Last reviewed 16 Sept 2026 · 8 min read
Topographic maps
- Show relief by contours — lines joining points of equal elevation.
- Contour interval — constant vertical difference between successive contours.
- Closely spaced contours → steep slopes; widely spaced → gentle slopes.
- V-shaped contours pointing upstream indicate valleys; pointing downhill indicate ridges/spurs.
- In India, topographic maps are published by the Survey of India.
Geological maps
A geological map shows the distribution of rock units (formations) at the earth's surface (or beneath soil cover), their boundaries (contacts), ages, and structures, usually superimposed on a topographic base.
Contents
- Rock units shown by colours/patterns, usually arranged by age in the legend (youngest at top).
- Stratigraphic column — vertical sequence and thickness of units.
- Geological boundaries — conformable contacts, unconformities, intrusive contacts.
- Structural symbols — strike and dip of beds, foliation and joints; fold axes (anticline, syncline, with plunge arrows); faults (with downthrow side marked); horizontal and vertical beds.
- Mineral deposits, quarries, springs, landslides (on thematic maps).
- Scale, north arrow, grid, and cross-section lines.
Common map symbols (conventional)
| Feature | Symbol idea |
|---|---|
| Strike and dip | Long line along strike, short tick on the down-dip side with dip angle |
| Horizontal beds | Cross (+) in a circle or special symbol |
| Vertical beds | Strike line with a short cross line on both sides |
| Anticline axis | Line with arrows pointing away from the axis |
| Syncline axis | Line with arrows pointing towards the axis |
| Fault | Heavy line; D/U or tick marks to show downthrown side |
(Exact symbols vary between agencies.)
Principles of relative dating
| Principle | Statement |
|---|---|
| Superposition | In undisturbed sequences, younger beds lie above older ones |
| Original horizontality | Sediments are deposited nearly horizontal; tilted beds were deformed later |
| Lateral continuity | Beds extend laterally until they thin out |
| Cross-cutting relationships | A feature (dyke, fault) that cuts across rocks is younger than the rocks it cuts |
| Inclusions | Fragments included in a rock are older than the host rock |
| Faunal succession | Fossil assemblages succeed one another in a definite order — correlation of strata |
Outcrop patterns
The outcrop of a bed is the intersection of the bedding plane with the ground surface; its pattern depends on the dip of the bed and the topography.
| Attitude of beds | Outcrop pattern |
|---|---|
| Horizontal beds | Boundaries parallel to contours (follow the contours); in valleys, V's point upstream exactly like contours |
| Vertical beds | Boundaries are straight lines along strike, unaffected by topography |
| Inclined beds | Boundaries cut across contours; shape follows the rule of Vs |
Rule of Vs (outcrops crossing valleys)
- Beds dipping upstream — the outcrop V points upstream, but is less pronounced than the V of the contours (the steeper the dip, the straighter the outcrop).
- Beds dipping downstream at an angle greater than the valley gradient — the V points downstream.
- Beds dipping downstream at an angle less than the valley gradient — the V points upstream and extends farther upstream than the contour V.
- Horizontal beds — V points upstream, parallel to contours.
- Vertical beds — no V (straight outcrop).
Outcrop width
The width of outcrop depends on the thickness of the bed, its dip, and the slope of the ground:
- Wider outcrop for thicker beds, gentler dips, and where the ground slope is in the same direction as the dip (nearly parallel).
- Narrowest when beds are vertical (width = thickness on level ground).
- On level ground, .
Recognition of structures on maps
| Structure | Map evidence |
|---|---|
| Anticline (eroded) | Beds repeated symmetrically on either side of an axis, oldest at the centre, dips away from axis |
| Syncline | Beds repeated, youngest at the centre, dips towards the axis |
| Plunging fold | Outcrops form V or U shapes closing in the plunge direction (anticline) or opposite (syncline) |
| Dome / basin | Roughly circular/elliptical outcrops; oldest (dome) or youngest (basin) at the centre |
| Fault | Abrupt displacement, termination or repetition/omission of beds along a line; beds on either side may not match |
| Unconformity | Younger beds cut across the boundaries of older beds; different dips above and below |
| Dyke | Narrow band cutting across other rocks |
| Overlap / outlier / inlier | Outlier — younger rocks surrounded by older; inlier — older rocks surrounded by younger |
Drawing geological cross-sections
- Draw the topographic profile along the section line using contour intersections (with suitable vertical exaggeration — preferably none for correct dips).
- Mark points where geological boundaries cross the section line.
- Determine the dip of each boundary — from dip symbols or strike lines (lines joining points of equal height on a boundary).
- Convert to apparent dip if the section is not perpendicular to strike.
- Project boundaries downward at the appropriate dip; show faults, folds and unconformities.
- Add ornamentation, legend and scale.
Strike lines: where a boundary crosses the same contour at two points, the line joining them is a strike line at that elevation; the perpendicular spacing between strike lines of successive heights gives the dip: .