consider the famous unconformity at Siccar Point, on the coast of Scotland (Figure11.15).
Figure 11.15: Hutton’s Unconformity on the Coast of Scotland. ( 17 )Based on figure 5, at least nine geological events can be inferred:
- A series of sedimentary beds is deposited on an ocean floor.
- The sediments harden into sedimentary rock.
- The sedimentary rocks are uplifted and tilted, exposing them above the ocean surface.
- The tilted beds are eroded by rain, ice, and wind to form an irregular surface.
- A sea covers the eroded sedimentary rock layers.
- New sedimentary layers are deposited.
- The new layers harden into sedimentary rock.
- These layers are tilted.
- Uplift occurs, exposing the new sedimentary rocks above the ocean surface.
Hutton realized that an enormous period of time was needed to account for the repeated
episodes of deposition, rock formation, uplift, and erosion that led to the formation of an
unconformity, like the one at Siccar Point. Hutton realized that the age of Earth should not
be measured in thousands of years, but millions of years.
Matching Rock Layers
Superposition and cross-cutting are helpful when rocks are touching one another, but are
useless when rocks are kilometers or even continents apart. Three kinds of clues help geol-
ogists match rock layers across great distances. The first is the fact that some sedimentary