Field Stuff
Fieldwork is one of the most important and exciting thing for a geologist. You get the adventure, happiness and most importantly you get the chance to "hear" the tales of the earth. I have made an attempt to give a list of fieldwork procedure for an amateur field geologist.
Things you need for Fieldwork:
Geological Hammer (Essential), Clinometer/Brunton, GPS, Pocket lens (10X/40X), Dilute hydrochloric acid (dil. HCl Pocket bottle),Stationary(Pen, Marker, pencil, scale), Meter Tape, Sampling Bags, Field Diary, Haversack to keep all the field stuff, Sledge Hammer(if possible) .
Pre-Fieldwork:
Read about the study area as much as possible through literature/internet.
Go through the Satellite Images/ Google Earth Images/ Toposheet of the map thoroughly.
By studying these images/maps you :
Will get to know the general elevation profile of your area
Can mark various lineaments, change in lithology, identify different structures on a regional scale.
Mark all the lineaments, structures, other features or any spot (one's your curious about) you can see on the images/maps. If possible note down the GPS reading of these spots from Google Earth.
It's important to note down all you can interpret from the maps because it will give you a sort of head start in analyzing geology your area.
You'll be in a better position to envision the geological history of the area.
Fieldwork:
Study the outcrop properly before u hammer the outcrop .Note down all the different features you can look at the outcrop .
To interpret the rock in the area, break the rock and look for a fresh surface of the rock.
The most important characters of a rock are: texture and mineral composition.
Identifying the texture is first and most important step in identifying rocks.
Identify the minerals with the help of lens (look for the diagnostic characters of minerals for eg: muscovite/biotite pearly lustre, one set cleavage ; quartz : vitreous lustre (they kinda look like oily grains), no cleavage; calcite : rhombohedron crystal, 3 sets cleavage ).
Based on the texture and mineralogy, identify the rock.
Take readings about the attitude of the beds/dyke/veins using a brunton/clinometer. Note down the thickness of the beds/dyke/veins using a meter tape.
Take structural readings such as bearing of lineation, plunge of fold, attitude of fault etc (if present)
Interpret the structures (if present) , write down in your own words a line or two about its significance.
Take a picture of the outcrop
Draw a sketch of the outcrop.
Sketch and Picture of the Listric Fault
Few Points to remember while taking a picture of the outcrop:
Don’t let your shadow or anybody’s shadow fall on the the outcrop while you are taking the picture.
If it’s not possible to avoid the shadow, see to it that the whole outcrop is covered with shadow while the picture is been clicked.
Always keep a scale(eg. Hammer, Pen, coin, Human ) in the picture, scale depending upon the size of the feature.
Always keep the structure/feature of the outcrop at the center and the scale at the side of the picture.Never let the scale block the structure/feature.
If the feature/structure is a linear one (eg. Micro-fault) keep scale such as pen, and if the feature is a circular/elliptical/rounded keep scale such as coin.
Field Photo enhanced using Photoshop
Few Points to remember while sketching the outcrop:
Always draw using a relative scale with respect to different features on the outcrop. Dont draw the more important features big and less important small.
Details of the important features would be appreciated.
If possible try drawing the outcrop with the scale (eg. Hammer, Pen,) on it too. It will help in visualizing the outcrop better. If drawing the scale isn’t possible, then draw trees/ shrubs and write down the approx height of the tree/shrubs below the sketch.
Sketching a vertical profile by visualizing and combing information gathered at different spots :
Sketching outcrops comes in handy while creating vertical sections of the area as we get a “sense” of the dimensions, distances between various features.
A vertical profile sketch can only be drawn through proper visualization of the area and its structures/features. All one needs to do is integrate all the different structures/features at different spots, visualize and then sketch it out.
Creating a vertical profile needs proper understanding of the outcrop and it helps in simplifying in making your interpretation about the area too.(as one hardly gets a typical vertical section outcrop)
A vertical profile sketch
Tips:
Hand Specimen Mineralogy (Diagnostic Characters of some common mineral/ Mineral Groups)
Quartz: Vitreous Lustre, no cleavage
Feldspar Group: 2 set of cleavage (cleavage angle 90), Specific Gravity lower than pyroxenes, usually light coloured
Calcite: Rhombohedron crystal form, 3 sets cleavage, will produce effervescence if treated with dil HCl
Pyroxene group Minerals: 2 set Cleavage(cleavage angle 90), Specific gravity higher than feldspar, usually dark coloured
Amphibole Group Minerals: 2 set cleavage (cleavage angle 120)
Mica Group Minerals: Pearly Lustre, Flaky/foliated, one set of cleavage.
To identify minerals quickly one needs practice and needs to know the properties of minerals, knowing at least the Diagnostic properties(two /three properties of minerals that are typical of that mineral and helps you identify quickly, each mineral has different diagnostic property) of common rock forming minerals.
Note down the properties that strikes you first(diagnostic properties) and then using these 2 or 3 properties, identify the mineral.
If one can’t identify the mineral still, then look for the following properties (order that I would prefer) and then identify:
Form
Cleavage
Hardness
Specific Gravity
Streak(if it is an ore mineral)
Lustre
Colour
Specific Gravity is a very important property and one needs to get “sense” of the approx sp.gr of the mineral while identifying , which needs more fieldworks and practice. It can give you an indirect indication of the chemical composition of the mineral if one combines it with other properties.
Sampling:
First see the strength of the the rocks approximately and then break the rocks accordingly.
Try not breaking huge chunks of the outcrop and cut down the huge samples to small and proper sample so that only the important and necessary amount of sample is being carried .
Always label the samples
Enjoy Hammering :)
Acknowledgement: Sushant Navarat (Sketches).
