GEOLOGICAL MAPS AND SECTIONS.
The writer does not propose to give an account of the intricacies of
geological mapping, for their right consideration requires a separate
treatise[22]; all he desires is to call attention to some of the uses
of geological maps as a means of conveying information. A geological
map may be looked upon as an attempt to express as far as possible in
two dimensions phenomena which possess three dimensions; this can be
done to some extent on the actual surface of the map, by conventional
signs, still more fully, by supplementing the map with sections; but
best of all by a geological model, which is cut across in various
directions in order to show the underground structure as well as that
of the surface.
[Footnote 22: The student is recommended to consult in particular,
Appendix I. "On Geological Surveying" in _The Student's Manual of
Geology_, by J. B. Jukes (Third Edition, Edited by A. Geikie), p. 747,
and _Outlines of Field Geology_, by Sir A. Geikie (Macmillan and
Co.).]
The ordinary geological map is one which shows the outcrop of the
strata, subdivided according to age, as they would be seen upon the
surface of the earth after stripping off the superficial
accumulations, and it is to be feared that the term 'geological map'
is associated in the minds of most students with a map of this
character and of no other. Nevertheless, a great many most important
observations other than those connected with the order of succession
of the strata are capable of representation upon a geological map, and
the possession of a large number of maps of any area upon the geology
of which a person is engaged--each map to be used for recording
observations of a particular kind--will save much writing in
note-books and, what is of more importance, will allow him to compare
observations which have been made at different times at a glance,
instead of causing him to search through a series of note-books.
Still, however well furnished with maps, the geologist will find a
note-book essential[23].
[Footnote 23: As a result of some experience, the writer recommends
every student to acquire some skill in the use of the pencil, and if
to such a degree that he can combine artistic effect with accuracy, so
much the better. An acquaintance with photography is invaluable: often
the possession of a camera would enable a section to be recorded,
which is otherwise lost to science.]
The earliest geological maps represented the variations in the surface
soils, or at most the general lithological characters of the rocks
which by their decay furnished the materials for the soils. We have
seen that the first chronological map was due to William Smith, and
most subsequent English geological maps have been based upon his map
of the strata of England and Wales. The order of succession of the
strata is represented in these maps to some extent by the use of
arrows to indicate the direction of dip of the strata, though this is
not an unerring guide where strata are reversed, and accordingly the
addition of a legend at the side of the map may be looked upon as
essential to the correct understanding of the map itself. The legend
is usually in the form of a section of a column, the strata being
arranged in right order, the oldest at the base and the newest at the
summit, the colours by which the strata are indicated being similar to
those placed upon the map. Other information besides the mere order of
succession of the strata may appear in the legend; thus their relative
and actual thicknesses can be indicated if the column is drawn to some
definite scale, and a brief description of the lithological characters
of the rocks may well be appended to the side of the column. On the
actual maps it is customary to exhibit the outcrop of the junctions of
all igneous rocks as well as of the sedimentary ones: the nature of
the metamorphism which sedimentary rocks have undergone at the contact
with igneous ones may be and often is indicated by suitable signs; the
position of faults is shown, and often also that of metalliferous
veins, the nature of the ore in the latter being further indicated in
some suitable manner, as by giving the recognised symbol for the
metal; and in many maps an attempt is made to show the variations in
dip and strike of the cleavage-planes.
The Geological Survey of the United Kingdom publishes two sets of
maps, one showing the 'solid geology' and the other the 'superficial
geology.' It is easier to understand these terms than to define them,
for in Britain there is a sharp line between the two everywhere except
near Cromer. The maps showing the superficial geology represent
gravels, glacial drifts and other incoherent accumulations of
geologically recent origin, which to a greater or less extent mask the
strata below which are usually composed of more or less solidified
material. The maps showing the solid geology display the outcrops of
these strata, though it is usual to insert alluvium upon these maps,
as it is often impossible to trace the junction-lines of the strata
below it. Attention has already been directed to the fact that these
maps of solid geology, though chronological, that is, having the
strata represented according to age, are founded largely upon
lithological differences, rather than upon included organisms; and it
has been stated that for theoretical purposes two sets of
chronological maps, one founded upon lithological differences, the
other upon difference of fossil organisms, would be extremely
valuable.
