The Native Americans have a fascinating legend regarding Mount Si origins. It fits your first impression of the landmass:

Mount Si figures prominently in a Prometheus story from the Snoqualmie tribe. According to the story it is the dead body of Snoqualm, the moon. Snoqualm had ordered that a rope of cedar bark be stretched between the earth and the sky. But Fox and Blue Jay went up the rope and stole the sun from Snoqualm. Snoqualm chased them down the cedar rope, but it broke and he fell to his death. Fox then let the sun free in the sky and gave fire to the people. A face like Snoqualm's is visible on the rocks near the summit.[8]