FORENSIC GEOLOGY
"At a glance the splashes on his trouser told me what part of London he had been to." (Sherlock Holmes)

    In many ways a geologist is like a detective, using their tools and talents to sift through evidence.  The famous storybook detective, Sherlock Holmes, presumably had an amazing grasp of forensic geology, being able to tell exactly where a person had been by looking at the splashes on their trouser.  In real life, earth science, of which geology is the richest subfield, has many disciplines, with forensic geology evolving out of Germany, mainly via a doctor called Georg Popp who, in 1904, devoted himself to classifying earth minerals and seeing to it that this kind of evidence got introduced in criminal trials.  In the United States, the FBI started taking an interest as early as 1935, and around 1973, a Rutgers professor named Raymond Murray (1975; 2004) is probably most well-known for jump-starting the field.  Today, most of the crime laboratories around the world can perform forensic geology, or know somebody they can contract for it.  Differences exist with respect to expert qualifications, training, and the quality of analyses, although the basic principles and methods are fairly well agreed-upon. 

    According to Murray & Tedrow (1975), forensic geology encompasses the analysis of evidence from an amazing variety of material: e.g., soils; clothing; hair; blood; shoeprints; fingerprints; toolmarks; metal; glass; paint; stains; organic (botanical or zoological) material; inorganic (mineralogical) material; wood; dust; cigarettes and ashes; paper; fibers; grease and oil; documents; containers and construction and/or packing material.  It is primarily concerned with soil.  SOIL falls into three general groups: zonal soil, which reflects the influence of climate and vegetation; azonal soil, which does not; and intrazonal soil, which reflects the influence of some other factor like age or relief (Clark 1998).  Soil material is formed by nature in one of two ways: residual or transported (Murray & Tedrow 1975).  Residual soil is formed in place, produced by the outcropping of solid rock at the earth's surface and where weathering has beat down the solid rock, turning it into soil.  Transported soil is soil that has been moved by nature, usually by rivers or streams, to another location as mud, sand, or dust.  Gravity itself can also be responsible for transport.  The "composition" of soil (a key term in forensic geology) tells a story about how "consolidated" (another key term) the layers of sand and gravel are in any given location.  Four (4) key components make up the forensic evidence:  (1) the soil itself; (2) rocks; (3) minerals; and (4) fossils.  With a sample of material, the most basic procedure is to conduct a series of "comparable similarity" tests using lights, microscopes and other instruments which eventually satisfy the examiner that the sample is similar to a place on earth AND dissimilar to other places on earth (Murray 2004).  Examination of rocks, minerals, fossils and other particles in the sample will help provide probabilities that the material might be found in exactly one place only.  In some cases, the probabilities become so high as to approach the level of fingerprint evidence.

    All forensic geology is designed to establish the degree of probability that a sample is derived from a given place.  However, there are no mechanized ways to produce probability estimates.  Professional judgment often substitutes for probability and is used to establish the degree of confidence in an expert opinion.  Professional judgment is greatly aided by such techniques as measuring the physical density, melting point, and the hardness (on something like Mohs Scale of Mineral Hardness).  There are no substitutes for systematic procedures combined with a forensic geologist's expertise in judgment and ability to decide on whether further examination procedures are needed to fully assess whether enough unusual combinations or consolidations are present.  Geologists are, by nature, trained to look for the unusual and rare, and they are often experts at microscopy. 

MINERALS, ROCKS, AND FOSSILS

    Minerals come in over 4000 varieties (see Complete List of Minerals), but only about 200 or so are ever discussed in mineralogy textbooks.  Further, about 20 or so minerals are considered "common" minerals found in most types of soil, and further still, the vast majority of soil samples will only contain 4 or 5 minerals.  Minerals are classified as light (usually salts and conditioners, like calcite, feldspar, or quartz) or heavy (usually ores or gemstones like magnetite, fluorite, or garnet).  Every mineral has an unique atomic structure with fixed physical and chemical properties or that vary within a definite range.  Non-mineral substances like coal and volcanic glass lack an orderly internal structure.  The characteristics of minerals are usually examined with the aid of a microscope to classify them in terms of qualities like: color, luster, cleavage or fracture, and streak.  Many minerals have magnetic properties, and some minerals only reveal their true color under polarized light.  Crystals come in six (6) different shapes: isometric (square); tetragonal (rectangular); hexagonal (stop sign shaped); orthorhombic (roughly squarish); monoclinic (rhomboid); and triclinic (diamond). 

    Rocks are simply aggregates of minerals formed by one of three processes: igneous (melting of older rocks); metamorphic (formed by pressure); and sedimentary (weathered).  Rocks are simply classified by size, shape, and the way the minerals fit together in the rock.  The coarseness of the grain can be also classified, but the most defining feature is the prevalence of an index mineral comprising the vast majority of the rock. Petrology is the name for the science of the study of rocks.  

