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and astrochronology (Hinnov and Hilgen 2012 ). The latest time scales of the
Neogene (23.0-2.59 Ma) and Paleogene (66.0-23.0 Ma) periods are entirely
based on astronomical calibrations and “floating” astrochronologies are available
for extended time intervals (multiple millions of years) extending through stages
in the geologic timescale belonging to the Triassic, Jurassic and Cretaceous
periods (Gradstein et al. 2012 ).
1.2.1 Map-Making
A field geologist is concerned with collecting numerous observations from those
places where rocks are exposed at the surface. Observation is often hampered by
poor exposure. In most areas, 90 % or more of the bedrock surface is covered
by unconsolidated overburden restricting observation to available exposures; for
example, along rivers ( cf . Agterberg and Robinson 1972 ). The existence of these
exposures may be a function of the rock properties. In formerly glaciated areas, for
example, the only rock that can be seen may be hard knobs of pegmatite or granite,
whereas the softer rocks may never be exposed. Of course, drilling can help and
geophysical exploration techniques provide additional information, but bore-holes
are expensive and geophysics provides only partial, indirect information on rock
composition, facies, age and other properties of interest. It can be argued that to-day
most outcrops of bedrock in the world have been visited by competent geologists.
Geological maps at different scales are available for most countries. One of the
major accomplishments of stratigraphy is not only that the compositions but also
the ages of rocks nearly everywhere at the Earth's surface now are fairly well
known. However, as Harrison ( 1963 ) has pointed out, although the outcrops in an
area remain more or less the same during the immediate past, the geological map
constructed from them can change significantly over time when new geological
concepts become available. A striking example is shown in Fig. 1.2 . Over a 30-year
period, the outcrops in this study area repeatedly visited by geologists had remained
nearly unchanged. Discrepancies in the map patterns reflect changes in the state of
geological knowledge at different points in time.
Many geologists regard mapping as a creative art. From scattered bits of
evidence, one must piece together a picture at a reduced scale, which covers at
least most of the surface of bedrock in an area. Usually, this cannot be done without
a good understanding of the underlying geological processes that may have been
operative at different geological times. A large amount of interpretation is involved.
Many situations can only be evaluated by experts. Although most geologists agree
that it is desirable to make a rigorous distinction between facts and interpretation,
this is hardly possible in practice, partly because during compilation results for
larger regions must be represented at scales of 1: 25,000 or 1: 250,000, or less.
Numerous observable features cannot be represented adequately in these scale
models. Consequently, there is often significant discrepancy of opinions among
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