It is a bit unusual to see a single rock being transported on the highway.
Large rock on I-5 north bound through the Chuckanuts
Large rocks are being transported from the Big Lake Quarry in Skagit County northward to to the Bellingham waterfront and from there to the North Jetty at the Columbia River.
Mouth of The Columbia River
The rock is greenschist - metamorphosed ocean floor basalt. It was mined from one of the terranes that make up the Northwest Cascades. The advantage of this rock is they can be broken into very large blocks, they have high density relative to other rocks and are resistant to both physical and chemical weathering. Good material for a jetty that must resist huge waves.
This has resulted in a proposal to mine additional greenschist in the Skagit Valley with a mine proposed at a greenschist outcrop near Marblemount.