Map interpretation

What do topographic contours represent on a map?

A contour line is a line that links heights of equal elevation and is always horizontal

What is the definition of a fault?

A fault is a planar feature across which displacement of the rocks on either side has occurred

What is an anticline?

A fold in which the oldest rocks are in the core of the fold

What is a syncline?

A fold in which the youngest rocks are in the core of the fold

What does it mean when we say that a fold is plunging?

A fold is plunging when the hinge line is inclined from the horizontal

What is a synform?

A folded surface that closes downwards

What is an antiform?

A folded surface that closes upwards

what is the movement on a normal fault?

A normal fault is where the hanging wall moves downward relative to the footwall

what is the movement on a reverse fault?

A reverse fault is where the hanging wall moves upwards relative to the footwall

What is a terrane?

A terrane is a fragment of crust that is bounded by faults and has a geological history distinct from its surrounding areas

Why is the strike always written as 3 digits?

As it is a compass bearing or azimuth

How do you attach a younging symbol to a bedding reading?

At 90 to strike direction

Where on the stereonet are bearings read and where are dips and plunges read?

Bearings are read around the primitive- outer edge of stereonet Dips and plunges are read along the equator

Name some sensible geological relevant planes that you could measure the orientation of

Bedding, cleavage, fault plane

Name some sensible geological relevant lineations that you could measure the orientation of?

Bedding/ cleavage intersection, fold hinge lines, slickenlines

What does azimuth mean?

Compass bearing

What do stratum contours represent on a map?

Contours that can be drawn on a geological surface, this is normally bedding surface but it could be a fault plane surface or the surface of a coal seam. They allow us to represent the 3 dimensional shape of the rock surfaces below the ground or even above before they were eroded to present land surface

What is the difference between exposure and outcrop?

Exposure is where the rock is exposed on the surface and you can stand on it and measure the orientation of the bedding planes Outcrop is what we see on a geological map and it shows where a formation would occur if it were exposed on the surface

What is a strike slip fault?

Fault that has movement parallel to the strike of the fault

What is a dip-slip fault?

Faults that have movement parallel to the dip of the fault

What is the relationship between bedding and cleavage in folded rocks?

Fold axial planes and cleavage both form perpendicular to the direction of maximum compression. When folding and cleavage form in the same stress field then the cleavage planes and the fold axial planes will be approx parallel to each other

What is the heave of a fault?

Horizontal component of displacement

Draw the stratum contours on an area with flat topography for a uniformly dipping plane, a non-plunging anticline, a plunging syncline and a steeply dipping dip slip fault

If the geological surface is planar then then stratum contours will be straight, parallel and equidistant the strike of the surface is parallel to the strike of the stratum contours If the stratum contours are curved then the strike of the geological surface will vary and follow the curves If the separation between the stratum contour varies then the dip of the geological surface varies

How would you estimate the position of a fold axis?

If you know the orientation of the profile plane them the fold axis is the pole to this great circle. The fold axial plane must pass through the fold axis and bisects the poles to bedding on the 2 limbs

What descriptive name do we give to a fold whose fold axis dips between 0-10 degrees?

Non-plunging

how do we recognise a fault on a geological map?

Normally drawn as a black line

In the context of map interpretation what is a geological contact?

On a geological map the line is drawn between 2 different formations is called the contact. We can show how confident we are about the position of a contact by using different symbols. A solid lines means we are absolutely confident. A dashed line means that there are some uncertainties and a dotted line is unsure

Where will the fold axes of non plunging folds plot on the stereonet?

On the primitive

What are slickenlines and why are they useful in interpreting the movement of a fault?

Some exposed fault surface may display linear features produced by frictional wear or scratching. They indicate the direction of displacement of the fault

What descriptive name do we give to a fold whose fold axis dips between 60-80 degrees?

Steeply plunging

How does the contact between two uniformly dipping surfaces crop out across a valley? What if the bed are horizontal? What if the beds are vertical?

The contact will "V" in the direction of the dip, but can point either upstream or downstream. The sharpness of the V depends on the dip of the bedding surface and so its shape is useful for working out dip of beds Horizontal beds follow stratigraphic contours Vertical beds- topography will have no effect on the outcrop of bed

What is cataclasite?

The crushed rock that forms in a zone between the fault planes

What are the strike and dip of a plane? Give an example of a strike/dip reading

The horizontal line (contour) which can be drawn on a dipping surface= the strike line ( this is also the same stratum contour). The dip of a plane is the angle which that plane makes with the horizontal measured at 90 to the strike 030/40SE

On a geological map how can you tell if a fold is plunging?

The limbs will not be parallel to each other and you may see the beds come around in a curve

What does the bedding/cleavage asymmetry relationship tell us?

The orientation of the cleavage will be constant across the fold but because the dips of the beds change across the limbs of the fold then angle between bedding and cleavage will change in a constant manner round the fold

What are the plunge and plunge direction of a lineation? Give an example of a plunge/plunge direction reading

The plunge of a line is the angle between that line and the horizontal. The plunge direction is the compass bearing towards which the line is tilted. 23/060

What is the pole to a plane and why do we use them?

The pole is an imaginary line at right angles (perpendicular) to the plane. Poles are much quicker and neater to plot and help to reveal patterns to show us the geometry of features like folds

What is the displacement of a fault?

The relative offset of points that were adjacent now on either side of the fault

What is a hiatus?

This is the amount of time represented by the unconformity, showing what is missing from the rock record

What is the throw of a fault?

Vertical component of displacement

How can you work out which is the downthrown side of a fault shown on a geological map?

When you cross the fault if you pass into younger rocks you are passing onto the downthrow

How would you recognise an unconformity on a geological map?

You would look for the younger beds being in contact with more than one of the older formations below it

What is an unconformity?

a gap in the geologic record

What does the great circle that contains the poles to bedding of a folded sequence of rocks represent?

profile plane of the fold