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S260 Geology Block One - Maps and Landscape
I found block one quite difficult as there's a lot to take in, and it's hard to visualise some of the geological features without seeing them on the ground in 3D. I hope to come back to this section once I've completed the summer school and have more experience of geological maps.
Introduction to block one
The block starts off with an introduction to geological time and geological maps. I found it fascinating to put 'life on Earth' in the context of the age of the planet. The ten mile geological maps contain a wealth of information, it's important to be systematic and precise when examining them.
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Landscapes and geology
This section starts with a video explaining the geology of Dorset, and introducing the cdrom activities. The course uses a virtual field trip that gives a 360° panoramic view of various locations around the Island of Skye. I've done a lot of general map reading in the past, but map reading in a geological context is very painstaking and exact - there are a lot of new concepts to learn but it's starting to come together. I'm keeping notes as I go, to help with revision.
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Strike and dip
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Gradients
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Unconformities - a structural boundary between two sedimentary rock sequences, usually with a significant difference in dip, caused by uplift and erosion between the two sequences. Unconformities usually represent a long period of geological time( millions of years).
inliers - older rocks completely surrounded by younger rocks (seen as an indent in cross section)
outliers - younger rocks completely surrounded by older rocks (seen as an outcrop in cross section)
Faults - deformation of rocks where the layers of rock fracture and move relative to each other.
Folds - bends or kinks in sedimentary strata
• syncline (trough)
• anticline (arch)
Dykes - vertical igneous rock intrusions
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The Moreton-in-Marsh sheet
Section three uses the Moreton in Marsh sheet to explain geological outcrop patterns in greater detail, and in relation to a map. Several general rules are introduced such as:
"Where geological boundaries between strata are parallel to the topographic contours, the strata must be horizontal."
"Outcrops of horizontal or gently inclined 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".
"An outlier is an outcrop of rocks separated from the main outcrop by the process of erosion and surrounded entirely byolder. strata. An inlier is an outcrop of rock that has been exposed by erosion of younger strata in such a way that it is entirely surrounded by the younger strata remaining."
"Where younger rocks lie against older rocks along a fault displaying vertical displacement, the younger rocks occur on the downthrow side."
For normal faults, the fault plane is either vertical or dips towards the downthrow block, while for reverse faults the fault plane dips towards the upthrown block."
The age of a fault must be younger than the youngest bed (or any other geological feature) which is unaffected by the faulting. In other words, the youngest rocks or features that are cut by a given fault give the maximum age of the faulting and the oldest rocks or features unaffected by a fault give the minimum age of faulting."
The cheddar sheet
The final section of block one focuses on the Cheddar map sheet (ST45). Various concepts from earlier sections were explained in detail,a nd it's slowly coming together.
Strike - the direction of a horizontal line on an inclined surface.
Dip - the angle of maximum slope of an inclined surface.
"Younger strata crop out in the direction of dip"
"Outcrop V's of dipping strata on geological maps typically point in the direction of dip."
A vertical stratum, or any other vertical geological feature, does not show an outcrop V-pattern as it crosses a valley. Instead it's outcrop will be straight."
Thickness and width of outcrop
true thickness (t) = width of outcrop (w) x sine of angle of dip (θ)
t = w x sin θ
vertical thickness (v) = width of outcrop (w) x tangent of angle of dip (θ)
v = w x tan θ
cos θ = t / v
v = t / cos θ
"Older strata are found towards the core of an anticline, and younger strata towards the core of a syncline"
"Folding is later than the deposition of the youngest sediment that had been folded and earlier than the oldest rock which is unaffected by the folding"
Exam revision - map work
20% of the final exam involves a map work question, so it's worth learning how to tackle the map work in a methodical way. Be familiar with, and able to quickly recognise geological features such as unconformities, synclines and anticlines.
***Get familiar with block one bookmark (allowed in exam)***
• Read question carefully - **check scale**.
• Identify unconformities and mark on map and cross section with a wavy line.
• Mark off vertical features (remember igneous rocks cut through sedimentary sequences).
• Don't waste time trying to work out the throw of a fault - treat each side of the fault as separate in the cross section and the throw should become clear when you see it in cross section.
• Carefully and accurately mark off outcrops (use a protractor if given dip angles), identify synclines and anticlines from dip markings or the rule of V's.
• Mark off rock units.
• Use question marks(?) to show unknown detail.
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URL of this page:[ http://www.larnach.info/S260/one.htm ]
General Geology links
Block one links
Ordnance Survey
UCMP time scale
Books
