Maps and Landscape

Rules for Interpreting Geological Maps

Interpreting the Moreton-in-Marsh Sheet

Geological History from the Cheddar Sheet

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Rules for Interpreting Geological Maps

The table below is a summary of the rules for interpreting geological maps, taken from Maps and Landscape.

Dip is towards younger strata.

Younger strata crop out in the direction of dip.

Where geological boundaries between younger strata are parallel to the topographic contours the strata must be horizontal.

Outcrops of horizontal strata usually form prominent V-shaped patterns that point upstream in valleys.

For horizontal or gently dipping strata of constant thickness, narrow strip-like outcrops occur on steep slopes and wide outcrops occur on gentle slopes.

Where younger rocks lie against older rocks along a fault displaying vertical displacement, the younger rocks occur on the downthrow side.

Outcrop Vs of dipping strata typically point in the direction of dip.

The more steeply the strata dip, the less pronounced is the V-pattern where the outcrop crosses a valley.

A vertical stratum does not show an outcrop V-pattern as it crosses a valley. Its outcrop will be straight.

The width of outcrop of a vertical stratum is always equal to its true thickness, and so can be determined directly from the map. It is unaffected by topography.

On overturned strata beds young in the opposite direction to dip (Ref. Fig. 4.14b).

Folding is later than the deposition of the youngest sediment that has been folded and earlier than the oldest rock which is unaffected by the folding.

Strike-slip faults can have the largest displacement of all faults (horizontal parallel to the direction of strike).

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Interpreting the Moreton-in-Marsh Sheet

Try the exercises in the table below. Instructions and questions are in the left-hand column.

Questions	Links to Answers
Locate the Moreton-in-Marsh sheet on the 10-Mile Map.	Do this one on your own!
On the 10-Mile Map, relate the dip & strike of the Jurassic to that of the rest of the geology of the south of England.	Go to answer
What are the oldest rocks known in the Moreton-in-Marsh area?	Go to answer
Are there any unconformities exposed in the Moreton-in-Marsh area? Does this mean that there was continuous deposition during the time when the Jurassic sequence in the Moreton-in-Marsh area was being laid down?	Go to answer
What does one call the exposure of the Forest Marble at SP 143279?	Go to answer
Why do sedimentary rock units vary in thickness?	Go to answer
Why do faults "give out"?	Go to answer
What kind of force acts on the crust to produce a normal fault? A reverse fault?	Go to answer
Is the rule that younger rocks occur on the downthrow side of a fault true of reverse faults as well as normal faults?	Go to answer
Why is it not possible to determine the ages of drift deposits with precision (both relative and absolute ages?)	Go to answer
How deep is the cross-section relative to the average thickness of continental crust?	Go to answer
How thick is continental crust relative to the radius of the Earth?	Go to answer
Locate Reading on the 10-Mile Map. What geological structure underlies the Reading area? Can you relate the geology of the Reading area to landforms?	Do this one on your own!

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Answers to questions about the Moreton-in-Marsh Sheet

Geological History from the Cheddar Sheet

Evidence	Event	Age	Interpretation
1. Portishead Beds	Deposition of clastic sediments	Devonian	Continental environment
2.	Marine transgression
3 Carboniferous Limestone series	Deposition of marine sequence (clastic & calcareous sediments)	Carboniferous	Shallow marine environment
4 Unconformity	Uplift & erosion	Namurian or post-Namurian
5. Folds & faults affecting Devonian & Carboniferous rocks	Folding & faulting	Namurian or post-Namurian	Tectonic compression
6. Mercia Mudstone Group & Penarth Group	Deposition of clastic sediments	Triassic	Deposition on uplifted land surface
7.	Marine transgression
8. Lower Lias	Deposition of marine sequence	Jurassic	Shallow marine environment
9. Gap in sedimentary sequence	Uplift & erosion	Jurassic to Tertiary
10. Faults affecting Triassic & Jurassic rocks	Faulting	post Lower Liassic (very early Jurassic)	Tectonic stresses
11. Drift deposits		Quaternary - Recent	Deposition of superficial sediments onto uplifted land surface

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On the 10-Mile Map, relate the dip & strike of the Jurassic to that of the rest of the geology of the south of England.	Broadly conforms to the NE/SW strike of the Jurassic strata across England. Return to question
What are the oldest rocks known in the Moreton-in-Marsh area?	Silurian limestones, from borehole evidence. Return to question
Are there any unconformities exposed in the Moreton-in-Marsh area? Does this mean that there was continuous deposition during the time when the Jurassic sequence in the Moreton-in-Marsh area was being laid down?	No; unconformities only known in borehole evidence. But continuous deposition cannot be assumed, from the nature of sedimentary processes. Return to question
What does one call the exposure of the Forest Marble at SP 143279?	An outlier, bounded by normal faults – Forest Marble has been downthrown against the Great Oolite & Chipping Norton Limestone. Return to question
Why do sedimentary rock units vary in thickness?	Wedging out can indicate (1) lateral changes in depositional environments; (2) variations in proximity and type of sediment sources. Also may explain lateral grading of one sediment into another, where they were deposited at the same time. Return to question
Why do faults "give out"?	The throw on a fault can vary from end to end. Return to question
What kind of force acts on the crust to produce a normal fault? A reverse fault?	Extensional force gives a normal fault; compressional force gives a reverse fault. Return to question
Is the rule that younger rocks occur on the downthrow side of a fault true of reverse faults as well as normal faults?	Yes! Return to question
Why is it not possible to determine the ages of drift deposits with precision (both relative and absolute ages?)	Drift deposits are superficial and unconsolidated. They are unlikely to be cut by igneous intrusions which can be dated. They are likely to lie across any hard rocks exposed at the surface. Return to question
How deep is the cross-section relative to the average thickness of continental crust?	The cross-section extends 2 km deep beneath sea level, compared to an average 35 km thickness for continental crust. The thickness of continental crust beneath mountains can reach about 75 km. Return to question
How thick is continental crust relative to the radius of the Earth?	The radius of the Earth is 6371 km. Return to question