Sources of geological information

What is an outcrop and why is it considered a direct source of geological information, as opposed to aerial photographs or maps, which are indirect sources?

Place the following tasks of a geological study of a region in order and justify the chosen sequence:

1. a) draw the final geological map;
2. b) visit outcrops and collect data in the field;
3. c) consult existing maps and aerial photographs.

A hiking group wants to know the altitude of the peaks and the gradient of a route through the Sierra de Gredos. Which type of map should they consult and which graphical feature will provide this information? Explain how to read this feature.

On a topographic map the contour lines pass through elevations of $400\ \text{m}$, $420\ \text{m}$ and $440\ \text{m}$. What is the contour interval of the map? Explain how you would estimate the elevation of a point located between two contours.

A map only shows the numerical scale $1:10\,000$. Explain what this figure means and calculate the real length of a path that measures $4,5\ \text{cm}$ on the map.

On a topographic map at a scale of $1:50\,000$, the distance between two towns is $6,4\ \text{cm}$. Calculate the actual distance between them, expressed in kilometres.

On a map at a scale of $1:25\,000$ the distance between two outcrops is $7,6\ \text{cm}$. Calculate the real distance in metres and, if the difference in elevation between them is $190\ \text{m}$, estimate the average gradient of the route as a percentage.

The diagram shows the contour lines of a hill with a contour interval of $100\ \text{m}$. Compare the gradient of the slope at point A with that at point B and explain, based on the spacing of the contour lines, which of the two you would climb with less effort.

Explain, step by step, how a topographic profile is constructed from the contour lines crossed by a straight line on the map, and what information the profile provides that cannot be seen directly in plan view.

A topographic map shows contour lines very close together at the head of a basin and very far apart on the valley floor. Interpret this pattern in terms of gradient and explain in which zone you would expect more intense fluvial erosion.

On a topographic map with a contour interval of $20\ \text{m}$ and a scale of $1:25\,000$, a north-south mountain range presents asymmetrical slopes: on the west slope, adjacent contour lines are $1\ \text{cm}$ apart on the map, whereas on the east slope they are $4\ \text{mm}$ apart. The geological series consists of alternating limestones and marls with a uniform dip of $18^\circ$ towards the east.

1. a) Calculate the topographic gradient (in percentage and in approximate degrees) of both slopes.
2. b) To build a road, an excavation is planned that will increase the cut slope angle to $30^\circ$ on the east slope. Evaluate the geotechnical stability of both slopes by classifying them as anaclinal or cataclinal and predict, with reasoning, what type of failure or instability this excavation will trigger on the east slope.

Explain in your own words the difference between an aerial photograph and a satellite image, and give a specific example of a geological application for each.

Aerial photographs are taken in overlapping pairs so that they can be viewed through a stereoscope. What advantage does this three-dimensional view of the relief bring to geological study?

In a satellite image of the mouth of the Ebro, a triangular plain with channels branching towards the sea can be seen. What sedimentary landform are you observing and which external geological process formed it?

In aerial photographs taken early in the morning, scarps and faults are easier to recognise than at midday. Explain what role shadows play in the photogeological interpretation of the relief.

In a false-colour satellite image, healthy vegetation appears in reddish tones and bare soil in greys. Explain how this contrast can be used to map rocky outcrops or areas burnt by a fire.

When comparing a frame from the 1956 photogrammetric flight with a current orthophoto of a sandy stretch of coast, a technician observes that the coastline has retreated $60\ \text{m}$ and hastily concludes that the mean sea level has risen at a linear rate of $1\ \text{m}/\text{year}$ due to climatic causes.

1. a) Refute their conclusion by arguing two sources of methodological error linked to the image capture conditions (such as tidal range or seasonal beach profile) that must be corrected before quantifying real retreat.
2. b) Assuming a real net retreat, formulate two hypotheses linking this sedimentary deficit to specific anthropogenic actions (in the river basin or along the coastal strip itself) and describe what evidence you would look for in the images to verify the alteration of longshore drift transport.