The use of geological materials as trace evidence in criminal cases has existed for approximately one hundred years. Murray (2004) provides an overview and reminds us that it began, as with so many of the other types of evidence, with the writings of Sir Arthur Conan Doyle. Doyle wrote the Sherlock Holmes series between 1887 and 1927. He was a physician who apparently had two motives: writing salable literature and using his scientific expertise to encourage the use of science as evidence (Murray and Tedrow 1992). In 1893 Hans Gross wrote his book Handbook for Examining Magistrates in which he suggested that perhaps one could tell more about where someone had last been from the dirt on their shoes than from toilsome inquiries. A German chemist, Georg Popp, in 1908 examined the evidence in the Margarethe Filbert case. In this homicide a suspect had been identified by many of his neighbours and friends because he was known to be a poacher. The suspect's wife testified that she had dutifully cleaned his dress shoes the day before the crime. Those shoes had three layers of soil adhering to the leather in front of the heel. Popp, using the methods available at that time, said that the uppermost layer, thus the oldest, contained goose droppings and other earth materials that compared with samples in the walk outside the suspect's home. The second layer contained red sandstone fragments and other particles that compared with samples from the scene where the body had been found. The lowest layer, thus the youngest, contained brick, coal dust, cement and a whole series of other materials that compared with samples from a location outside a castle where the suspect's gun and clothing had been found. The suspect said that he had walked only in his fields on the day of the crime. Those fields were underlain by porphyry with milky quartz. Popp found no such material on the shoe although the soil had been wet on that day. In this case, Popp had developed most of the elements involved in present day forensic soil examination. He had compared two sets of samples and identified them with two of the scenes associated with the crime. He had confirmed a sequence of events consistent with the theory of the crime and he had found no evidence supporting the alibi.
Rocks, minerals, soils and related materials have evidential value. The value lies in the almost unlimited number of kinds of materials and the large number of measurements and observations that we can make on these materials. For example, the number of sizes and size distributions of grains combined with colors, shapes and mineralogy is almost unlimited. There are an almost unlimited number of kinds of minerals, rocks, and fossils. These are identifiable, recognizable, and can be characterized. It is this diversity in earth materials, combined with the ability to measure and observe the different kinds, which provides the forensic discriminating power.
There have been many contributions to the discipline over the last 100 years. Many have been made by the Laboratory of the Federal Bureau of Investigation, in Washington D C., McCrone Associates in Chicago, The Centre for Forensic Sciences in Toronto, Microtrace in Elgin, Illinois, the former Central Research Establishment at Aldermaston, Kenneth Pye Associates Ltd in Great Britain, The Japanese National Research Institute of Police Science, The Netherlands Forensic Institute, as well as other government, private and academic researchers.
Because much of the evidential value of earth materials lies in the diversity and the differences in the minerals and particles, microscopic examination at all levels of instrumentation is the most powerful tool. In addition, such examination provides an opportunity to search for man-made artifact grains and other kinds of physical evidence.
Individualization, that is, uniquely associating samples, from the crime scene with those of the suspect to the exclusion of all other samples is not possible in most cases. In this sense earth material evidence is not similar to DNA, fingerprints and some forms of firearms and tool mark evidence. However, in a South Dakota homicide case, soil from the scene where the body was found and from the suspect’s vehicle both contained similar material including grains of the zinc spinel gahnite. This mineral had never before been reported from South Dakota. Such evidence provides a very high level of confidence and reliability.
One of the most interesting types of studies is the aid to an investigation. There are many examples of cases where a valuable cargo in transit is removed and rocks or bags of sand of the same weight are substituted. If the original source of the rocks or sand can be determined, then the investigation can be focused at that place. In a high visibility case, DEA agent Enrique Camarena was murdered in Mexico (McPhee 1997). His body was exhumed as part of a cover-up staged by members of the Mexican Federal Judicial Police. When the body was found later, it contained rock fragments that were different from the country rock at that place and represented the rocks from the original burial site. With the combination of petrographic examination of those rocks and a detailed literature search of Mexican volcanic rock descriptions, the original burial location was found and the cover-up exposed.
