Exercises: Tools of representation

What is a stratigraphic column and what information does it provide about rock layers? State at least three items of data that must appear on it.

Explain the difference between a geological map and a geological cross-section. What can be seen in the cross-section that cannot be seen on the map?

A flowchart represents the sequence weathering → erosion → transport → deposition. What is the advantage of representing a geological process with a flowchart compared to a written description?

What is the difference between the numerical scale and the graphic scale of a map? Explain why the graphic scale remains useful when the map is photocopied enlarged or reduced.

What is a geological cross-section? Describe the steps to draw one from a geological map and a chosen section line.

On geological maps, the orientation of strata is represented with a "T"-shaped symbol. Explain what its long stroke and its short stroke with its figure indicate (strike and dip).

What is a conceptual model or diagram in geology? State one advantage and one limitation of simplifying reality in a diagram.

What is the difference between the stratigraphic column of a specific outcrop and the composite column of a region? Explain how the latter is built from several partial columns.

The figure shows the stratigraphic column of an outcrop with five undeformed layers, numbered from bottom to top. State which layer is the oldest and which is the youngest, and state the geological principle you have applied.

In the key of a geological map, colours, patterns and acronyms like "Pz" or "K" appear. Explain what each of these three graphical elements represents.

In a schematic geological cross-section, the vertical scale is exaggerated five times relative to the horizontal. Explain what this vertical exaggeration means and what error in interpretation it can cause when estimating terrain gradients.

In a stratigraphic column, marls pass gradually into limestones with abundant shallow-water marine fossils. Interpret this change in terms of sea-level variation and justify whether it corresponds to a transgression or a regression.

In a geological cross-section, a dyke of igneous rock cuts across all the strata and also across a fault. Which principle allows you to order the dyke, the strata and the fault in time? Could the dyke be older than any of the layers? Justify your answer.

The legend of a geological map orders the units from the oldest at the bottom to the youngest at the top. Explain which principle this order reflects and why it makes the map easier to read and interpret.

On a map, the contacts between strata draw parallel bands that repeat in symmetrical order on both sides of a central band. What tectonic structure do you interpret? What additional data do you need to decide whether it is an anticline or a syncline?

On a geological map, a contact cuts obliquely across all the units it traverses. Explain which two types of contact (intrusive or fault) can show such a trace and which observations on the map and in the field allow them to be distinguished.

The schematic cross-section shows strata A, B and C, an erosional surface (unconformity), strata D and E, fault F and igneous dyke G. Place the eight geological events represented (including the erosion) in chronological order from oldest to newest, and justify each relationship using the applied principle.

A stratigraphic column is drawn at a vertical scale of $1:2\,000$. A marl formation measures $7,5\ \text{cm}$ on paper. Calculate its actual thickness in metres.

Design, describing its stages and arrows, a flowchart explaining how the erosion of granite in a mountain range can end up forming a sandy beach. Indicate which agent is involved in each stage.

A cross-section shows, from bottom to top: slates (A), an undulating erosion surface, conglomerates (B), sandstones (C), a fault that displaces B and C, and silts (D) that cover the fault without being affected by it.

1. a) Order all the events from oldest to most recent;
2. b) state the principle that justifies each relationship;
3. c) explain what type of unconformity separates A from B.

On a map, a limestone stratum draws a "V" where it crosses a valley.

1. a) Explain the relationship between the shape of the contact on the map, the relief and the dip of the stratum (the rule of V's);
2. b) describe how the contact would be traced if the stratum were horizontal;
3. c) and if it were vertical?

You can do it on paper and send a photo with your answer.

You must summarise the geological history of a region in a block diagram.

1. a) List the elements that cannot be missing (units, ages, structures, processes);
2. b) explain how you would mark cross-cutting relationships and unconformities;
3. c) justify why this diagram complements, rather than replaces, the map and the stratigraphic column.

The profile of a road cutting is drawn with a vertical exaggeration of ×10.

1. a) What apparent inclination will strata with a true dip of $5°$ have in the drawing?
2. b) What interpretation error can this exaggeration cause when assessing the stability of the slope?
3. c) How would you present the information in a technical report to avoid that error?

You can do it on paper and send a photo with your answer.

Design, describing its boxes and arrows, a flowchart that summarises how a buried clay sediment can become transformed into a schist and, finally, into a granitic magma. State at each stage the dominant process and the agent that drives it.