Ye Olde Cobblestones of Pike Place

Tourists and locals alike flock to Seattle's famed Pike Place market to buy everything from tulips to salmon. If you're into coffee the flagship Starbucks is just around the corners (just don't call it the original) or you can take photos with the famous statue of Rachel the Pig or the Public Market signage. There are plenty of sights and sounds to draw your attention in a hundred different directions - this earthcache challenges you to tune those out and look down to your feet. 

As a reminder, this is an Earthcache! If it's your first time completing one, an earthcache is a container-less geocache. Rather than search for a container, you'll need to read through the earth science below, make on site observations, and send your answers to the below questions to the CO in order to log your smiley. 

The Stones Beneath Your Feet

The posted coordinates take you to the convergence of Pike Place and Post Alley. Pike Place is covered by its characteristic red bricks, but Post Alley bosts an older set of cobblestones that date back to the years following Seattle's Great Fire. The streets of Seattle used to be nothing more than mud, dirt and logs - these roads worked relatively well for those transiting north and south along the waters of Elliot Bay. When transiting east towards Capitol Hill area however these roads became nearly impossible to transit during rainy days (imagine trying to hike up Yesler Street from 1st to 6th with nothing bud mud beneath your feet). 

“If you came to Seattle in the 1850s up through the 1880s, even well into the 20th century, a lot of our streets were just dirt and mud,” historian Feliks Banel.

Machinery to lay asphalt weren't around yet, so locals decided to try brick - an example of which you can see to east of GZ on Pike Place. This brick provided much more traction but when they got wet, they still produced a slick surface which made them not ideal for hills. For hilly surface streets a rougher hewn material was reuqired. One which was durable, relatively water resistent, and which would provide significant traction ----- Enter the cobblestone!

Choosing The Right Stone

Early Seattlites knew they had to choose a stone that was durable, non-porous, and that was in relatively close vicinity to the city. Their first choice was a sandstone quarry approximately 45 miles to the south of the spot you're standing on now. In the shadow of Mt. Tahoma (Rainier) the tiny town of Wikeson produced a quarryable stone with unique qualities was quarried to supply the burgeoning city with cobblestones for its roads. 

Wilkeson sandstone has a compact grain configuration which makes it have incredibly low porosity (about 10%) which makes this batch of sandstone exceedingly durable and it required no waterproofing. Sandstone is alos much softer than granite, which meant that in the early 1900s when all of production was being done by hand, it was much faster to mine and shape large quantities of stone.

In later years Seattlites would turn to index granite for their cobblestone needs as it was a harder stone, but in the early years this hardness was detrimental to its use as a paving stone because it was so difficult to quarry and shape with hand tools. Index grainte is significantly darker in color with larger grains than sandstone.

Sandstone Formations

This sedimentary rock, which formed during the early to middle Eocene epoch, provides a window into the environmental conditions and processes that shaped the Pacific Northwest millions of years ago. Wilkeson Sandstone is primary composed of well-cemented, fine- to medium-grained sand grains. It is predominantly an off white to pale yellow color but can also harbor thin drak veins of coal.

The formation of Wilkeson Sandstone can be attributed to a complex interplay of geological processes. Approximately 50 million years ago, the region experienced uplift due to tectonic forces, leading to the formation of the Cascade Range. Subsequent erosion of these moutnains provided a vast amount of sediment that accumulated in river systems and deltas. Over time, the weight of the overlying sediments (and during certain epochs, glacial weight) compressed the sand, and the presence of mineral-rich groundwater facilitated cementation. This reuslted in the solidificiation of the sand grains into durable sandstone.

Questions

In order to log this earthcache as "found" you must make observations at the posted coordiantes and provide your answers to the below questions. The reading above along with your observations will provide you with enough information to answer the questions! You may send the answers to the CO via the message center or email. Logs without answers will be deleted after a warning to the CO. 

1. The Seattle Department of Transportation (SDOT) works closely with neighborhoods to maintain the 50 best preserved of the remaining 93 cobblestone streets in Seattle. It does so by periodically replacing worn cobblestones with those from the other 43 roads and from roads that have been previously torn up. This can result in the intermingling of different types of cobblestones. Do you see indications of multiple types of cobblestones at GZ? How can you tell?
2. Look closely at any one of the lighter colored stones at your feet. Characterize the grain size. Does this align with what you'd expect from Wilkeson Sandstone?
3. Post alley was laid with Cobblestone while the nearby Pike Place was laid with brick. What physical attributes of post alley required cobblestones?
4. Post a Photo with your log - This photo can be of yourself, or some other item that distinguishes you as a geocacher (a travel bug, a sheet of paper with your user name on it, etc.) near GZ. Feel free to use any of the iconic backdrops around you to make your photo pop!

References

• https://mynorthwest.com/1429333/seattle-cobblestone-streets-here-to-stay/

• https://geologywriter.com/streetsmartnaturalist/stories-in-stone-blog/good-old-cobblestones/

• https://www.dnr.wa.gov/Publications/ger_washington_geology_1993_v21_no4.pdf

• https://www.osti.gov/biblio/5024880

• https://www.qahistory.org/articles/cobble-cobble-cobblestones