Des Moines Beach erratic

Article sent in by long-time reader Bud Hardwick with additional material by Dave Tucker. Photos submitted by Bud Hardwick and Sandy Bowman.

Des Moines Beach Park form the Marina. Bud Hardwick photo. Click to enlarge any image.

An 8 ft x 6 ft x 4.5 ft high granitic erratic squats on the beach at Des Moines Beach Park. (Don’t mistake this for the French term that is coincidentally spelled the same way- this place is pronounced “Dezmoynz”). This isn’t a significantly large rock, but it is in a nice location. It clearly shows in Google Earth. The measurements obtained with tape measures are an interesting check on the ability of Google Earth’s  measuring tool (the little ruler button at the top of the GE view). Dimensions using that are close: 8 ft x 4; however, depending on the photo you use in GE, parallax distortion is significant: in one view, length is only 6 feet. Height can not be determined within accuracy of GE.

The Des Moines Beach Erratic is located only a short distance north of the Des Moines Beach Park located at 22030 Cliff Avenue South in Des Moines, WA, on the north side of the Des Moines Marina where Des Moines Creek enters the Sound.

Getting there:

I-5 is on the far left (east). The boulder is in the NW corner. Note that there is an even larger erratic at Highline Community College (bottom of image). Someone should investigate that for us.

The most direct route from I-5 is via Exit 149B (from the south) or Exit 148 (from the North).

  • Go west on WA 516 (Kent-Des Moines Road)
  • 1.2 miles- north on 509  (Kent-Des Moines Road)
  • 2.0 miles- merge with Marine View Drive South in Des Moines
  • 2.3 miles- turn west (toward the water) onto South 223rd Street. Continue on this street as it curves north parallel to the water and becomes Cliff Avenue South.  There are three parking options for the short stroll up the beach to the erratic:  1) at the Marina, 2) at the foot bridge in the park (2.6 miles from I-5, 0.3 miles from Marine View Drive), or 3) continue along Cliff Avenue South as it becomes Jordan Avenue, which loops around and reaches the beach north of the foot bridge via Nebo Blvd.

The field trip:

The granitic erratic at Des Moines Beach PArk. Birds like it. Bud Hardwick photo.

Walk to the beach on the north side of the creek.  There’s a rough gravel boat launch on the north side of the bridge over the creek and the boulder is less than 50 yards up the beach to the north, standing out only a short distance from the high tide shoreline. The GPS coordinates are 47°24.250′N 122°19.848′W but you won’t need that to find this obvious boulder.

The granitic erratic is of a distinctly different makeup than the numerous boulders accumulating beneath the eroding sand and gravel cliff above this section of the beach. (DT says: Biotite mica is the obvious large dark mineral, but some of the smaller black grains look like hornblende. The white minerals look like plagioclase feldspar. There are a few smoky quartz veins in some of the photos I saw).

Consuelo Larrabee at the Des Moines erratic. Photo by Sandy Bowman.

This area is interesting to visit for a number of other reasons. Notice the erratics artfully located at the south entrance to the pedestrian bridge that crosses the mouth of Des Moines Creek (one has a bronze dedication plaque mounted on it).  Further along the road entering the Heritage Park (just past the yellow “Slide Area” caution sign) the bluff is eroding to the point of endangering a multi-story housing structure built along its lip.  Attempts are being made to stabilize this slide area using various techniques.  Locations like these occasionally make headlines as buildings dramatically topple down these unstable slopes.  The valley surrounding Des Moines Creek is typically steep-sided but a substantial alluvial flat has formed in the valley floor.  This area has long been used by Native peoples and more recently was the site of early commercial logging, a wood mill, a private park, a dance pavilion, a church retreat, and now a public beach and Heritage Park as well as the terminus of the Des Moines Creek Trail.  Prompted by age and recent flood damage, many of the old buildings are being restored and are unusual in that the open creek is channeled beneath some of them.  The adjacent Marina offers a long fishing (and viewing) pier as well as unusual “totem poles” at the entrance.  The freshwater creek and saltwater of the Sound mingle among near-shore sand and gravel bars that change dimension with the rising and falling tide making this an interesting birding location.

To visit more glacial erratics in the area, go to the glacial erratics page on this website.

The dark mineral grains are biotite mica and (probably) hornblende. Bud Hardwick photo.

A fine-grained inclusion in the boulder is at left. This is probably a chunk of faster-cooled magma that fell back into the magma chamber represented by the coarser grained rock. Sandy Bowman photo.

3 Responses

  1. […] Last spring I put out a call for intrepid readers to investigate a reported Jackass conglomerate erratic on the beach at Des Moines, between Burien and Federal Way. Thanks to Sandy Bowman and Bud Hardwick for independently investigating. Turns out the large rock is granitic, not conglomerate. Read the article. […]

  2. As a child growing up in Des Moines, we were not allowed to play near the cliff face next to what is now the North end of the marina because it was unstable and constantly sluffed. Imagine my surprise when they chose to build condos right on the edge. It seems like it is only a matter of time for this cliff to slide and take the condos with it.

    We used to play on a rock down the beach from our house. Creative children that we were, we called it “The Big Rock.” I wish we had pictures. It was probably 12 x 12. It was big enough that there were several routes to the top. It is now under the Anthony’s Home Port at the South end of the marina. When the marina was put in, we were as upset about losing The Big Rock as we were about losing our beach.

    • Robin,
      Thanks for your comments. There are a lot of places along coastal bluffs that have been developed. Occasionally they et hit by slides, too, often with tragic results. But if people purchase property unaware of hazards because they haven’t taken Geology 101, or weren’t paying attention the day of the lecture on ‘hazards’, or aren’t being observant and properly sceptical, and if cities/counties do not enforce hazardous areas ordinances, people are stuck with what gravity and geology dish out.

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