A lot of initial observations and attempts to reconcile those with the biblical record, but that didn't add up. Lyell establishing gradualism. Lord Kelvin noting that thermodynamic cooling only allowed for about 30 million years of Earth history (pretty clearly not consistent with the structural geological record). Rutherford, Soddy, and Hardy establishing radioactivity and radiodating. Plate tectonics being proposed (initially as "continental drift"), rejected as patently absurd, and adopted some 50 years later as the fundamental organising principle of geology.

Added gloss from discoveries such as the Chixulub impactor, etc.