Other phenomena are often best represented upon separate maps, for if
all observations are crowded upon one map the result will be very
confusing. Special glacial maps showing the contour of the country,
with the portions between the contour lines coloured differently
according to altitude, say the country between sea-level and 500 feet
light green, that between 500 and 1000 dark green, that between 1000
and 1500 light brown and so on, exhibiting the direction of all
observed glacial striae, the distribution of boulders so far as it is
possible, and any other glacial phenomena which can be noted upon the
map, will be valuable to the student of glaciation[24].
[Footnote 24: For examples see Tiddeman, R. H., "Evidence for the
Ice-Sheet in North Lancashire and the adjacent parts of Yorkshire and
Westmorland," _Quart. Journ. Geol. Soc._, vol. XXVIII. pl. XXX., and
Goodchild, J. G., "Glacial Phenomena of the Eden Valley" &c., _Quart.
Journ. Geol. Soc._, vol. XXXI. pl. II.; and for a map of distribution
of boulders, Ward, J. C., "Geology of the Northern Part of the English
Lake District" (_Mem. Geol. Survey_), pl. IV.]
Various structural features may be well displayed on separate maps.
The trend of the axes of folds will be useful, and may be accompanied
by other information of cognate character[25]; maps of the
distribution of joint planes may be given in combination with those
showing the folding of the strata if it be desired to exhibit the
relationship between these; or with the physical features of the
country, if the dependence of physical features upon joint structure
be under consideration[26]. Much information concerning cleavage may
be acquired from a map showing anticlinal and synclinal axes of
cleavage[27], or the actual strike of the cleavage over different
parts of a map may be represented, and its relationship to the
geological structure of the district exhibited[28].
[Footnote 25: See Bertrand, M., "Sur le Raccordement des Bassins
Houillers du nord de la France et du sud de l'Angleterre," _Annales
des Mines_, Jan. 1893, Plate 1.]
[Footnote 26: See Daubrée, A., _Études Synthétiques de Géologie
Expérimentale_, 1^{ère} Partie, Plates III.-VI., for an example of the
latter, which is also interesting as showing the utility of a map on
transparent paper super-posed on another, when illustrating the
connexion between two sets of structures.]
[Footnote 27: Ward, J. C., _Geology of the Northern Part of the
English Lake District_, Plate IX.]
[Footnote 28: Harker, Alfred, "The Bala Volcanic Series of
Caernarvonshire" (_Sedgwick Essay_ for 1888), Fig. 5.]
Maps exhibiting changes in physical geography appertain to the
geologist as well as to the geographer. The position of ancient
beaches, former lakes, representation of the changes in the courses of
rivers and kindred phenomena may be shown upon maps, and will prove
useful[29].
[Footnote 29: For examples of maps of this kind, see Kjerulf, Th.,
_Die Geologie des südlichen und mittleren Norwegen_.]
A perusal of the maps to which reference has been made above will give
the student some notion of the extent to which maps may be utilised to
represent geological structures, and may suggest other methods by
which they may be utilised.
A geological section is usually drawn in order to exhibit the lie of
the rocks, as it would be seen if a vertical cutting were made in that
part of the earth's crust which is under consideration. The character
of the section will depend upon circumstances. The Geological Survey
of Great Britain issues two kinds of sections which are usually spoken
of as vertical sections and horizontal sections, though each is in
truth a vertical section; but whereas in the former the horizontal
distance represented is small as compared with the thickness of the
strata, in the latter the rocks of a considerable horizontal extent of
country are exhibited in the section, and the section is not carried
down to a great depth below the earth's surface. There is no essential
difference between the two kinds of section, and often sections are
drawn which cannot be definitely classed as belonging to either kind,
but in extreme cases the vertical section is a representation of the
order of succession as it would appear if the rocks were horizontal,
no matter how disturbed they may be in reality; whereas the horizontal
section represents the strata as they actually occur, with all the
folds and faults by which they are affected. The accompanying figure
(Fig. 6) represents a horizontal section on the left side of the
figure with a vertical section of the same rocks on the right side.
