We will suppose by way of illustration that near us flows a river, on
the rising ground above which is a pit that we propose to visit for the
purpose of putting our apparatus into practical operation. When we have
reached the floor of the pit, and stand looking up at the section before
us, we are at first rather puzzled as to what the beds, which we see
before us, are; for as the pit has not been worked for some time, its
sides are partially overgrown with grass, and in places bits and pieces
of the upper beds have fallen down and form a heap beneath which the
lower ones lie buried. We must therefore make our way to those spots
where the beds are left clear, and find out, if possible, what they are.
By climbing up one of the heaps of fallen earth (_talus_) we reach the
top, where, first of all, under the roots of the grass and shrubs, we
find the mould in which these grow, and which is formed of the broken up
(_disintegrated_) rocks forming the still higher ground above, and which
the rains, frosts and snows, aided afterwards by the earthworms, have
converted into mould. This, geologically speaking, is called _surface
soil_, and is here about two feet deep. Just below it we find a layer of
coarse gravel; the pebbles of which this is composed are of all sorts,
sizes, and shapes, and are stained a deep brown by oxide of iron. Most
of them are flints, and by diligent search you may find casts and
impressions in these of sponges, shells, spines of sea urchins, etc.
Flints, whether from gravel or their parent rock the chalk, are easiest
broken by a light smart tap of the hammer, though when it is desired to
shape them for the cabinet a soft iron hammer should be used, and the
piece to be shaped placed on a soft pad on the knee, for when struck
with a steel hammer flints splinter in all directions, and often through
the very portion you most desire to preserve. In one spot we find a mass
of sand included in the gravel; this mass is thickest in the middle, and
tapers away towards each end, its total length being about fifty feet.
Could we see the whole mass, we should probably find it to be a patch
lying on the gravel and thinning out all around its edges; in other
words it would be shaped like a lens--"_lenticular_" as geologists term
it. When we examine this mass more closely, we find that the layers of
sand do not run parallel with the bed, but are inclined in different
directions, sometimes lying one way, sometimes another. This _false
bedding_ is due to the sand having been thrown down in waters agitated
by strong currents that swept over the spot, now in one direction and
now in another, scattering at one moment half the sand they had just
piled up one way only to redeposit it the next minute in another. In the
gravel also may be observed a similar though less marked arrangement,
owing to the larger size of its constituents, which of course required a
still stronger current action to wash them down.
Amongst the sand we now see some shells, and set to work to dig them out
very carefully, for they are exceedingly brittle. The best specimens are
to be obtained by throwing down masses of the sandy material and
searching in it; but only the stronger and finer examples will bear such
usage. We next notice that these shells are precisely similar to those
still found with living occupants in the river below, only they are no
longer of a brownish colour, but owing to the loss of the animal matter
of the shell have an earthy, dirty-white appearance. To carry these home
they should be packed in bran in one of your tins with a note as follows
made on a piece of paper and placed just inside--"Sand in gravel:
topmost bed ---- pit, August 2nd, 188-." Then if you are not able to
work them out at once on reaching home, you will not forget whence they
came. From the appearance of these sands and gravels, and the presence
in them of shells exactly like those in the river below, it may
reasonably be inferred that they once formed a portion of the bed of
that river long ago, before it had scooped out its valley to the present
depth. There is, however, something else in this sand-bed--a piece of
bone protruding; clear away the sand above it, and dig back until the
whole is visible. It is broken through in one or two places, but
otherwise is in fair condition; remove the pieces carefully one by one,
and wrap them in separate pieces of paper, and then proceed to search
for others. These bones, which are plentiful in some of our river valley
gravel-beds, are the remains of animals that once roamed in the forests
which at that time covered the country; they were probably either
drowned in crossing the water, or got stuck in the mud on the banks on
coming down to drink. A fine collection was made at Ilford by the late
Sir Antonio Brady, and is now in the British Museum (Natural History) at
South Kensington. Besides the bones of animals, you may expect to find
examples of all, or nearly all, the different rocks in which the river
has cut its valley, and samples of these may be picked out and taken
home. Each specimen should be wrapped in a separate piece of paper to
prevent its rubbing against others, care being taken to note the
locality either by writing it on the paper or by affixing to the
specimen a number corresponding to one in your note book against the
description you have written of the bed. The gravel, with its
accompanying bed of sand, may be traced down, by scraping away the
surface, for about ten feet, when you will discover that it rests
unevenly upon the beds below, which, instead of being horizontal, slope
(_dip_) in a N.N.E. direction, making an angle of about 45 deg. with the
floor of the pit; the gravel therefore rests successively upon the
upturned ends of the lower beds, and, geologically speaking, is
"unconformable" to them. Now as these underlying rocks were of course
originally deposited in an horizontal position, they must have been
pushed up and the upper parts worn away (_denuded_) before the gravel
was deposited on them, for the accomplishment of which process an amount
of time must have elapsed that it would be impossible to reckon by
years.
When we come to examine these lower beds, we find first a stratum of
stiff dark-brown clay containing fossils disposed in layers: those near
the outer surface have been rendered so brittle by the weather, that it
is necessary to make use of the pick end of the hammer and dig a little
way into the face of the section before we come upon some which will
bear removal by cutting them out with a knife. Pack them in a tin with
bran, or, where much clay still adheres to them, wrap them in paper.
The true top of this bed is not visible, being concealed beneath a heap
of earth in the corner of the pit, but we can see and measure about six
feet of it.
