Here is the famous unconformity at Siccar Point:
This is where James Hutton realised the concept of ‘Deep Time’ - way back in 1788! Note that the unconformity surface is uneven. There older layers below are marine sandstone, which after lithification became subject to strong deformation, then became emergent and subject to erosion. They were deformed in the Caledonian orogeny, a phase of continental collision and mountain building as a consequence of the closure of the Iapetus Ocean that once existed between what was essentially North America on the one hand, and the Southern part of the Brisith Isles and the Baltic shield on the other.
All this of course happened a long, long time ago…
Younger sediments were eventually deposited on top. These are actually terrestrial fresh-water conglomerates and sandstones, sourced from the land mass that arose out of the Caledonian orogeny. They were gently deformed and tilted in turn.
We now know that the time gap represented by this unconformity is about 65 million years. Not by measuring the ages directly here, but by dating elsewhere and careful correlation and mapping.