[Illustration: Fig. 6.]
Vertical sections are extremely useful when it is desirable to compare
variations in the strata over wide extents of country: this can be
done by drawing a series of columns of the strata, each showing in
vertical section the lithological characters and thicknesses of the
strata in one place, whilst the relationship between the strata of
two different places may be indicated by joining the beds of the same
age by dotted lines as shown in Fig. 7[30].
[Illustration: Fig. 7.]
[Footnote 30: It is useful to adopt conventional symbols for the
representation of strata of different lithological characters, and so
far as possible to adhere to the same kind of symbol for any one kind
of deposit. Those which are generally in use, are rough pictorial
representations of the characters of the deposits, as shown in Fig. 7.
The conglomerate is indicated by circular marks representing
cross-sections of the pebbles, a breccia by triangular marks
signifying that the fragments are angular and not rounded; a sandstone
is indicated by dots to represent the grains of sand; a mud, clay or
shale by continuous or broken horizontal lines, which reproduce the
appearance of the planes of lamination so frequent in beds of this
composition; a limestone is usually marked by the use of regular
horizontal lines illustrating the pronounced bedding, with vertical
lines at intervals to represent the regular jointing which occurs in
so many limestones: the nature of the bedding may be further shown by
drawing the lines comparatively far apart when the limestone is a
thick-bedded one, nearer together when it is thin-bedded. Igneous
rocks are represented by crosses or irregular V-shaped marks,
illustrating the absence of stratification and presence of joints.
Volcanic ashes are sometimes represented by dots, at other times by
signs somewhat similar to those which are used for true igneous rocks.
Sedimentary rocks which are composed of more than one kind of material
may be further shown by a combination of two symbols, thus the
existence of a sandy clay may be shown by means of a combination of
horizontal lines and dots, and so with other combinations. The
practical geologist should become accustomed to the use of these
symbols in his note-book; if used, they will save much writing.
These symbols are used in some of the later illustrations to this
book.]
The horizontal section is one which is in constant use by the
practical geologist: the results of the first traverse of a district
may be jotted down in his note-book in the form of a horizontal
section (with accompanying notes), and the written memoir on the
geology of any district composed largely of stratified rocks will
almost certainly require illustration by means of these sections.
Perhaps nothing more clearly marks the careful observer than the
nature of the sections which he makes, and geological literature is
too frequently marred by the publication of slovenly sections. A badly
drawn section not only offends the eye, it may and frequently does
convey inaccurate information.
[Illustration: Fig. 8.]
In the above figure (Fig. 8) taken from Sir Henry de la Beche's
"Sections and Views Illustrative of Geological Phænomena," Plate II.,
the lower drawing represents a section drawn to true scale, while that
above shows one which is exaggerated. The student who saw this would
infer that the uppermost beds on the left side of the upper section
rested unconformably upon the dotted beds beneath, and once abutted
against them in that portion of the figure where the beds have been
removed by denudation in the deep valley, whereas an examination of
the section drawn to true scale shows that the unconformity does not
exist (although there is one at the base of the deposits marked by
dots), and that there is room for the higher deposits to pass above
those marked by dots at the place where the former have been removed
by denudation. Whenever possible, horizontal sections should be drawn
to true scale, the vertical heights being on the same scale as the
horizontal distances. Sections which are so drawn represent the nature
of the surface of the country as well as the relationship of the
strata, and often illustrate in a marked degree the influence which
the character of the strata has exerted upon the nature of the
superficial features of a country. If it be impossible to draw a
section in which the elevations and horizontal distances are
represented upon a true scale, the former ought to be drawn on a scale
which is a multiple of the latter; thus the vertical heights may be
shown on 2, 3, or 4 or more times the scale chosen for the horizontal
distances; when this is done, it will often be necessary to show the
strata with an exaggerated dip, and accordingly the exaggerated
section loses some of its value, though if vertical and horizontal
scales bear some definite proportion it will still be more valuable
than a rough diagram which is not drawn to any scale.
