Two-layer sea cliffs — a resistant cap rock sitting on a softer, more erodible base — retreat in a repeating cycle: waves hollow a notch into the soft layer at the waterline, the harder rock above is left overhanging with nothing to support it, and once the overhang outgrows what the cap rock can carry, it collapses and the cliff face jumps landward. This 2D cross-section companion to the 3D coastal erosion simulator draws that cycle side-on — soft rock, hard cap, notch and collapsing block — while wave energy and rock hardness sliders drive how fast the notch grows and how large an overhang the cap can hold before it gives way. Watch the live readouts to see the average long-run retreat rate settle toward wave energy divided by rock hardness, however irregular the individual collapses look.