50 GEOLOGICAL FIELD MAPPING records strike / dip angle / quadrant, as in 330/30 SW The other gives dip angle/dip direction, as in 30/240 The latter is advised It has fewer components, is less prone to error, and is in the same form as lineation data Use of Field Notebook The notebook is the essential constant companion to the field map While the map depicts the important readings, boundaries, and formations in spatial relationship, the notebook holds further readings, detailed descriptions, drawings, and much extra information The order of use differs amongst geologists, but a common practice is to make a thorough examination of the exposure, record it in the notebook, and then plot the salient features on the map An example of notebook layout and entries appropriate to a mapping survey is shown in Figure Corresponding explanatory notes are also given, extended in a few cases to cover other geological situations Should the field map be lost, it would be possible to reconstruct much of it from the notebook, although not boundaries drawn directly from field observation Drawings should always be made in pencil, to be inked subsequently when the correct line has been achieved Notes can be in pencil or ink Use of Field Map Mapping involves placing a record on the map of: (1) the presence of rocks at the surface (exposures); (2) the type of rock(s) exposed; (3) the orientation of bedding, contacts, and tectonic structures; (4) the nature of superficial cover; and (5) landform features, such as river terraces Information from various sources is combined to allow: (6) boundaries to be drawn on the map, thus giving the ‘outcrops’ of mappable units At each location, proceed as follows Determine the position on the map by GPS, by reference to nearby features and contours, or by resection using two or three compass bearing lines 60 –90 apart Outline the limits of the exposure Identify the rock type(s) and formation, and colour the exposure on the map Record the locality, grid reference, rock descriptions, measurements, drawings, etc., in the notebook If a formation contact is present, plot it on the map Scan surrounding country for clues to extend the line Measure the orientation of bedding and plot the symbol on the map, precisely where the reading was taken Record measurements in 1.5 mm figures Plot other features, such as cleavage, drawing the symbols to the centre of the bedding bar In complex areas, plot the most important structures, leaving others in the notebook No reading should be plotted in a position which was not the point of measurement Add locality number and relevant notes, particularly for key stratigraphical, fossil, and structural localities Notes can be written on unused areas, such as the sea, and linked to the exposure by a dotted line Move to another locality, observing continually If unexposed ground is thought to be of the same formation, shade it more lightly with the same colour If the next exposure is of an entirely different formation, there must be a boundary somewhere in the intervening ground Whether it is sedimentary, igneous, or tectonic is commonly inferred from its geological setting Its position is judged after assessing 30 m cliff of sandstone clearly belongs to a sandstone formation (d) Underline key words (Use the formation colour employed on the map.) (e) Give size, shape, colours, and proportions of the different rock constituents, not just a rock name or mineral list (f) Contacts demand particular attention Is the contact sedimentary, igneous, or tectonic, sharp, gradational, sheared, discordant, etc.? What are the age relationships? With igneous contacts look for features such as chilling, baking, xenoliths, and veins (g) Note the nature of ground between localities, e.g., continuous, patchy or no exposure, type of vegetation (h) Record specimen numbers Collect typical rocks: abnormal ones are additional Make notes if a specimen has been oriented (marked in the field such that it can be oriented identically in the laboratory) (i) Plotting symbols and figures in the margin as they will appear on the map provides a check against possible map errors, allows easy data retrieval, and provides a visual record of changes in orientation (j) Include deductions or implications arising from the geological observations (k) Proportions and thicknesses are more informative than descriptions such as ‘thinly bedded sandstones and siltstones’ If fossils are present, record their identity, size, state, distribution, and abundance (l) In deformed areas, record all planar and linear elements (bedding, cleavages, axial planes, fold axes, lineations) Always draw folds, because readings alone can never convey shape Draw accurately, label the rock types, and avoid ornamental lines that might be mistaken for geological fabric Incorporate a scale and an orientation (N arrow for plan views, viewing direction for sections) It helps to show readings on the drawing (m) Note geological features in view, as well as those at the exposure (n) Record any evidence, or lack of it, for a deduced boundary (o) Conglomerates indicate which rocks were exposed to erosion at a particular time The proportions and shapes of the constituents provide evidence on transport and depositional environment (p) The measurement of bedding and truncated foresets shows younging of the sequence and gives information to calculate the current flow vector Other sedimentary structures have similar potential