Section-drawing cannot be satisfactorily accomplished without some
practice, and the student is strongly advised to acquire the art of
drawing good sections; the writer can assert as the result of
considerable experience in the conduct of examinations of all kinds,
that slovenly sections are the rule in candidates' papers, and good
sections very rarely appear. Study of the six-inch maps and horizontal
sections (drawn on the same scale) of the Geological Survey of the
United Kingdom will enable the student to familiarise himself with
admirable sections, and it should be his aim to produce sections like
these. He is recommended to take some of these six-inch maps which
show contour-lines as well as the disposition of the strata, and to
draw sections on the scale of six inches to the mile, vertical and
horizontal, exhibiting the proper outline of the ground and the
arrangement of the strata, and afterwards to compare them with the
published sections. The sections should be drawn as far as possible at
right angles to the general strike of the strata. Some datum-line is
taken for the base of the section (say sea-level) and offsets drawn
vertically from this where the section crosses a contour-line or
recorded height. The height is marked on these offsets; thus if a
recorded height of 2700 feet (just over half a mile) occurred on the
line of section a height of somewhat over three inches is marked on
the offset, and so with the other points where the section crosses
contours or recorded heights. By joining these points on the offsets,
giving the connecting lines curves similar to those which are likely
to occur in nature, the general character of the surface of the ground
is represented. The geology of the district is next shown. Wherever a
dip is marked on the map, the direction and amount of dip is shown by
a short line on the section, and where dips are not actually seen
along the line of section, the dips which are nearest to that line on
the map must be considered, and marked on the section. The lines of
junction between the various deposits shown by different colours upon
the map are inserted on the section as short lines, the inclination
being judged by study of the nearest dips; faults and igneous rocks
must be marked off, and any indication of the hade of the fault or the
slope of the edges of the igneous rock which the map affords will be
taken into account. The section will then appear somewhat as shown in
the following figure:
[Illustration: Fig. 9.]
and sufficient indication of the trend of the rocks will be obtained
to shew that they form portions of curves which may then be filled in
as shown in Fig. 10 and the section will be complete.
[Illustration: Fig. 10.]
It will be noticed that the small dyke of igneous rock on the right of
the main dyke is joined to it lower down, though no indication of this
is given along the line of section; but the requisite information for
this and evidence of the existence of the small dyke proceeding from
the left-hand side of the main one may be obtained by the study of
the rocks in a valley on one side or other of the line of section.
After the student has become conversant with the nature of geological
maps and sections, and has read Sir A. Geikie's _Outlines of Field
Geology_, he should on no account omit to learn something of the art
of making geological maps, by going into the field and attempting to
produce a map, for the art of geological surveying does not come
naturally to any one, and some acquaintance with the methods of
surveying is a necessity to everyone who wishes to make original
geological observations, though all cannot expect to afford the time
and acquire the skill necessary for the production of maps vying with
the detailed maps of the Government Survey. Before actually attempting
to draw lines on a map on his own account, he will do well to tramp
over a portion of a district with the published geological map in his
hands, selecting a country which is not characterised by great
intricacy of geological structure, and he can then attempt to
represent the geology of another portion of the same district without
consulting the published map. Of all the districts of Britain with
which he is acquainted the writer believes that the basin of the river
Ribble, in the neighbourhood of the town of Settle in the West Riding
of Yorkshire, is best adapted for studying field geology in the way
suggested above, for the main geological features are marked by
extreme simplicity, and the exposures are good, whilst the presence of
an important fault-system and of a great unconformity relieve the area
from monotony. Anyone who stands on the summit of Ingleborough or
Penyghent will grasp the main features of a portion of the district
without any difficulty, for it lies beneath his feet like a geological
model, and when the student has mastered and mapped in the leading
features, he can find bits of country with geology of varying degrees
of complexity amongst the Lower Palæozoic rocks of the valleys which
run down to Ingleton, Clapham, Austwick and Settle.
The biologist is supplied with laboratories at home and abroad, where
he may study his science under the best conditions. Would that some
munificent person would found, in a district like that referred to
above, a geological station where Cambridge students would have the
means of acquiring a knowledge of field-geology under conditions more
favourable than those presented by the flats around the sluggish Cam!