Green Lake is a kettle lake formed by glaciation

Green Lake is a kettle lake created by the retreat of the Vashon Glaciation approximately 14,000 years ago; it occupies a closed depression left by melting ice blocks. The lake's shape, shallow depth, and surrounding parkland are characteristic of kettle-lake topography produced by Pleistocene glacial processes. Green Lake Park preserves this glacial landform within the city and is a well-documented example of glacial geomorphology in Seattle.