What is a geological map?
A normal map shows roads and rivers. A geological map shows the rock that lies at the ground surface, just under the soil.
Each rock unit gets its own colour and a short letter code. A legend (key) lists the units in order of age, the youngest at the top.
Common symbols
- Coloured areas: different rock units (for example sandstone, limestone, granite).
- Thin lines between colours: contacts, where one rock meets another.
- Thick or red lines: faults. Teeth or a small "D/U" may show which side went down or up.
- T-symbol with a number: strike and dip of a layer.
- Brown contour lines: height of the land (topography).
Layers, age and the law of superposition
Sediment settles in water and builds up like pages in a pile of newspapers. The first page is at the bottom. So, if layers are not turned over, lower layers are older. This is the law of superposition.
Two more simple rules help:
- Original horizontality: layers start almost flat. If they are tilted or folded now, something happened later.
- Cross-cutting: a fault or a lava dyke that cuts layers is younger than the layers it cuts.
Stratigraphic column
A stratigraphic column is a tall bar that lists the layers in order: oldest at the bottom, youngest at the top. The height of each block shows the layer's thickness, and a wavy line marks a gap in time (an unconformity).
Strike and dip: describing tilted layers
Imagine a tilted roof. Put a level line on it (a line that does not go up or down). Its compass direction is the strike. Now let a marble roll straight down the roof. The angle of that slope from the horizontal is the dip, and the marble's direction is the dip direction. Dip direction is always at 90° to strike.
On a map we draw a T: the long bar along the strike, a short tick pointing down-dip, and the dip angle written next to it, for example 30°.
How tilt shows on the map
- Flat layers: the contacts follow the contour lines.
- Vertical layers: the contacts are straight lines, whatever the hills do.
- Gently tilted layers: wide colour bands. Steep layers: narrow bands.
Faults, folds and drawing a cross-section
A fault is a crack along which rocks have moved. On a map, colour bands stop at the fault line and continue somewhere else. The amount of movement is the throw. A fold bends layers into an arch (anticline, oldest rock in the middle) or a trough (syncline, youngest rock in the middle).
Drawing a cross-section
- Draw a line A–B across the map.
- Draw the ground profile from the contour lines.
- Mark where each contact meets line A–B.
- From each mark, draw the layer going down at its dip angle.
- Add faults, colour the layers and write a title and scale.
Flow diagrams of geological history
After reading a map and section, we write the story as a flow diagram: layer A deposited → layer B → layer C → tilting → faulting → erosion → new layer on top. Each arrow is one event, oldest first.
Try it at home
Stack 4 colours of clay or play-dough in a bowl, press them flat, tilt the block and slice it with a thread. The top is your map; the cut face is your cross-section.
Key formulas and definitions
- Law of superposition: lower layer = older (if not overturned)
- Strike ⟂ dip direction (always 90° apart)
- Dip = angle between the layer and the horizontal (0°–90°)
- Cross-cutting: a feature that cuts a layer is younger than it
- Map symbol: T with dip angle, e.g. ⊥30°
- Anticline: oldest in the core; syncline: youngest in the core
Worked examples
1. A map shows layers P (bottom), Q and R (top), all flat. A fault cuts P and Q but not R. Put the events in order.
P deposited → Q deposited → fault → R deposited. R is not cut, so it came after the fault (cross-cutting rule).
2. A strike line runs north–south and the dip tick points east with 40° written beside it. Describe the layer.
Strike N–S, dip 40° towards the east. If you walk east on the surface, the layer goes deeper under your feet.
3. On one map, contacts follow the contour lines exactly. On another, contacts are straight across hills and valleys. What does each tell you?
Contacts that follow contours mean flat (horizontal) layers. Straight contacts across hills mean vertical layers.
Common mistakes
- Thinking the top of the legend is the oldest rock. The legend lists the youngest at the top.
- Mixing up strike and dip. Strike is a direction of a level line; dip is an angle plus a direction downwards.
- Drawing the dip direction parallel to the strike. They are always at right angles.
- Forgetting that a fault is younger than the layers it cuts.