reference below- DT). There, tafoni commonly occur in canyon walls in zones where moisture is migrating through the sandstone as a mini-aquifer. At other sites, cavities develop at the lower levels of large talus blocks, where moisture arrives as dew. Some of the best evidence for microclimate effects comes from studies of tafoni in snow-free regions of Antarctica, where researchers have placed small data loggers within cavities to record temperature and humidty.
      There is a fair amount of literature on tafoni, and although various theories have been advanced, there is no explanation that can account for all the various occurrences. The morphologies of the cavities are quite variable. At some sites, cavities show a definite orientation (e.g., cavities occur on north-facing outcrops in preference to other orientations, etc.). At some sites, the back walls of large cavities contain patches of honeycomb weathering. I’ve observed this in sandstone outcrops at coastal sites in the Pacific Northwest, and and I’ve seen photos of similar honeycomb-within-tafoni in crystalline igneous rocks in the Adirondack Mountains. These complex geomorphic features are evidence that cavity development is not always a simple process.
      As a final general perspective, ” tafoni” is a name for a feature, not a description of a process. On a much larger scale, a “mountain” is a feature that can be created many different ways: as a volcano, a result of tectonic uplift, or by extensional basin and range faulting, to name only a few processes. The Matterhorn doesn’t look like Mount Baker, which in turn doesn’t look like Mount Whitney. Likewise, tafoni don’t all look alike.

      Mustoe, G. E, 1983, Cavernous weathering in the Capitol Reef Desert, Utah: Earth Surfaces Processes and Landforms, v. 8, p. 517-526.

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