Santarosae — the joined superisland of the northern Channel Islands — was separated from the California mainland during the terminal Pleistocene by the Santa Barbara Channel.

The Channel is a genuine piece of ocean, not a sheltered harbor or a river crossing.

Its waters are cold, driven by upwelling from deeper Pacific depths, and traversed by currents.

Even at its minimum width during the terminal Pleistocene — when sea level was substantially lower and more of the continental shelf was exposed — the crossing from mainland to island required navigating several miles of open water.

The specific minimum distance has been the subject of detailed research.