Given their size, it's amazing that the ocean has the force to break these boulders apart; but it's doing at a current pace that's removing an average of 100' per year off the end of the jetty, according to a U.S. Army Corps of Engineers report quoted in the Tillamook Headlight Herald of March 24, 2015. The Corps says the jetty has lost a total of 900' since 1979, and that it now "meets the completely degraded condition criteria." Tillamook County and the Port of Garibaldi hope to get federal funding to rebuild it soon.
The north jetty was repaired in 2010. As the story of Bayocean Park's demise makes clear, it's critical to keep the two jetties in balance. Building just the north jetty in 1914 prevented the summer replenishment of Bayocean sand that had been scoured away during winter storms. We understand this now, but at the time many reasoned the slow loss of sand could have been just part of a generational ebb and flow. By the time the north jetty had been lengthened to its current length of 5700' in 1931, beach erosion had accelerated dramatically. In 1932 Bayocean's natatorium, which sat right on the beach, had been undercut to the point it had to be abandoned. One house after another fell into the sea until a 1952 storm created a breach a mile wide at the southern end, and left Bayocean an island until the Corps build a dike to close in 1957. Beaches started rebuilding a little, east of the original location, but the pace of accretion increased dramatically as soon as construction of the south jetty began in 1969. It took three phases of funding and work to get it to 8000'. See Oregon Coastal Atlas and Bayocean Then and Now to get an idea of how dramatic the changes have been over the last century.
Getting back to the boulder at Kincheloe Point: as you can see there is a square carved out of the upper right side of it. This once held a commemorative plaque that was stolen about 10 years ago. Until it can be replaced we can at least see what it looked like thanks to Walter Van Camp, who provided a photo of it. You can watch a video about the building of the South Jetty produced by Anchor Pictures for the Port of Garibaldi.