    Fossils are used by forensic geologists primarily to determine the age of rocks.  A simple distinction when examining fossils is to look for any hard parts of animals.  If there are hard parts, the fossil comes from a recent period (no more than 600 million years ago); if there are no hard parts or only plant life, the fossil comes from much earlier (3 billion years ago).  Paleontology is the science of fossils, and it is sometimes commonly involved in forensic geology.

SOIL AND RELATED MATERIAL 

    Examination of color is the simplest way to identify characteristics of minerals and soils.  Almost all colors of the spectrum are represented in such material.  The dominant color of a sample usually indicates the native soil color.  However, after a period of time and/or leaching, a sample may change color, but even then, the color of the coating may provide a "color history."  A dark color coating usually indicates the presence of organic material (also called humus, which may be oxidized off using hydrogen peroxide to reveal true color).  Sifting a sample will also change the color, as coarser fragments will typically be gray and yellow as opposed to finer fragments which tend to exhibit more red or reddish brown colors.  Forensic samples are usually dried at 100 degrees centigrade and then examined for color and classification according to the Munsell Color System (the famous color wheel everyone learns in school).  Microscopic examination and other tests follow once color has been classified.      

    Soil samples are often examined for their properties reflecting biogeochemical reactions, which means the examiner treats the sample as if it were a sample of living tissue.  The science of soil study is called pedology and it is a common part of forensic geology.  To a pedologist, soil is seen as a combination of interactions between living and non-living material, kind of like anatomy, and it is amazing how many living things can live in places (like inside rocks) which you wouldn't think support life.  For example, there is a science of micro-organisms called geomicrobiology which specializes in finding living things on asteroids and other planets.  Soil constantly has biologic and geochemical processes taking place.  More biologic (or organic) processes take place at layers near the surface, but once you dig down about fifteen inches or more, there may be less than 1% organic matter.  This "layering" pattern of mature soil (with an organic/non-organic mix at various depths) is called the soil profile.  A deeper cut into the ground might reveal the soil horizon, which shows how the layering process makes soil.  Each soil profile has an individual set of characteristics, such as appearance, color, texture, and saturation (with the amount of water saturation being factors in density and weight).  The profile, however, is not the most important forensic characteristic.  The soil structure is, and this refers to the clustering arrangement of the particles (sand, silt, or clay) which make up the soil, as well as the size and shape of these particles.  Particle clustering may be granular, flat, blocky, angular, or columnar (under a microscope), and soil structure stays the same regardless if you are dealing with a ground sample or a piece of mud splashed on a suspect's pants.

    Geochemical analysis of soil involves determining via chemical tests the major trace elements of a soil sample.  The major trace elements found in soil are silica, iron, and aluminum.  Silica-rich soil is generally found in cooler climates, and iron and aluminum are generally found in warmer environments.  Each state of the U.S. and all of its countries have had the trace mineral content of the soil mapped and indexed.  The FBI and most crime labs have access to this information, which helps to "trace" where a soil sample came from.  In fact, a World Reference Base exists for all the world's soils (Bridges 1997).  Most soil scientists agree that no two points on the surface of the globe have precisely the same soil (Murray & Tedrow 1975).  Peat, which is found in marsh-like, swampy areas or coastal regions, is somewhat different, having properties unique to the vegetation (which made up the peat) as well as the tidal flow and degree of salinity.  Likewise, soil containing vast amounts of pollen and spores (one tassel of corn contains 50 million pollen grains) may throw off precise "tracing" because this material travels hundreds of miles.  The science of palynology involves the study of spores and pollens, and it is an emerging part of forensic geology which is steadily making progress (Moore 1991). 

INTERNET RESOURCES
Forensic Geology Case Histories
Forensic Geology in Military or Intelligence Operations
Geoforensics, Inc.
Necrosearch International
Ray Murray's Home Page

PRINTED RESOURCES
Bridges, E. (1997). World soils, 3e. NY: Cambridge University Press.
Clark, A. (1998). Penguin dictionary of geography, 2e. NY: Penguin Books.
Moore, P. et al. (1991). Pollen analysis, 2e. NY: Blackwell.
Murray, R. & Tedrow, J. (1975). Forensic geology: Earth sciences and criminal investigation. New Brunswick, NJ: Rutgers Univ. Press.
Murray, R. (2004). Evidence from the Earth: Forensic geology and criminal investigation. Missoula, MT: Mountain Press.

Last updated: Jan 26, 2007
Not an official webpage of APSU, copyright restrictions apply, see Megalinks in Criminal Justice
O'Connor, T.  (Date of Last Update at bottom of page). In Part of web cited (Windows name for file at top of browser), MegaLinks in Criminal Justice. Retrieved from http://www.apsu.edu/oconnort/rest of URL accessed on today's date.