Most examinations involve comparison. Comparison aims to establish a high probability that two samples have a common source, or conversely that they do not have similar properties and thus are unlikely to have come from the same source. In comparison studies of soils, it is difficult to overestimate the value of findings artifacts in the soil or some other unusual type of evidence. In an Upper Michigan rape case, three flowerpots had been tipped over and spilled on the floor during the struggle. It was shown that potting soil on the suspect's shoe had a high degree of similarity with a sample collected from the floor and represented soil from one of the pots. In addition, small clippings of blue thread existed both in that flowerpot sample and on the shoe of the suspect. The thread provided additional trace evidence which supplemented the soil evidence.
In a New Jersey rape case, the suspect had soil samples in the turn-ups of his trousers. In addition to glacial sands grains that showed similarity with those in soil samples collected from the crime scene, the soil contained fragments of clean Pennsylvania anthracite. Such coal fragments are not uncommon in the soils of most of the older cities in eastern North America. However, in this sample there was too much coal when compared with samples collected in the surrounding area. Further investigation showed that some 60 years earlier the crime scene had been the location of a coal pile for a coal burning laundry. Again, the combining of soil-evidence with an investigation of an artifact and local industrial history increased the evidential value.
A new and evolving type of study is one done for the purpose of intelligence gathering. An example might involve identifying mineral material on an individual who had claimed to have recently been to a particular location. In such a case the question would be asked whether the mineral material supports the claim and could have come from that location. Identification of the mineral material alone can be useful in the case of mine fraud, gem fraud and art fraud by providing information that demonstrates the fraud.
The alertness of those who collect samples, and the quality of collection, is critical to the success of any examination. If appropriate samples are not collected during the initial evidence gathering, they will never be studied and never provide assistance to the court. There is the case in which an alert police officer happened to look at an individual arrested for a minor crime. He observed, "that is the worst case of dandruff I have ever seen." It was not dandruff but diatomaceous earth, which was essentially identical with the insulating material of a safe that had been broken into the previous day.
The future of Forensic Geology holds much promise. However that future will see many changes and new opportunities. New methods are being developed that take advantage of the discriminating power inherent in earth materials. Quantitative x-ray diffraction could possibly revolutionize forensic soil examination. When developed to the point that this or similar methods become routine laboratory techniques, it will be possible to do a quantitative mineralogical analysis that is easily reproducible. However, the microscope will remain an important tool in the search for the unusual grain or artifact. Sampling methods, plus the thorough and complete training those people who collect samples for forensic purposes, will be improved. Soils are extremely sensitive to change over short distances, both horizontally and vertically. Soil sampling in many cases is the search for a sample that matches. The collection of all the other samples serves only the purpose of demonstrating the range of local differences. In collecting soil samples for comparison, we are searching for one that would have the possibility of matching. Screening techniques applied during sampling that eliminate samples that are totally different are often appropriate. For example, a surface sample offers little possibility of matching with material collected at a depth of four feet in a grave.
Studies that demonstrate the diversity of soils are important. One approach is to take an area that one would normally assume was fairly homogenous in its soil character and collect a hundred samples on a grid. Each pair of samples would then be compared with each other until all the pairs are shown to be different. Starting with colour and moving on to size distribution and mineralogy, different methods are used to eliminate all of these pairs as appearing similar. Junger (1996) performed several such studies and suggested methods for soil examination.
The qualifications and competence of examiners are a very major problem. How do you learn to do forensic soil examinations? This requires a thorough knowledge of mineralogy and the ability to effectively use a microscope and the other techniques used in earth material examination. It is also important that examiners be familiar with the other kinds of trace evidence plus the law and practice of forensic examination.
REFERENCES
Junger, E. P. 1996. Assessing the Unique Characteristics of Close-Proximity Soil Samples: Just How Useful is Soil Evidence? Journal of Forensic Sciences, 41 27-34.
McPhee, J. 1997. Irons in the Fire. Farrar, Straus and Giroux, New York.
Murray, R.C. and Tedrow, J. 1992. Forensic Geology. Prentice Hall, Englewood Cliffs, N.J.
Murray, R. C. 2004, Evidence from the Earth, Mountain Press, Missoula, MT
Presented at the International Conference on forensic Geology, London, 2003
