A geological cross-section is a vertical slice through the ground constructed from a map by transferring the topography and the boundaries of the rock units onto a profile and drawing the beds at their measured dip; a graphic (sedimentary) log records a vertical sequence of beds to scale, showing thickness, grain size, rock type and structures; both turn observations into a diagram from which the order of beds, the structures and the geological history can be read.

What this dot point is asking

Eduqas wants you to construct and read two key geological diagrams. A geological cross-section is a vertical slice through the ground, built from a map by drawing the topographic profile, transferring the rock-unit boundaries, and drawing the beds at their measured dip. A graphic (sedimentary) log records a vertical sequence of beds to scale, showing thickness, grain size, rock type and structures. Both turn observations and map data into a diagram from which the order of beds, the structures and the geological history can be read. Cross-sections and logs appear directly in Component 2.

The answer

What a cross-section is and how to build it

A geological cross-section is a drawing of what you would see if you cut down vertically through the ground along a line. It is built from a geological map in a set order:

1. Draw the topographic profile. Along the section line, read where each contour crosses, plot the heights at the chosen vertical scale, and join them to draw the ground surface.
2. Transfer the geological boundaries. Mark on the profile where each rock-unit boundary crosses the section line on the map, projecting it up to the surface.
3. Draw the beds at their dip. From each surface boundary, draw the bed downwards at its measured dip (angle and direction), so the beds tilt correctly underground. Keep beds parallel unless a fold or fault says otherwise.
4. Finish with the key and scales. Colour or label each unit to match the map key, show any folds or faults, and label the horizontal and vertical scales.

The result reveals the structure at depth and the order of beds, which a flat map alone cannot show fully.

What a graphic log is and how to build it

A graphic (sedimentary) log is a vertical column drawn to scale recording a sequence of sedimentary beds, oldest at the bottom and youngest at the top. For each bed it records:

• thickness (drawn to scale on the vertical axis);
• grain size (often plotted on the horizontal axis, so coarser beds stick out further);
• rock type (with a standard symbol or colour);
• sedimentary structures (cross-bedding, ripples, graded bedding) and any fossils.

A log packs a whole vertical sequence into one readable diagram.

Reading them: order, structures and history

Both diagrams are made to be interpreted:

• The order of beds follows from superposition (oldest at the base), so a log or section reads as a sequence of events from bottom to top.
• A cross-section shows the structures (the dip of beds, folds and faults) and how the units relate at depth.
• A log shows how the environment changed through time: a fining-upward trend (grain size decreasing upward) suggests deepening water or a transgression, while a coarsening-upward trend suggests shallowing or a regression; the structures and fossils refine the picture.

So a cross-section answers "what is the structure here?" and a log answers "how did conditions change over time here?".

Examples in context

Example 1. A cross-section through a fold. Drawing the beds at their dip on a section reveals an anticline that the map only hinted at, with the oldest beds arching up in the core, making the structure obvious at depth.

Example 2. A river log. A graphic log of a river deposit often shows repeated fining-upward cycles (coarse channel sand fining to overbank mud), each recording a flood event, which the log makes visible at a glance.

Try this

Q1. State the first step in constructing a cross-section from a geological map. [1 mark]

• Cue. Draw the topographic profile along the section line from the contour heights.

Q2. Explain what a fining-upward sequence on a graphic log suggests. [2 marks]

• Cue. Decreasing energy and usually deepening water (a transgression), as grain size decreases from the base to the top.

Q3. State which bed is the oldest on a graphic log. [1 mark]

• Cue. The bed at the base (by the law of superposition).