Spalling Erosion
The Holyoke Building that towers over you on the south side of Spring St, was under construction in 1889 when the great Seattle fire burnt through the young city. Thankfully, as it was constructed out of stone rather than wood, it survived the fire and was one of the first buildings completed in the months following the conflagration. Like most of the buildings built in the aftermath of that fire, it's constructed with one of three local sandstones - in this case the brownish rocks of Chuckanut.
As you arrive at the posted coordinates, you'll want to face south towards the windows on the Holyoke building. Here we will examine an interesting erosion feature: spalling. While spalling can happen in many different types of sedimentary stone it is exceedingly common in sandstone due to the nature in which it was deposited. In order to log this earthcache you'll need to read this description, make some observations, and then message or email me the answers to the questions at the bottom of this listing!
Chuckanut Sandstone
The local sandstone from Chuckanut began its life relatively recently in geologic terms - approximately 50 million years ago. At this time, Western Washington lacked the dramatic topography of mountains that now dominate the region. Neither the Cascades nor the Olympic Mountains existed, and what we now call the I5 corridor was right on the ocean. Rivers flowed from the mountains along the Washington-Idaho border across vast plains. As the rivers ran to the ocean they deposited beds of sand layer by layer which, over time became sandstone.
Above: The Chuckanut Formation (Annotated by "CK" on the map)
Original estimates put the depth of sandstone at up to 6,000 meters (20K feet) making it one of the largest sandstone deposits in North America to come from a non-marine source. The vast amount of sandstone coupled with its relative closeness to Seattle made the Chuckanut Sandstone a natural choice for building material at the turn of the century. Sandstone is easily cut and transported and can withstand erosion quite well when quarried correctly - for evidence of this you only have to look at the detailed reliefs and carved capitals of the Holyoke building. However, there is one major drawback to sandstone - spalling.
Spalling
Spalling is a term for a specific type of erosion wherein water seeps between layers of stone depositing salt crystals which, over time, slowly grow and wedge the layers apart. In addition to spreading caused by salt intrusion, during Seattle's winters when freezing weather happens, water expands by up to 9% as it crystalizes into ice, further expanding and degrading the stone.
While water is the primary cause for spalling to occur, building technique can increase the likelihood of it happening as well. As a general rule of thumb, spalling typically occurs first on ledges where the stone is less protected and more exposed. Additionally, if a builder stacks sandstone blocks with their beds vertical and parallel to the building's surface (like an upright book stacked with its cover face out on a book shelf) then these erosive agents can weaken the stone so that the beds peel off one layer at a time. If however the beds are laid horizontally (like a book flat on a shelf) water has a harder time penetrating the layers.
Below: An example of spalling occuring after a fire in the wilderness. While spalling in an urban environment is typically caused when water acts as an erosive agent, in the wilderness fire and extreme heat can cause the same effect.
Logging Requirements
To log this earthcache you must answer the following questions and send your answers to the CO. Please do not include your responses in your log. Once you've sent off your answers you are free to log this earthcache!
- Examine the wall of the Holyoke Building. Do you see any spalling? Which part of the wall has it?
- Why do you think this part of the stone is eroding this way?
- How thick are the layers that are separating?
- Post a photo of yourself or a personal geocaching item at GZ.
References:
- Seattle Walks: Discovering History and Nature in the City by David B Williams.
- Vine, James D. (1969), Geology and Coal Resources of the Cumberland, Hobart, and Maple Valley Quadrangles, King County, Washington, vol. Professional Paper 624, U. S. Geological Survey