Reconstructed setting· The columns of time
Sturtian Snowball Earth
717 Ma → 660 Ma
Ice to the horizon, with narrow open leads rather than an ice-free ocean, and nothing on the ice. The planet is white for fifty-six million years.
What the evidence shows
The Sturtian glaciation ran from 716.5 to 660.98 million years and reached equatorial latitudes, which is what makes it a Snowball rather than an ice age. The narrow leads are drawn on purpose. Whether the tropical ocean froze solid, the hard snowball, or kept a thin belt of open water is the central unresolved question about these episodes, and drawing either extreme would settle it by picture. Nothing is drawn on the ice because complex life was marine and microscopic, and animals appear later.
Bounds and conventions
The dates are unusually precise for this depth, and the physics is not. How a planet covered in ice, which reflects most of the light that reaches it, ever warms up again is the difficulty, and the backdrop deliberately leaves the question open where the evidence does.