Outcrop Sketch and Field Description
Photograph the face, draw the bed boundaries with a finger, describe each bed — and get an annotated sketch and a measured section.
The field sketch is the oldest tool of the geologist and still the best record of an outcrop. Here it is drawn on a photo of the face or on a blank sheet: bed boundaries, faults, joints and veins are traced with a finger, fossils, samples and dip notes are placed with a tap, and the scale is set from an object of known length. The beds between the boundaries are numbered from the base and described with lithology, colour, grain size, bedding thickness, veins, fossils, joints, sedimentary and tectonic structures and landform. The thickness of each bed is measured on the picture. The result is an annotated sketch and a measured section (log) with lithology patterns and grain-size widths, a bed table and a project file.
Overview
Drawing forces observation: to trace a boundary you must find it, and to number a bed you must decide where it begins and ends. The tool keeps that discipline and removes the chores: the numbering, the thickness, the neat copy and the log.
It works without a network, and the sketch is kept on the phone as you draw. The dip and strike measured with the field station can be written on the sketch.
What you enter · What you get
What you enter
- Photo of the outcrop or a blank sheet
- Lines drawn by finger or mouse: boundaries, faults, joints, veins, scale
- Symbols: fossils, samples, dip and notes
- Description of each bed
What you get
- Annotated sketch with scale and viewing direction
- Measured section (log) with lithology patterns and grain size
- Bed thicknesses from the picture
- Bed table (CSV) and project file
How it works in the field
Method and equations
Scale
A line drawn over an object of known length L gives the size of a pixel on the face.
Bed thickness
At the centre of the picture the gap between the two boundaries is measured and corrected for their inclination in the picture, giving the thickness perpendicular to bedding in the plane of the photo.
Description
Grain sizes follow the Udden–Wentworth scale and bedding thickness the terms of Ingram (1954); the log is drawn with the width proportional to grain size, as is the convention for sedimentary logs.
Accuracy and error limits
- Thickness from the photo is within about 5–10 % when the photo is taken square to a vertical face and the scale object lies on the face; it is the apparent thickness in the plane of the photo, so where the beds dip toward or away from you it differs from the true thickness. The description is as good as the observation.
- What is not covered: perspective on oblique or tall faces (the scale changes from the bottom to the top of the picture), beds thinner than a few pixels, and the correction of apparent to true thickness for dip into the face.
- To reduce the error: stand square to the face and far enough to fit the section with little tilt; put the staff on the face at mid-height; split tall faces into several photos; measure key beds with a tape and enter the thickness; and correct for dip with t = apparent × sin of the angle between the face and the bedding where it matters.
Who uses it
Standards and references
- Coe, A.L. (ed.) (2010). Geological Field Techniques. Wiley-Blackwell.
- Tucker, M.E. (2011). Sedimentary Rocks in the Field: A Practical Guide, 4th ed. Wiley-Blackwell.
- Compton, R.R. (1985). Geology in the Field. Wiley.
- Stow, D.A.V. (2005). Sedimentary Rocks in the Field: A Colour Guide. Manson Publishing.
- Wentworth, C.K. (1922). A scale of grade and class terms for clastic sediments. Journal of Geology, 30(5), 377–392.
- Ingram, R.L. (1954). Terminology for the thickness of stratification and parting units in sedimentary rocks. GSA Bulletin, 65(9), 937–938.
- Federal Geographic Data Committee (2006). FGDC Digital Cartographic Standard for Geologic Map Symbolization. FGDC-STD-013-2006.
At a glance
- Category
- Field geology
- Runs on
- Phone, tablet and desktop browser
- Export
- Annotated sketch and log as PNG, JPG and PDF; beds as CSV; project file (JSON); full PDF report
- Access
- Free
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