Location information and photos by Bud Hardwick. Geology by Dave Tucker February 21, 2012.
A large granodiorite erratic lies on the campus of Highline Community College. It measures 21 ft x 12 ft x 9 ft high. Mineral grains are large and pronounced though surfaces are weathered and crusted with microlichens. The boulder’s surface is smoothed and planed by glacial erosion- not by the ice itself, but by the slurry of sand and finer grit contained in the ice that delivered the rock.
(Eric Baer, geology professor at Highline, adds this: This erratic was supposed to be blasted for the construction of the Higher Ed Building at Highline, but the facilities folks realized its value and had it moved about 20 feet and incorporated into the front steps. There are several other small erratics nearby that were from excavating the basement of the building.)
Thin white ( or ‘leucocratic’) dikes cut the rock from edge to edge. Unlike the granitic rock of the boulder, these are devoid of dark mafic minerals such as biotite or hornblende. I don’t know what the precise mineralogy of these dikes is, but if the percentage of orthoclase feldspar is much greater than plagioclase, the dike rock would be called ‘alaskite’, or alkali-feldspar granite. These dikes are nominally younger than the salt-and-pepper host rock. Notice that the dikes are broken into en echelon segments- there are several long segments, and their ends overlap. These closely resemble a common pattern taken by fractures forming as a rock (cold or hot) is stressed by compression (squeezed) in one direction- the fractures form along the direction of the squeeze. If this compression occurs before the magma is completely solidified, the remaining low viscosity magma can fill the fractures. Note that there is a fracture (unfilled? hard to say because of the moss) immediately adjacent, and parallel to, the skinny dike. Just to be honest, Tucker has never been to this place, so if anyone sees errors or has more knowledge to impart, please do so via this email:
Getting there: The Community College is located
just west of I-5 in Des Moines (that’s ‘Dez Moynz’ for folks foolishly tempted to employ a French accent in this neck o’ the woods). Link to Google Maps. The erratic is located at the north edge of the large parking lot entered off South 240th between 24th and 25th Streets. No parking permits are necessary if visiting on Saturday or Sunday. Otherwise contact College Campus Safety office located on the lower level of Building 6 or call (206) 878-3710, ext. 3218. Refer to the college website for details and a detailed map of campus (NOTE- the campus map linked from the campus website has NORTH at the RIGHT, though no arrow indicates this http://maps.highline.edu/directions.php. The erratic is in front of the ‘Higher Education Building’. N 47 23.240 W122 18.101