Geoscience Reference
In-Depth Information
environments. However, subsequent changes can also add
to, rather than wholly remove, the previous Earth history.
Thus sand grains deposited in subtropical Triassic deserts,
lithified and tectonically displaced northwards, eroded by
Quaternary glacial ice from their north-west England
location and redeposited in the Midlands can retain
distinctive aeolian, glacial and meltwater micro-abrasion
marks. Each event superimposed its marks on fading
traces of earlier ones and can be seen by scanning electron
microscopes. The potential and limitations of a broad
range of diagnostic and analytical techniques will now be
explored.
stratigraphy
of broader rock assemblages of all types, to
unravel the nature, origins and sequence of past events.
Processes also take time to occur, creating direct or
indirect measures of their age, rate and duration.
Chemical, structural and other properties of Earth
materials provide three-way opportunities; as descriptors
of their present condition, diagnostic of their previous
history and as the basis for predicting their future response
to changing conditions (
Table 23.1
).
The present is thus
seen as the key to the past and, together, they provide our
best clues to environmental futures (see box, p. 560).
However, science is rarely that simple! Geological
records also contain gaps representing periods of
inactivity or erosion of parts of the record - often of
indeterminate length.
Diagenesis
(see p. 263) occurs
through compaction, dewatering, chemical alteration and
reworking, which create diagnostic features of their own.
Relocation of materials between rocks also risks distorting
their respective histories. Leaching of dissolved minerals
depletes the source rock, and contaminates its recipient;
wind may transport pollen grains, volcanic ash or
snowflakes long distances from their origin into 'alien'
Stratigraphy depends on the availability of surface
exposures, preferably in vertical or near-vertical
strati-
graphic sections
, or our ability to recover or
log
(record)
subsurface rock profiles. Erosion exposes sections,
typically at the coastline, in river or glaciated valleys, on
steep slopes and artificial excavations (quarries, rail/road
Geochemistry
Texture/Fabric
Structural properties
Geotechnical properties
Mineralogy
Particle system
Internal structure
Strength
Chemistry
Size
Bed forms
Bulk density
Isotopic content
Shape
Lamination
Dry density
Sorting (size range)
Grading
Crystallography
Packing arrangement
Cross-bedding
Cone resistance
Structure, shape and size
Packing density
Dunes
Compressibility
Cleavage
Orientation
Ripples
Compressive strength
Clast/matrix ratio
Drapes
Overconsolidation ratio
Point load strength
General properties
Void ratio
Palaeocurrent fabric
Shear strength
Colour
Porosity
Imbrication
Tensile strength
Lustre
Specific gravity
Cohesion
Taste
Dry density
Disturbance structures
Hardness
Flame structures
Water content
Specific heat capacity
Collapse structures
Natural water content
Optical properties
Tool marks
Plastic limit
Solubility
Flutes
Liquid limit
Magnetic property
Bioturbation marks
Plasticity index
Undrained shear strength
'External' structures
Sensitivity
Discontinuity geometry
Permeability
Schistosity
Foliation
Fissility
Cleavage
Notes: The list is not exhaustive and some terms, such as texture, fabric and structure are interchanged by some users. Major groups of properties
are shown in
bold
and secondary properties, which are subdivisions of or dependent on primary properties, are shown in italics. Particle
characteristics can also apply to grains and crystals and some structural properties can apply to single particles/crystals at micro-scales. Entries
in each column do not read across to adjacent columns
