How it works

The land is a grid of cells. Each cell holds a rock height, a layer of loose sediment, a depth of water and the silt that water is carrying. Many times a second, every cell runs the same four rules on your graphics card.

Water flows

Neighbouring cells are joined by virtual pipes. Water accelerates through them according to the difference in surface height (the shallow-water pipe model of Mei, Decaudin and Hu, 2007).

Rivers carry

Fast water on a steep bed can hold more silt. Below that capacity it tears rock loose; above it, it drops its load. That one rule makes gorges, alluvial fans, braided channels and deltas.

Slopes fail

Any slope steeper than its material can hold slides downhill. Rock stands steep; loose sediment and anything under water slumps sooner.

Layers differ

Rock hardness changes with depth, so canyon walls step back in ledges where a hard band protects the soft one below.

What is not real

• The clock is a storyteller. One step is counted as a fixed number of years, chosen so the pace feels geological. It is not calibrated to any rock.
• Plants and snow are painted from slope, height and wetness. They do not grow, hold soil or melt.
• There is no uplift, no groundwater and no ice. Water is a thin sheet with no vertical structure.
• Heights are exaggerated. Real mountains are far flatter than they feel.

Keys

space pause · [ ] speed · 1–5 tools · N view · M sound · P picture · F fullscreen · H hide interface · right-drag always looks