Unit 4: Basics of Geological Field Work

Uses of Field Tools

Geological field work is the foundation of geology. It requires a specific set of tools for observation, measurement, and collection.

Clinometer

A Clinometer is an instrument used to measure vertical angles. This includes:

• The angle of a slope.
• The dip angle of a rock bed.
• The plunge of a lineation.

It works using a simple pendulum or a weighted rotating wheel. When you align the instrument's sight with the slope, the pendulum hangs vertically (due to gravity), and the angle between the sight and the pendulum can be read. In most geological compasses, this function is built-in.

Brunton Compass

The Brunton Compass is the most essential and versatile tool for a geologist. It is a high-precision magnetic compass that combines three instruments in one:

1. Magnetic Compass: For navigation. The needle points to magnetic north, allowing you to orient your map and find your bearing.
2. Sighting Compass: Using the sights and mirror, you can measure the precise horizontal bearing (azimuth) from you to a distant object. This is used for mapping and traversing.
3. Clinometer: It has a built-in clinometer (a "bubble level" attached to a rotating scale) used to measure vertical angles, most importantly, the dip angle of a rock layer.

Planning Traverse

Traverse: A traverse is a pre-planned route or path that a geologist walks to make observations and create a geological map.

Why Plan? You cannot just wander randomly. A planned traverse ensures you cover the area systematically and efficiently.

Principles of Traverse Planning:

• Goal: To observe the maximum number of rock types and structures.
• Best Practice: Plan traverses that run perpendicular to the "strike" (or "grain") of the geology. This ensures you walk across all the different rock layers in sequence.
• Practical Routes: Traverses are often planned along existing paths for accessibility, such as:
  • Road cuts
  • River or stream beds
  • Ridgelines
  • Hiking trails
• The route is drawn on a toposheet, and observations are recorded along the way.

Locating Outcrops

Outcrop (or "crop"): An exposure of bedrock at the Earth's surface. It is a place where the rock is not covered by soil, vegetation, water, or man-made structures.

Outcrops are the "data points" for a geologist. The rest of the geology is inferred (guessed) by connecting the data from outcrops.

Where to Find Outcrops:

• Natural: River banks, sea cliffs, steep mountain slopes, ridges.
• Man-made: Road cuts, railway cuttings, quarries, mine pits.

Locating on a Map:

When you find an outcrop, you must record its location accurately:

• Modern Method: Use a GPS to get the precise latitude and longitude (or grid reference) and plot it on the map.
• Traditional Method: Use triangulation (sighting bearings to two or more known landmarks like mountain peaks or temples) or resection.

Concept of Bed, Dip, and Strike

These are the fundamental measurements used to describe the orientation of planar structures (like sedimentary beds, faults, or metamorphic foliation) in 3D space.

Bed

A "bed" is a single, distinct layer of rock. In sedimentary rocks, this is a layer of sand, mud, or limestone deposited at one time. We measure the orientation of the bedding planes (the top and bottom surfaces of the bed).

Strike

Strike: The compass direction (bearing) of a horizontal line on the surface of an inclined plane (the bed).

• Analogy: If you have a tilted roof, the "strike" is the direction of the ridge line at the peak.
• How to Measure: Place the *long edge* of the Brunton compass (held horizontally) against the bedding plane. The direction the compass needle points is the strike line.
• Example: N45°E (or 045°).

Dip

Dip: The maximum angle of inclination of the bed, measured perpendicular to the strike.

Dip has two parts:

1. Dip Angle: The angle (from 0° to 90°) between the bedding plane and a horizontal plane.
   • A horizontal bed has 0° dip.
   • A vertical bed has 90° dip.
   • This is measured using the clinometer.
2. Dip Direction: The compass direction *towards which* the bed is sloping (the "downhill" direction). This direction is always 90° (perpendicular) to the strike direction.