Local rocks, global time
Lithostratigraphic names — formations, groups, members — are deliberately local. They describe a particular body of rock at a particular place. That is exactly why they cannot, on their own, be compared between continents. What can be compared is age. The global framework of age is chronostratigraphy, and it is organised into a strict hierarchy: eonothems, erathems, systems, series and stages (the rock), corresponding to eons, eras, periods, epochs and ages (the time). The Eocene, the Permian, the Triassic — these are chronostratigraphic units, shared by everyone.
The golden spike
The hard part is defining exactly where one stage ends and the next begins. The modern solution is the GSSP — the Global Boundary Stratotype Section and Point, informally the “golden spike.” For each stage boundary, an international body chooses a single, well-exposed rock section somewhere in the world and drives a conceptual spike into one precise level in it, usually marked by the first appearance of a chosen fossil species. That physical point is the definition of the boundary; every other section on Earth is correlated back to it. (For the deep Precambrian, where good fossils are lacking, boundaries are instead fixed at chosen ages.)
The ICS chart
All of this is gathered into the International Chronostratigraphic Chart, maintained by the International Commission on Stratigraphy. The chart is the world’s master reference: a single, colour-coded table of every eon, era, period, epoch and stage, with their agreed ages and defining points. It is revised as new GSSPs are ratified and dating improves. When this site places a Pakistani formation in “the Early Eocene (Ypresian),” it is speaking the language of that chart — the same time scale used from Argentina to China.
How a local unit joins the global chart
A formation earns its place on the global chart through correlation, and the bridge is usually biostratigraphy. Index fossils tie a local bed to a biozone; the biozone is calibrated to a stage defined by a GSSP; radiometric dates and the magnetic-reversal record sharpen the tie. Through that chain, the Sakesar Limestone or the Chorgali Formation can be placed on the same axis of time as rocks anywhere in the world.
Pakistan’s contributions
Pakistan is not just a consumer of the global chart; its sections have helped build it. The Salt Range holds some of the classic Gondwanan sections spanning the Permian–Triassic boundary — the interval of Earth’s greatest mass extinction — and its Chhidru and Mianwali formations have been studied for generations as reference sequences for that catastrophe and its aftermath, even though the boundary’s defining golden spike lies elsewhere. The Triassic ceratite zones of the Salt Range, the Tethyan carbonate successions of the Karakoram, and the superbly dated Siwalik molasse all feed into the international effort to calibrate deep time.
Why a single chart matters
The point of a shared timescale is simple but profound: it makes Earth history comparable. An extinction, a climate swing or a sea-level change recorded in Pakistan can be lined up, to the stage and often finer, with the same event on another continent. Local names give the rocks their identity; the international chart gives them a common clock — and only with that clock can the country’s record contribute to a global story.
See how the dataset’s ages map onto that clock on the time scale.