Atlas Trail
How geologists learned to read time in stone: six stops from a Danish anatomist's simple rule about flat-lying sediment to the calibrated calendar of the geologic time scale. This is the trust-and-methodology story of the Earth Atlas: not what the planet did, but how anyone knows.
Stop 1 of 6.
Concepts
In the 17th century Nicholas Steno states a rule a child can test: sediment settles flat. Every folded and tilted rock face is therefore a record of something that happened afterward, and reading rocks as history becomes possible.
Stop 2 of 6.
Concepts
Steno's second rule orders the pages: in undisturbed strata the oldest layer lies at the bottom. Depth becomes age, and a cliff face becomes a stack of dated pages with no dates written on them yet.
Stop 3 of 6.
Concepts
Around 1800 the surveyor William Smith notices that fossils succeed each other in the same reliable order everywhere. Now strata can be matched across a country sight unseen, and Smith draws the first geological map of an entire nation from it.
Stop 4 of 6.
Disciplines
Stratigraphy gathers the principles into a discipline: layers sorted by rock type, by fossil content and by age. The rock record becomes an archive with a filing system, read the same way on every continent.
Stop 5 of 6.
Concepts
Radioactive decay finally writes numbers in the margins. Radiometric dating turns relative order into years, and the pages Steno and Smith ordered can be dated in millions and billions.
Stop 6 of 6.
Concepts
The geologic time scale is the finished calendar: eons, eras, periods and epochs, kept and refined by international agreement. Every fossil, eruption and ice age this atlas describes hangs on the framework these six stops built.
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