The first major Basque stop is geological and it is genuinely arresting.

The flysch at Zumaia is a sequence of sedimentary rock layers exposed in the coastal cliffs that records sixty million years of Earth's history in visible stripes of rock running from the cliff top to the waterline. The layers are tilted almost vertical by tectonic movement — what were once horizontal seabed deposits now stand on end, running in dark and light bands across the cliff face like the pages of a book held upright.

One of these layers — a thin dark band of iridium-rich rock — marks the precise moment of the mass extinction event that ended the Cretaceous period. The one that took the dinosaurs. You can stand in front of it and put your finger on it if the tide is out.

The cliffs are best seen from the water — boat tours run from Zumaia harbor and give you the full scale of the formations from the sea side, which is considerably more impressive than the cliff-top view. The tour takes about an hour and runs in most weather conditions.

If the boat tour does not fit the day, the cliff-top path from Zumaia toward Deba runs above the flysch formations and gives aerial views of the rock bands and the coastline. The path is about four kilometers one way and well marked.

Practical note: the boat tours run on a schedule that changes by season. Check times in advance and arrive at the harbor early enough to secure a place.