The next bed in order is a light brownish band of sandy clay that
splits along its layers into thin pieces or "_laminae_," whence we may
describe it as a sandy, _laminated_ clay. On the freshly split surface
of one piece we see scattered a number of small darker brown
fragments; an examination with a pocket lens clearly shows that these
are little bits of leaves and stems, with here and there a more
perfect specimen. These beds must have been deposited in the still
waters just off the main stream of a large river which brought the
plants floating down to this spot, where they became water-logged and
sunk; so, too, if you examine the shells in the bed immediately above,
you will see that they are very like though not the same as those
which at the present day love to dwell in the mud off the estuaries of
big rivers in warmer parts of the globe; hence we discover that at
some far distant period a big river, but one which had no connection
with that running close by, once flowed over this very spot. On
tracing the leaf-bed down, we come all at once, at about three feet
from its upper surface, upon a narrow band one or two inches thick of
a substance composed of numerous bits of sticks and stalks closely
matted together and partially mineralized. Vegetable matter in this
form is known as lignite, and is one of the first stages towards the
formation of coal out of plant remains. Below this lignite band we
find our leaf-bed getting sandier and sandier, and losing all trace of
the plants by degrees till it becomes almost pure sand. Here and
there, however, it contains some curiously shaped masses, which, when
broken through with the hammer, seem composed of nothing but the same
grains of sand cemented together into a hard mass. In one there is,
however, a curiously shaped hollow, which, upon examining it closely,
you will see is a perfect cast of a small shell that has itself
disappeared. A drop of acid on it fizzes away and sinks in between the
grains of sand which in this spot become loose. A mass of sand or
particles of clay thus cemented together, be it by iron, lime, or any
other substance, is termed a "_nodule_" or "_concretion_," and in this
particular instance has been formed as follows:--The rain-water
falling on the sand where it comes to the surface sinks in and filters
through the bed. Now there is always a certain amount of carbonic acid
in rain-water, and this acid acted on the carbonate of lime of which
the shell was composed, dissolving and dispersing it amongst the
neighbouring grains of sand where it was re-deposited, cementing them
together as we have seen. The bottom of this bed of sand we find to be
just fifteen feet from the lignite band when measured at right-angles
to the bed, and it is succeeded by a hard greyish rock, which requires
a smart blow of the hammer to break it, but the surface of which,
where it has been exposed to the weather, is much crumbled
("_weathered_"), and breaks readily into small pieces. It is easily
scratched with the point of a knife, and therefore is not flint;
moreover, it fizzes strongly when touched with acid--hence there is a
great deal of carbonate of lime in it, and we know that it is
limestone.
Limestones are very largely, sometimes almost entirely, made up of the
calcareous portions of marine creatures, such as the hard parts of
corals, the tests of sea-urchins, the shells of mollusca, etc.,
welded, so to speak, into one mass by the heat, pressure, and chemical
changes which the bed has undergone since its deposition at the bottom
of the sea. There would be every reason, therefore, one might suppose,
to expect a number of fossils in this bed; but, alas! disappointment
awaits the young explorer, for with the exception of chalk and a few
other limestones, these rocks are generally of such uniform texture
that on being struck with the hammer they split through fossils and
all, the fractured surface only too frequently showing nought save a
few obscure markings. But what we fail to accomplish in our
impatience, nature effects by slow degrees, and if you will turn over
the weathered pieces and blocks lying about, you will soon find plenty
of fossils sticking out all over them; by a judicious use of hammer
and chisel any of these may be detached and added to your stock, each
being separately packed in paper and the locality written on the
outside. Some seventy or eighty feet is all that is visible of this
limestone; the rest is unexcavated.
Before leaving the pit, it will be as well to select such rock
specimens as you wish to place in your cabinet, trimming them to the
required size on the spot, for should you, as is not unlikely, spoil
two or three, you can readily pick a fresh one. Having secured our
specimens, we will take a look at our note-book, to see if we have
noted all the details we require. If so, our entries should run
something as follows:--First, we have made a rough sketch of the
position of the beds, carefully numbering each one; then follow our
notes on the individual beds, preceded by numbers corresponding with
those in the sketch, thus:--
1. Surface Soil 2 ft.
2. River Gravel, including a lenticular mass of }
3. Sand, with land and fresh-water shells and bones of } 10 ft.
animals }
4. Stiff dark-brown clay, with estuarine shells 6 ft. seen.
5. Light-brown sandy clay, with leaves and stems of plants 3 ft.
6. Band of Lignite 2 in.
7. Same as 5, passing into-- }
8. Pure Sand, with layers of concretions containing casts }
of shells } 15 ft.
9. Dark-Grey Limestone, with numerous fossils 80 ft. seen.
Beds 4 to 9 dip at an angle of 45 deg. to the N.N.E.
Our imaginary pit is of course only a sort of geological Juan Fernandez,
but it will serve in some degree to illustrate the method of dealing
with various rocks and fossils when met with in the field, and how they
may best be collected and carried home. A few additional suggestions
where to look for fossils may, however, be given here. To begin with, I
never neglect to search the fallen masses, especially their weathered
surfaces, or to look carefully over the heaps of quarried materials,
whatever they may happen to be, piled on the floor of the pit. In
working at the beds themselves, remember that fossils frequently occur
in layers which of course represent the old sea-bottom of the period; to
find these, it is necessary to follow the beds in a direction at right
angles to their stratification, till you arrive at the sought-for
layers, or _zones_.
Do not be surprised, when collecting from a formation you have never
before studied, if the fossils are not at first apparent, though many
are known to be present. The eye requires a few days in which to become
accustomed to its fresh surroundings, and when the same spot has been
carefully hunted over every day for a week, it is astonishing what a
quantity of fossils are discernible where not one in the first instance
was recognised.