Beautiful rock features: Honeycomb weathering along the Chuckanut Coast
Posted January 11, 2010, by Dave Tucker.
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
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.
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