Yellow Island

Yellow Island sits in San Juan Channel, one of the small Wasp Islands between Orcas and San Juan Island. Like all of these islands, it consists primarily of metamorphic bedrock and most of the shoreline is marked by rocky cliffs. But also like on many of them, there may be patchy deposits of glacial drift, which form an erodible substrate that forms small bluffs in the few places it reaches to the water.

AERIAL VIEW

At each end of the island, small spits extend out to rocky ledges, creating tombolos, with small gravel beaches on both sides of each. In addition, there is also a pocket beach below a low bluff on the south shore of the island. 

On some rocky islands, the dearth of erodible sediment and the abundance of shellfish and barnacles on the resistant substrate gives rise to beaches consisting primarily of shell  debris. But as far as I can see, the beaches here are primarily gravel, presumably derived from the glacial deposits -- although there's not an obvious active source of this material, aside from occasional erosion of the toe of the bank associated with particularly major storm events. Of course, what we don't know is whether there might be a source, or at least a reservoir, of sand and gravel in shallow water offshore.

According to Phil, who has been watching this place carefully for a long time, the eastern tombolo is subject to significant changes from year to year. In particular, the cobble on the south beach is sometimes exposed (as on this trip), but may be covered in gravel some years. The fact that the beach changes significantly from year to year is not surprising, but understanding what drives this is more challenging. On a low tombolo like this, there may be large transfers of gravel back and forth across the berm during high tide storms from one direction or the other.

For more on the Preserve:
Yellow Island: The Nature Conservancy

And for a first hand account of life on Yellow Island:
Island Time on Yellow