Geologic Material as Physical Evidence

By Finley, Joseph A., Jr. | The FBI Law Enforcement Bulletin, March 2004 | Go to article overview

Geologic Material as Physical Evidence


Finley, Joseph A., Jr., The FBI Law Enforcement Bulletin


Whenever two objects come into contact, there always is a transfer of material. The methods of detection may not be sensitive enough to demonstrate this, or the decay rate may be so rapid that all evidence of transfer has vanished after a given time. Nonetheless, the transfer has taken place. (1)

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The field of geology and its relationship to forensic science has remained shrouded in mystery for many years. Although the interaction of humans with the environment long has been the object of much interest to physical scientists, published literature on this topic is relatively limited.

Geologic material, commonly used as physical evidence in both criminal and civil cases, can play an important role in forensic science. Geologic material, as with all physical evidence, contributes scientific support that can assist in establishing the guilt or innocence of an individual. In general, the usefulness of geologic material as physical evidence depends on the number of significant variations in the material and the ability to compare and contrast these variations. The inorganic nature of geologic material can make qualitative identification relatively straightforward. Combined with the scientific objectivity of the analysis and testimony of the expert witness, geologic material has a great advantage as physical evidence. To this effect, investigators must ensure that they use proper collection and preservation techniques for processing and analyzing various materials.

SOIL

Today, most major crime laboratories throughout the world, both public and private, study soils. The FBI was one of the first forensic laboratories in the United States to extensively use soil and mineral analysis in criminal cases. (2) As early as 1935, the FBI Laboratory worked with soils; by early 1939, heavy mineral separations and mineral identifications were standard practices for the FBI Laboratory in soil cases. (3)

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Formation

Soil material generally is formed by nature in one of two ways, residual or transported. This loose material (soil) is composed of fragments of minerals and rocks generated from the breaking up or dissolving of the earth's solid rocks. Residual soil material forms in places where solid rock is exposed in outcrops at the earth's surface. This solid rock endures the natural weathering processes, which, over time, break up and dissolve the rock. This procedure turns the outcrop into a mass of fragments and removes some of the material in solution (dissolving of the minerals normally occurs in rain or groundwater).

Transported soil materials occur where fragments of minerals are produced elsewhere and transported to the location where they are found. Fragments of rocks and minerals created by the weathering of a rock outcrop may be carried away by rivers and deposited as sandbars, gravel, or fine mud. Wind can move vast quantities of fragments great distances, depositing them as dunes and dust layers. Wave and current actions break down rock and mineral formations along shores of seas and lakes, transporting this material and depositing it as beaches or undersea sediment. The force of gravity may cause landslides and move tons of material down a slope, thereby producing a mass of newly transported soil on the land below.

Composition

Characteristically, soil is a very complex system composed of certain quantities of solid, liquid, and gaseous materials. The unconsolidated mineral matter on the earth's surface has been subjected to and influenced by genetic and environmental factors, such as parent materials, climate (including moisture and temperature effects), macro- and microorganisms, and topography. Over a period of time, these factors produce a product (soil) that differs from the material from which it derived in many physical, chemical, biological, and morphological properties and characteristics. …

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