A simplified geological map shows the distribution of rock units at the surface using colours and a key, with a scale, a north arrow and grid lines; features are located using grid references (four-figure for a square, six-figure for a precise point), and the map is read together with topography to identify the rock units present, the order of the beds, and structures such as folds and faults shown by the outcrop pattern.

What this dot point is asking

Eduqas wants you to read a simplified geological map: to know what it shows (rock units in colour with a key, a scale, a north arrow and grid lines), to locate features using grid references (four-figure for a square, six-figure for a precise point), and to read the map together with the topography to identify the rock units, the order of the beds, and structures (folds and faults) revealed by the outcrop pattern. Reading a simplified geological map is the central skill of Component 2, which is built around such a map.

The answer

What a simplified geological map shows

A geological map shows the distribution of rock units at the surface (what rock you would find if you scraped off the soil). Its standard elements are:

• Colours (sometimes with letters or symbols) for the different rock units, explained in a key (legend).
• A scale, so distances on the map convert to real distances on the ground.
• A north arrow, giving orientation.
• Grid lines, the numbered squares used for grid references.
• Often dip symbols (showing the direction and angle beds tilt) and the topography (hills and valleys, shown by contours or shading).

Grid references: locating features

Features are located with grid references, read from the numbered grid lines. The rule is eastings first, then northings, remembered as "along the corridor, then up the stairs":

• A four-figure reference takes the easting line to the left of the point and the northing line below it, giving two digits each (for example 3 2 1 7). It names the whole grid square.
• A six-figure reference divides that square into tenths: estimate how far across (eastings) and how far up (northings) the point lies, adding one digit to each (for example 3 1 5 2 1 4). It names a precise point.

A four-figure reference is fine for a large area, but a small feature (a quarry, a spring) needs a six-figure reference, which on a typical map locates it to about 100 m.

Reading the rock units and the order of beds

From the key you identify which rock unit each colour represents. To work out the order of the beds (which is oldest), you combine the map with the principles of dating: in undeformed, tilted beds the oldest outcrop is generally on the side the beds dip away from (and the dip symbols help), while in folds the age pattern is symmetrical (see below). The map thus shows not just where each rock is but how the rocks are arranged in time.

Structures from the outcrop pattern

The shape of the outcrop bands reveals the structures:

• A fold makes the beds repeat in a mirror image either side of the fold axis. An anticline shows the oldest beds in the core with younger beds outward on both sides; a syncline shows the youngest in the core. The symmetrical repeat is the giveaway.
• A fault appears as a line that offsets or truncates the outcrop bands: the pattern of rock units does not match across the line, because the beds have been displaced.

Reading these patterns lets you interpret the geological history of an area from the map alone.

Examples in context

Example 1. A dipping sandstone bed. On a map, a sandstone band that runs across hills and valleys in a V-shape pointing in a particular direction reveals the bed is dipping, the V points down-dip in valleys, which a Component 2 question may ask you to read with the topography.

Example 2. A faulted coalfield. On a coalfield map, a coal seam that suddenly jumps sideways across a straight line shows a fault has displaced it, important both geologically and for mining the seam on the other side.

Try this

Q1. State the order in which you read the two parts of a grid reference. [1 mark]

• Cue. Eastings first (along the bottom), then northings (up the side).

Q2. Explain how a fault can be recognised from the outcrop pattern on a geological map. [2 marks]

• Cue. The outcrop bands are offset or truncated along a line, so the pattern of rock units does not match across it, showing the beds have been displaced.

Q3. State what a four-figure grid reference identifies. [1 mark]

• Cue. A whole grid square (not a precise point, which needs six figures).