Glacial Boulder--Des Moines Beach, Puget Sound area, Washington

This video features the Des Moines Beach Erratic, in the South Seattle metro area, Washington (using 360 degree photography). Glacial erratics are rocks of any size (although usually applied to larger size boulders). These rocks have been transported by a glacial ice sheet or in a glacial iceberg--to a locality far from the source area of the rock. This means that the rock type associated with the glacial erratic does not match the rocks common to the area to where it was deposited. Although an erratic could be any rock type, most erratics are highly competent rock (such as an igneous or metamorphic rock), that can resist intense abrasion and weathering. Because of the weathering on this granitic rock, it is difficult to identify the exact rock type (without breaking it--which I prefer not to do). Note that there are no granites or diorite rocks in the Des Moines area. So this boulder probably came from either British Columbia, Canada, or the northern Cascade Range Mountains of Washington State.
The large size and weight of the boulder infers to the immense energy associated with glaciers to easily move boulders of this size (and much larger). Boulders moved by glaciers can be house-size, so this boulder is relatively small in that context. Incidentally, the adjacent rocks--around this boulder are a mixture of two rock types. Most rock on the beach consists of sand and rounded gravel--as seen in the video. Many of these rounded beach rocks may be smaller equivalents to the Des Moines Beach Erratic. However, there are also another rock type too. These are mostly laying on top of the beach sands and gravels. The other rocks are larger in size, angular-shape rocks, that tend to be a brownish- to- reddish-brown color. These basalt rocks were transported by truck, and placed along the nearby shoreline with the other larger boulder size rockery rocks, or armor rock. These rocks were meant to limit shoreline erosion in this area, but many of the rocks have fallen from the the embankment and moved the short distance to this locality. The imported basalt rock is from the nearby Tukwila area.
Identification information for this rock:
Again, an erratic can be a rock of any composition, but was probably moved a significant distance by glacial ice. This boulder does not match the composition of the rocks in the locality of deposition. Additionally, glacial erratics usually are competent rocks that resist weathering and wear. Glacial erratics are typically somewhat rounded, although they can break into pieces during glacial transport too. Erratics also may be small as a pebbles or gravels, but house- and building-size erratic boulders are possible too.
Because of amount of weathering and mass on the surface of this boulder, it is difficult to identify the exact rock type without breaking off part of the boulder (since this is a special rock from glacial times, I would rather not do that). However, by its general black and white color, it is either a granite or a diorite.
Note the white barnacle shells on the lower areas of the Des Moines Beach Erratic. Barnacles live in sea water, but can survive for short times when tide water is temporarily low. In general, the barnacles mark the high tide line in the locality. Although extreme high tides may be higher, extreme high tides do not occur consistently enough to allow barnacles to live higher on the boulder--especially if the upper areas of the boulder are exposed to more evaporation from direct sunlight.
References:
1) geologic map and descriptions: pubs.usgs.gov/sim/2004/2855/d... , 2) erratic description: nwgeology.wordpress.com/the-f....
3) other nearby erratics: kids.kiddle.co/Glacial_errati...
Hello Rock People! This is Dave the Geology Dude. The focus of GeologyDude is to compliment other online geology education channels. GeologyDude is not a clone of other CZcams channels. The focus is two things: 1) to teach about geology and rocks--preferably in a non-traditional way (mostly oriented to students and hobbyists), and 2) to influence about how to use 360-degree videos for quality science education. Note that there are other videos with more rock identification information on this GeologyDude CZcams channel. Don't forget that: Geology isn't Rocket Science--it's Rock Science! Please "like" and subscribe to this CZcams channel!
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Hello Rock People! This is Dave the Geology Dude! There is more to this video than just a spinning boulder. There are detailed notes in the description area of this video. If you want more, longer length rock identification videos are on my CZcams homepage. Check them out, or leave a comment here.