Why a portion of that channel is now Kitsap and Mason county uplands while another part of it is now the floor of the canal remains a geologic mystery. So are the sills left by the last glacier that are now covered by the waters of Hood Canal. The sill is not bedrock, too hard to have been wiped away by the most recent glacier. Rather it is made of unconsolidated sediments that accumulated over the past few million years. But Hood Canal isn t a freak in that regard.
Many fjords have a sill.