How come no sandy beaches in Puget Sound?

Dear Dr. Rock,
We live in Colorado. The family loves to go to the beach. Since we live in Colorado, this means a road trip. Any excuse for a family road trip with our motor home is a good one, and a trip to one coast or the other is always a hit. Last summer we traveled to the Northwest and spent a week visiting friends in Seattle. We were surprised to find that sandy beaches were virtually nonexistent. The dang water was too cold, anyway. Everything is rocky or covered in gravel, or even boulders. Why is this? So much saltwater, and no beaches. Our friends didn't know.

Marv and Julie Harris
Denver, CO

Marv and Julie,

A typical cobble beach along Puget Sound. Each and
every one of these rocks is a glacial erratic. mostly carried south
from Canada.

As a native westerner Washingtonian living near one of those shores and who likes rocks, I'll only half-heartedly apologize. It's not my fault. I hope you had a good time in the area anyway!
Why are beaches in Puget Sound so rocky? It is our glacial legacy. Great glaciers have advanced from the interior of British Columbia down into the Puget Lowlands at least 6 times over the past 2 million years. These glaciers carried a lot of rock mixed in with the ice. Much of that rock load was rounded gravel and cobbles scoured out of northern river beds. All that rock got dumped when the climate warmed and the ice melted. It left a thick fill of glacial deposits covering the landscape. Technically, this is composed of glacial erratics- glacially transported rock material that differs from the underlying bedrock (see the January 2013 post about glacial erratics).

As the ice melted off the land surface, global sea level rose to flood the area we now call Puget Sound. At high tides and especially during storm surges, waves erode away at the base of the bluffs that line much of the shoreline. These bluffs consist of the glacial debris- small stuff like sand and silt, and larger stones: pebbles, cobbles, and big boulders.  The steep bluffs provide a ready-made supply of rocks. Wave energy can easily wash the fine sediment into deeper water, but is rarely powerful enough to carry the rocks off the beach. The result is a 'lag deposit'- the fine material has been removed, the heavier rocks are left behind. Voila, a rocky beach that is not much fun to lie around on, unless you find a smooth drift log or a nice big smooth glacial erratic to stretch out on to enjoy the skin-softening northwest drizzle. Beats skin cancer.


A fine sandy beach south of Bellingham.

There are some natural sandy beaches in the inland waters of the Northwest, but they require a special set of circumstances to last through the rough winter storms and waves. There has to be a supply of sand to start with, such as a sandy bluff with few cobbles in it nearby, and just the right combination of tidal energy and current direction to concentrate the sand on the shore above the low tide line.
You are liable to find these same cobble beaches anywhere along the shoreline in areas that have been glaciated: Maine and the rest of New England, and in places in the Great Lakes, too.