Migratory Mortar - A Seattle Earthcache
How often do we examine the world around us - I mean really and truly examine it? This Earthcache asks you to stand on a busy street corner of Seattle which is filled with people, fast moving cars and busses, and towering buildings with signs that demand to be read... and ignore them all. Instead today we're going to focus in on a narrow portion of the building in front of you - not the rocks, but the mortar that holds it together.
This thin mortar layer has traveled a long way and time to wind up on the busy streets of Seattle. Today we'll examine the geological history of this interesting and essential building material!
As always, please note that this is an Earthcache so there is no container or log to find here. Rather to claim this Earthcache you must read the description, make observations at GZ and send your answers to the CO.
Working Backwards in Time
Most recently our mortar originated from the San Juan Islands: from your current location the Lime Kilns of San Juan Islands are approximately 73 miles to the Northwest. From the 1860s to the 1920s, San Juan Island was a principal lime supplier to the cement industry in Washington. Production began in 1859 with the British using beds of limestone on the west side of the island and continued through various owners into the middle 20th century. It was quite exciting to find a rich deposit of limestone on the San Juans, as the original geoglogic report noted:
The value of these discoveries can better be appreciated from the fact that up to the time of the discovery of limestone on this island it was not known to exist at any point on Puget Sound, within United States territory, and for building purposes it was necessary to procure all the lime used, from California or Vancouver's Island
The process is straightforward. Take limestone, which by definition is a rock composed of calcium carbonate (CaCO₃), or calcite, and burn it at high temperature, which drives off the CO₂ and leaves behind CaO, also known as quicklime.
When quicklime is mixed with water, a process known as slaking, the reaction produces lime putty, calcium hydroxide (Ca(OH)₂). Lime putty can be used by itself or mixed with an aggregate to make mortar and cement. Because lime putty gets better with age, the Romans, who were premiers users of cement, waited at least three years to use theirs.
Mortar in the Making!
On the island, the kilns were huge structures made of brick and rock. Using wood, which led to deforesting San Juan and other islands, the workers heated up to 15 tons of limestone at a time to 1800°F. Within 24 hours, the limestone had become lime. Workers then collected it in barrels weighing 200 to 250 pounds and transported it to warehouses where it remained until shipping. The cement boom in Seattle began after its Great Fire of 1889, when the city passed ordinances requiring stone and brick as building materials. With these ordinances, lime from the San Juans became more common in Seattle and other cities around Puget Sound for use as mortar and/or cement, as well as subsequent use of concrete for buildings and structures.
Tracing the Mortar Back Even Farther...
Part of what makes the local lime interesting, is that the limestone on the San Juans did not form there. Fossil fusulinids (a type of Foraminefera) from the Lime Kiln quarries indicate that the limestone formed in the western Pacific Ocean (near Asia) around 248 million years ago.
Because basalt surrounds the Lime Kiln limestone, which occurs in discrete lenses, geologists propose that the original depositional
environment was a seamount atoll ringed by a reef. The basalt erupted into the water, cooled into pillow-like shapes, then tumbled into deeper water, where it was periodically covered in unconsolidated, calcite-rich, lightly fossiliferous mud, which eventually became the San Juan limestone. Millions of years later—sometime between 84 and 100 million years ago—plate tectonics carried the basalt and limestone (as part of what is known as the Farallon Plate) east and accreted, or attached, it to North America.
From Asia across the Pacific Ocean to North America, where it minded its own business for tens of millions of years, then from San Juan Island down the Puget Sound just to end up on this busy street corner. So next time you’re wandering downtown take a minute to appreciate the concrete and mortar in the older buildings and the lives entombed within. Their stories began a long, long time ago in an ocean far, far away!
Earthcache Questions
In order to log this cache as found, you must make your observations and send your answers to the CO. If you have any questions please feel free to reach out!
Please use the above figure to help orient yourself at GZ and to answer the questions below.
- Examine the photo above. The building's mortar has been repaired over the years. The area in red is the original mortar from the San Juan Lime Kilns. Compare this mortar with the newer one in blue.
- Which of the two mortars used an aggregate material?
- What benefits do you think using an aggregate material as a binder may provide to the mortar?
- Would you expect to be able to see any fossils in the mortar if given a large enough sample? Why or why not?
- Post a photo at GZ and include it with your log or your message to the CO!
References:
- https://www.historylink.org/File/10935
- https://streetsmartnaturalist.substack.com/p/well-traveled-cement
- Urban Geology by David B. Williams