Urban Stalactites

Last week I wrote about looking down. This week I want to write about looking up, though my first example is of looking slightly down and across.

Riding Light Rail the other day, I noticed a curious geological feature at the Tukwila Station. Hanging down from the platform were stalactites, those classic cave structures. The urban ones in Tukwila were a half inch to several inches long and resembled soda straws, another term for them. They are also known as neoformations and calthemites (for the Latin calx, meaning lime; the Latin théma, meaning deposit; and the Latin -ita, a suffix indicting a rock or mineral) and are wide-spread across urban landscapes. 

The key to their formation is the weathering of concrete. As I noted in a previous newsletter about lime kilns, concrete consists of cement (lime) and an aggregate. When water penetrates the concrete and seeps along fractures, it can pick up and carry calcium hydroxide. If the water reaches a surface in contact with air, the calcium hydroxide mixes with atmospheric carbon dioxide and leads to the formation of urban stalactites made of calcium carbonate, also known as the mineral calcite, and by the way, the second ingredient in Tums, after sugar.

For those who are interested, here’s the equation. From Garry K. Smith, Calcite Straw Stalactites Growing from Concrete Structures, one of the best papers on the subject. One point of caution, the solution that forms is very alkaline (pH13) and will burn your eyes or skin.

Water in concrete, particularly where calthemites form, comes from precipitation, gutters that leak, air conditioners, sewer pipes, and the like, says Garry K. Smith, an Australian caver and expert on calthemites. He found that soda straw growth depends on drip rate with maximum growth (2 mm/day) occurring when there are 11 minutes between drips. That growth rate is up to 360 times faster than occurs in caves. The longest calthemites are up to a meter long but most are less than eight inches. Smith told me that if the rate is too fast, on the order of 1 drip/minute, no stalactite forms though a stalagmite may develop below the drip. 

Image also from Garry’s paper, listed above.

Calthemites are quite varied in texture and color, though most are white to taupe. The different hues result from metals encountered by the water during its travels through the concrete. Copper pipes lead to shades of green and blue and iron pipes impart orangish-reds. Halite (table salt) can also affect calthemites but mostly in the surface topography. If left undisturbed, urban stalactites can last forever, says Smith, though they are hollow and could break due to wind or being bumped by a person or beast. He notes that another problem is that the alkaline, or basic, chemistry of the drip can damage car paint, which would probably trigger removal, or at least staunching water movement.

From: Paul L. Boughton, “Morphogenesis and Microstructure of concrete-derived calthemites,” Environmental Earth Science (2020) 79:245.

In Seattle, calthemites form in a variety of locations, including tunnels, overpasses, the undersides of bridges, parking garages, and basements. (With all of the graffiti covering these types of locations, they’re sort of like our own small scale modern Chauvet Cave.) The key, as noted above, is concrete, as well as water. As we Seattleites try to deal with this summer’s heat wave, what could be better than exploring for urban stalactites in a dark place, underground or under a big concrete structure? Have fun.

Calthemites under I-5 where it crosses over Weedin Place. Longest stalactite looks to be about 6-7 inches. 


I would like to thank Garry K. Smith, who provided helpful information and who has written about calthemites here and here

Please let me know if you know of/find any of these splendid little formations.

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My Favorite Maps

I freely admit that I have probably spent too much time looking down at my feet while exploring Seattle. I have probably spent too much time looking up, too. But whether I gaze skyward or sidewalk-ward, I am often rewarded. For instance, I think I am one of the few people to notice several duck tracks in concrete and to connect them to the duck in terra cotta just a few blocks away. I guess I just have an affinity for making brilliant deducktions. (I know, groan, but I couldn’t pass it up.)

Far better known than the tracks are the hatch covers throughout downtown. (Since personhole cover is a disgusting term and manhole a problematic term, hatch cover or utility cover are the preferred names.) The idea for Seattle’s hatch cover art began in 1975 after Seattle Arts Commissioner Jacquetta Blanchett traveled to Florence and saw the city’s hatch art. Working with Department of Community Development director, Paul Schell, Blanchett donated enough money to fund 13 covers, with six more paid for by other donors. Each cost $200 and weighed 230 pounds. There are now numerous designs adorning our streets.

My favorite hatch cover, of course, is a map, first put in place in April 1977, on the north edge of Occidental Park. It is still there.

Anne Knight designed the map. A map was natural because Seattle had such a “graphically interesting street pattern,” she said. Anne thought that the map covers would make an excellent teaching guide, as well as a guide to the city. If you look at the covers (except one), you will see there is no compass. A City of Light employee had told Anne that one thing he didn’t want was a compass on the map. Otherwise, the crews would have to orient the cover correctly each time. So there is no compass, though there is a map, which one would hope wouldn’t be too challenging for the City of Light crews to orient. Apparently it is. Despite Anne including a small welded bead on the outer ring of each cover to facilitate easy alignment, nearly all of the covers are misaligned at present.

Anne told me that a police officer had heard about the project and called her to say he was concerned that pedestrians would be stopping in the middle of the street to look at the maps. She told him not to worry as she had only chosen spots on corners.

Each of the maps includes city landmarks, such as the post office, the Seattle Public Library, King Street Station, and Denny Park. You can figure out which one is which by looking at the key on the map’s outer edge. All of the landmarks still remain except for the Kingdome. “At the time it had just been built and I thought naively that it looked like it was a structure that would be there forever,” Anne said.

Curiously, the manhole cover on Second Avenue between Spring and Seneca has a unique, post 1975 landmark on it. That landmark is the Seattle Art Museum. In order to fit that landmark on the key, Harborview Hospital was dropped. The map is also the only one not on a corner. (Anne does not know the exact origin of that cover; a special one was made for the Seattle Art Museum but it was removed when the Hammering Man fell and damaged it.)

As you can see from the accompanying annotated map, not all of the original covers exist. I could locate 14, plus the later one added on Second Avenue. One is now in Kobe. (This one had been moved to an alley and a City of Light employee had removed it and happened to have it in his truck when he attended a sister city meeting and suggested sending the cover to Kobe.) Several others have been moved and/or removed with their whereabouts unknown. Another that was on the original list apparently was never in that location and there is one at the Seattle Center at the northwest corner of the fountain lawn at what would be the SE corner of Republican St (August Wilson Way) and Second Ave N.

These manhole maps are one of the delights of Seattle. So next time you are out walking around downtown, take a look down at your feet. You might be amazed at what you find. And, watch out for that terra cotta duck. Who knows where she’ll land next.


I also know of one other map hatchcover. It’s at Waterway 15, at 4th Avenue NE and NE Northlake Way, on Lake Union.

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