A primary geological structure is one formed
Answer: A
Bedding, cross-bedding, ripple marks and columnar jointing all originate with the rock and record its environment. Folds and faults are secondary, being imposed afterwards by stresses acting on rock that already existed.
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Bedding in a sedimentary rock is classed as a
Answer: B
It records successive episodes of deposition and so is part of the rock's formation. It nonetheless acts as a plane of weakness in engineering works, which is why a primary structure can matter as much as a secondary one.
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The dip of a bed is the angle between the bed and the
Answer: C
Correct answer: horizontal, measured in a vertical plane
Dip is the angle of steepest descent of the bedding surface below the horizontal, and it must be measured in the vertical plane containing that steepest line. Measured in any other vertical plane the angle comes out smaller, which is the apparent dip.
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The strike of a bed is
Answer: D
Correct answer: the horizontal line lying in the plane of the bed
Strike is the one direction in the plane of the bed along which there is no rise or fall - the intersection of the bed with a horizontal plane. It is recorded as a compass bearing, and together with the dip it fixes the attitude of the bed completely.
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The angle between the strike and the direction of true dip is
Answer: A
Correct answer: 90 degrees
True dip is the direction of steepest descent, and that is necessarily perpendicular to the horizontal line in the same plane. The relation is a matter of geometry rather than of geology, and it is why a strike and dip can be plotted as a single T-shaped symbol on a map.
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An apparent dip is always
Answer: B
Correct answer: less than the true dip
True dip is the steepest possible line in the plane, so any section not perpendicular to the strike cuts the bed obliquely and shows a gentler angle: \[ \tan\delta_{app} = \tan\delta \cos\theta, \] where \( \theta \) is the angle from the true dip direction. The apparent dip falls to zero along the strike itself.
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A horizontal bed on a geological map has an outcrop that
Answer: C
Correct answer: follows the topographic contours
A horizontal bed has a constant elevation, so it crops out wherever the ground reaches that elevation - which is the definition of a contour. The outcrop trace and the contour of the same height therefore coincide, and this is the quickest way to identify horizontal strata on a map.
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The one attitude whose outcrop trace is unaffected by the relief is a bed that is
Answer: D
A vertical bed has no horizontal extent to be cut differently at different heights, so its trace is simply the strike line drawn straight across the map. Every other attitude has its outcrop deflected by valleys and ridges.
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The rule of Vs states that a dipping bed crossing a valley forms a V pointing
Answer: A
Correct answer: in the direction of dip
Where a dipping bed crosses a valley the outcrop is deflected up-valley, and the apex of the resulting V points in the direction of dip. It is the most useful rule on a geological map, since it lets the dip direction be read from the outcrop pattern alone with no symbol present. Vertical beds cross valleys in a straight line.
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For a given bed thickness, the width of outcrop on level ground is greatest when the dip is
Answer: B
A gently inclined bed is cut at a shallow angle by the ground surface and so crops out over a wide strip, while a vertical bed crops out over its own thickness. The relation is the basis of estimating thickness from a map.
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