Ancient Walls II EarthCache
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This cache will take you on a walking tour of downtown Portland illustrating many different kinds of stone from a variety of locations and geologic periods. A hand lens or magnifying glass will let you take in even more of the beauty of these stones.
Information is courtesy of the Geological Society of the Oregon Country (see above related web page).
The Geological Society of the Oregon Country (GSOC) is a non-profit organization based in Portland, Oregon. The society is dedicated to the study of geology in the Pacific northwest and is open to persons with all levels of education and professional backgrounds.
Our walking tour begins where Ancient Walls I left off in front of the Oregon Basalt and Tenino, Washington sandstone making up the walls of the First Congregational Church. If you are doing this one on a different day, go ahead and start at waypoint S1.
1. Pacwest Center (1984)
The Pacwest Center’s lower walls and curved railing are “Blue Pearl” larvikite from Finland. This stone is roughly the intrusive equivalent of the (extrusive) basalt seen earlier in the Congregational Church where we started this portion of the tour. The difference is that you can see the crystals in the larvikite whereas you cannot see them in the basalt without magnification. Stone formed in an intrusive process cools underground over a very long period of time while extrusive stone is expelled from a volcano and cools more quickly above ground.
2. Oregonian Building (1948) The rock face of the Oregonian Building is 1.8 billion years old and is a gneiss transformed over a much longer period of time resulting in the swirls you see and the greater separation of the mineral bands.. The main bands you see are mica and feldspar. Notice how wide they are, I'll ask you to compare them later.
3. Unitas Plaza (1965) The low courtyard walls and stairs at Unitas Plaza are white travertine, a type of sedimentary rock encountered so far. Travertine is a type of inorganic limestone resulting from the precipitation of calcium carbonate in hot springs and caves. Thermophilic (heat-loving) bacteria may also play a role in its structure.
4. Wells Fargo Center (1972) Portland’s tallest building (546 feet) is clad in white Carrara marble. This very pure marble from Carrara, Italy, is the same type of marble used by Michelangelo. The lower walls are rough granite.
5. KOIN Center (1984) The limestone walls around the KOIN center contain chert, shale and many fossils, including Echinoid sand dollars and tiny starfish fragments.
This biogenic limestone — in comparison to the inorganic travertine above — is composed of tiny marine organisms, such as foraminifera (a marine plankton). The base of courtyard fountain is rare orbicular granite made up of concentrically layered spheroids. Though the exact process is unknown, the spheroids may be formed around a nucleus in a cooling magma chamber. (A more accessible outcrop of orbicular granite can be viewed at the Bank of American Financial Center, SW 2nd and Alder, while you’re waiting for the #15 bus.)
6. Hatfield Federal Courthouse (1997) The walls and floor tiles of the outside of this federal courthouse are a swirled grey-and-white gabbro under blocks of light brown oolitic limestone.
The tiny egg shaped ooids are made up of concentric rings of calcium carbonate which formed around sand grains or other small fragments as they were rolled by wave action on a shallow tropical sea floor.
7. Morton Gneiss at 731 SW Morrison (across from Nordstrom) Morton gneiss, from Minnesota, is the oldest rock on our tour and a distinctive building stone seen throughout the United States. It is a gneiss which has been dated at 3.6 billion years old or one quarter the age of the known universe,
Question at Waypoint 1 : What is the color of the shiny crystal cleavages?
Question at Waypoint 2 : Which minerals (colors) do you see separated into bands?
Question at Waypoint 3 : Of the two distinguishing features, the holes and the lines, which do you think is more likely to have been caused by the bacteria and why?
Question at Waypoint 4 : In the rough granite blocks on the lower wall, what is the largest crystal you can find?
Question at Waypoint 5 : What kind of (tiny) fossils can you see here?
Question at Waypoint 6 : What is the size of the ooids (don't measure just compare to something you know)?
Question at Waypoint 7 : How broad are the color bands? Are they broader or narrower than the bands in the gneiss at Waypoint 2, a younger gneiss?
Additional Hints
(No hints available.)