(Seaside, OR)  --  The Seaside Aquarium is trying to figure out what caused thousands of live sand dollars to wash up on Seaside Beach over the weekend.  The aquarium says the sand dollars washed up with Sunday's afternoon high tides on the south end of the beach near Tillamook Head.  The aquarium says it's not yet clear what caused the incident or whether this is happening on other beaches.  Sand dollars can only live for a few minutes out of the water.