Guide
New to stratigraphy? Start here
A short, plain-language course in how geologists read the layers of the Earth — the ideas behind every formation on this site. No background assumed.
What is stratigraphy?
Stratigraphy is the branch of geology that studies rock layers — strata — and the order in which they were laid down. Sedimentary rocks accumulate layer upon layer over millions of years, so a cliff face or a borehole is really a stack of pages: read them in the right order and you recover the history of an ancient landscape — the seas, rivers, deserts and deltas that once stood where the rock now sits.
That history is what makes stratigraphy so practical. It underpins the search for oil, gas, coal, groundwater and minerals; it dates the fossils that record the evolution of life; and it lets geologists correlate rocks from one valley to the next, and from one country to another.
Three ways to slice the record
Every rock unit can be viewed three ways at once — by what it is, by the fossils in it, and by its age. Each has its own illustrated guide.
Lithostratigraphy
Divides rocks by their physical character — the actual lithology. Its units (groups, formations, members, beds) are what you map in the field and what this site catalogues.
Read the full guide →Biostratigraphy
Divides rocks by their fossil content. Because life evolves and species appear then go extinct, fossil assemblages let geologists correlate and date strata between distant areas.
Read the full guide →Chronostratigraphy
Divides rocks by age — the time-rock framework of systems, series and stages that underlies the geological time scale. It answers “when”, not just “what”.
Read the full guide →The principles of stratigraphy
A handful of common-sense principles, most first stated by Nicolas Steno in the 1600s and William Smith in the early 1800s, let anyone put a sequence of rocks into order. Learn these seven and you can read almost any outcrop.
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1Superposition
In an undisturbed sequence, each layer is younger than the one beneath it and older than the one above.
This is the foundation of reading rock. The bottom of a cliff is the oldest page of the story; the top is the most recent. It only holds where layers have not been overturned by intense folding.
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2Original horizontality
Sediments are laid down in nearly horizontal layers.
So when you see tilted, folded or vertical strata — as in the Salt Range or the Sulaiman fold belt — you know they were deformed after they were deposited. The tilt itself is evidence of later tectonics.
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3Lateral continuity
Layers extend outwards in all directions until they thin to nothing or meet a barrier.
Matching beds exposed on opposite sides of a valley were once a single continuous sheet, later cut by erosion. This lets geologists correlate outcrops separated by gaps.
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4Cross-cutting relationships
A feature that cuts across rocks is younger than the rocks it cuts.
A fault, dyke or igneous intrusion must be younger than the strata it slices through — you cannot cut something that is not yet there. A key tool for ordering events.
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5Inclusions
Fragments contained within a rock are older than the rock that contains them.
Pebbles in a conglomerate, or fragments of older rock caught in a granite, must have existed first to be included. The host is always the younger of the two.
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6Faunal succession
Fossil species succeed one another through time in a definite, recognisable order.
Because that order is the same everywhere, a distinctive “index fossil” dates the rock it sits in and links strata across continents. This principle turns palaeontology into a clock — and it is why Pakistan’s fossil-rich Siwaliks are so valuable.
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7Uniformitarianism
The present is the key to the past.
The same slow processes we watch today — rivers depositing sand, deltas building out, seas advancing and retreating — operated in the past at broadly similar rates, so modern environments help us read ancient ones.
How rock units are organised
Group — Two or more related formations grouped together — e.g. the Siwalik Group of the Potwar Plateau.
Formation — The basic unit of mapping, and the one this site is built around. Consistent in character and traceable across an area.
Member — A distinctive part of a formation that is worth naming but not mappable on its own.
Bed — A single layer — the smallest formal unit, sometimes a marker thin enough to walk along.
Gaps in the record: unconformities
The rock record is never complete. Surfaces where time is missing — because nothing was deposited, or because rock was eroded away — are called unconformities, and they are some of the most informative features in a sequence.
Disconformity
A gap between parallel layers — deposition paused or erosion occurred, but the beds above and below stay horizontal.
Angular unconformity
Older strata were tilted and eroded before younger, flat-lying beds were laid across the bevelled surface — a clear record of a tectonic episode.
Nonconformity
Sedimentary layers rest directly on eroded igneous or metamorphic basement, such as Cambrian sandstone lying on Precambrian rock.
How to read a stratigraphic column
A stratigraphic column is the standard diagram geologists use to summarise a sequence. Read it from the bottom up — oldest to youngest. The vertical axis is rough time (and thickness); the patterns and width of each layer encode lithology and grain size, with coarser, more resistant rocks often drawn wider. Wavy lines mark unconformities, little symbols flag fossils, and the contacts between units tell you whether deposition was continuous or interrupted. The geological time scale gives the column its calendar — and you can see live columns on each province page.
Key terms
Strata
Layers of rock. A single layer is a stratum; “stratigraphy” is literally the study of layers.
Formation
The fundamental mappable rock unit — a body of rock with consistent character, large enough to show on a map and traceable across an area.
Type locality
The reference location where a formation is formally defined and best displayed; its “official” example.
Facies
The set of characteristics (rock type, fossils, structures) that reflect the environment in which a rock formed.
Lithology
The physical make-up of a rock — its composition, grain size and texture (sandstone, shale, limestone, and so on).
Index fossil
A fossil that is widespread but lived only briefly, making it ideal for dating and correlating the rocks that contain it.
See the full glossary for more terms.
Now apply it to Pakistan
Pakistan is an unusually good place to learn stratigraphy: the Salt Range exposes a near-complete section from Precambrian to Recent, the Siwaliks hold one of the world’s richest fossil-mammal records, and the Himalaya and Karakoram show deformation on a spectacular scale. Our long-form article puts the whole picture together.
Read: Stratigraphy of Pakistan →Questions
Do I need a geology background to follow this?
No. The principles here are deliberately written in plain language. If you can picture layers of sand building up at the bottom of a sea, you already have the intuition stratigraphy is built on.
How should I use this for exam preparation?
Learn the principles here first, then browse formations by province, save the important ones to Favourites, and add your own notes. Use the Time Scale to fix the chronological order in your mind, and review your favourites list the night before.
Can I use it offline in the field?
Yes. Install the site to your home screen and the reference data is available without a signal — handy for fieldwork in remote areas.
Where does the data come from?
Primarily GSP Memoir Vol. 24, “Stratigraphy of Pakistan” (Malkani & Mahmood, 2017), published by the Geological Survey of Pakistan, alongside the wider literature. See the About page for full attribution.