Station A: History of Forensic Soil Examination

Real and fictional investigators have been using soil samples to identify criminals since the late 1800s. Between

1887 and 1893, Sir Arthur Conan Doyle wrote about the use of geology (the study of soil and rocks) in the

investigation of crime in his novels. His character, Sherlock Holmes, used soil and mud samples to help link an

individual to a specific location where a crime had been committed.

An Austrian, Dr. Hans Gross, is believes to be one of the first investigators to apply science to crime

investigation. His book Criminal Investigation, written in 1893, contained groundbreaking material in this new

science. Dr. Gross, a university professor, founded the Institute of Criminology in Graz, Austria. He firmly

believed in the value of trace evidence, including soils found at crime scenes.

A German investigator, Georg Popp, is credited with being one of the first forensic scientists to use soil

evidence to solve a crime. During a murder investigation in 1904, he examined a handkerchief left at a crime

scene and found it contained bits of coal and particles of hornblende, a mineral. Popp linked this evidence to a

suspect who worked in a gravel pit that contained hornblende. Popp found soil samples taken from the suspect’s

trousers were consistent with samples collected at the crime scene. When confronted with all of the evidence,

the suspect admitted his crime.

Station B: Soil Composition

Soil is part of the top layer of Earth’s crust, where most plants grow. Soil contains minerals from weathered

rocks, decaying organisms, water, and air in varying amounts. Soil texture describes the size of the mineral

particles that make up soil. There are three main soil textures: sand, silt, and clay. Sand is the coarsest texture,

and clay is the finest texture. Most soil samples are mixtures that contain a combination of sand, silt, and clay.

Loam is a fertile type of soil composed of approximately equal amounts of sand and silt, and about half as much

clay.

Soil also contains organic material, or humus, such as decaying plants and animals, and mineral particles. Soil

with more than 20 percent decaying organic material is called peaty soil, which is acidic. Chalky soil is alkaline

and contains various-sized pieces of a soft rock called chalk. The unique texture, amount and type of organic

material, and chemistry of a soil are what make soils so useful in forensics.

Station C: Soil Profiles

Sand, silt, or clay that is deposited by wind or water is called sediment. Sediment on dry land will settle into

soil horizons, or layers, that are more or less parallel to Earth’s surface. The soil in each horizon has

characteristic properties, such as soil in a given area having a unique soil profile, or sequence of layers. Each

soil horizon within the profile is labeled with an uppercase layer—commonly O, A, E, B, C, and R, from top to

bottom.

What is the forensic value of studying the layers of a soil profile? Humans disturb these layers when they dig

into the soil—when they are burying a body, for example. When crime-scene investigators search for a buried

body in an area, they look for disturbed soil. Topsoil and humus may be lower in the profile and the subsoil and

rocks may be on the surface. When a gravesite is filled after a burial, the soil profile is changed. Also, the layers

of an area’s soil profile may still be detectable on equipment or clothes used to dig a grave, leaving clues to a

specific location. Databases of soil profiles are readily available to forensic investigators who need to compare

the soil profile of the evidence to find locations in which to search.

Directions. 1) Complete the puzzle to differentiate between the layers of soil, and draw the completed figure of

a soil horizon under the “Soil Profiles” section of your paper. Be sure to label the layers.

2) In the space provided on your paper, explain the forensic value of studying the layers of a soil profile.

Station D: Sand

Sand is formed by the action of wind and water on rocks, called weathering. As wind and water move rocks,

they collide with other rocks. These collisions break the rock into smaller pieces until small grains, called sand,

remain. Sand grains can be anywhere from 0.5 mm to 2 mm in diameter. Grains can be rounded or angular,

depending on the amount of weathering and the mineral composition of the rocks that formed the sand. During

weathering, if rocks in wind or water stroke one another along their edges, the edges may break off. As the

edges break off, the rock pieces become more rounded. The rounding process may take place over millions of

years. Sand grains carried by water become rounded more slowly than wind-blown sand grains because water

acts as a buffer.

Sand grains are classified as immature, young, old, or mature. Immature sand contains a large portion of clay,

and the grains have a high percentage of fragments. Immature sand is found near the rocks from which it was

formed, usually in areas not exposed to waves or currents, such as the bottom of bays and lagoons, or in

swamps or river floodplains. Mature sand does not contain clay and has fewer jagged edges. Mature sand is

found in areas, such as beaches and desert dunes, where much weathering has taken place.

Sand has four primary sources: continental, volcanic, skeletal (biogenic), and precipitate. Sand from different

sources contains different combinations of minerals. The most common mineral found in sand is quartz. Many

other minerals, such as feldspar, mica, and iron compounds, many be present in smaller quantities.

Continental Sand

Continental sand is composed mostly of quartz, mica, feldspar, and dark-colored minerals like hornblende and

magnetite. If feldspar is present, the sand probably came from a temperate or polar climate or from a high

elevation. In warm, tropical climates, feldspar weathers quickly. A high percentage of quartz means the sand is

very old, because quartz weathers very slowly and often remains after other minerals have weathered away.

Volcanic Sand

Volcanic sand is usually dark as a result of the presence of black basalt or green olivine. The source of this sand

is mid-ocean volcanoes or hot-spot volcanoes like those found in Hawaii. It sometimes contains volcanic

cinders or other volcanic debris. Volcanic sand is very young and contains little or no quartz, except for black

obsidian particles.

Skeletal (Biogenic) Sand

Skeletal sand is made of the remains of marine organisms, such as microorganisms, shells, and coral. It takes

less time to form than other types of sand. Skeletal sand occurs all over the world. Skeletal sand derived from

shells or from coral remains occurs only in tropical regions. Because of the high amounts of calcium carbonate,

skeletal sand gives off bubbles when mixed with a few drops of acid.

Precipitate Sand

Water contains dissolved minerals. When water evaporates, these minerals precipitate, or come out of, the water

solution. Calcium carbonate will sometimes precipitate out of seawater, forming layers of hard particles that

resemble layers of an onion. These layers eventually form small, round structures called oolites. Oolite

formation is an example of deposition, the depositing of layers of material. Sand containing oolites is called

oolitic sand and can be found in various places, including the Great Salt Lake in Utah.

Soil Chemistry

Soil chemistry influences and is influenced by the plant and animal life living and decomposing in the area. The

amounts of nitrogen, phosphorous, potassium, and calcium in the soil influence plant growth. These nutrients

also affect the pH of the soil—its level of acidity or alkalinity (basicity). The pH scale ranges from 0-14. A pH

lower than 7 is acidic, while a pH higher than 7 is alkaline (basic). The pH of soil plays a part in determining

which organisms can survive. For example, most bacteria require a neutral pH, so soil pH can affect the rate and

manner of decomposition.

When a body is buried in soil, it affects the soil chemistry in complex ways. Toxic chemical products of

decomposition, along with nutrient-rich decomposition fluids, seep out of the body and enter the soil. A

cadaver decomposition island (CDI) is formed by these concentration decomposition products.

Station E: Soil Evidence

Proper procedures for collecting, labeling, and packaging soil evidence (including sand) includes: 1)

Photograph and document the crime scene; 2) The artist should sketch the evidence as the evidence collector

notes the locations and types of evidence; 3) Collect at least four tablespoons of material from several locations

at the crime scene (surface soil to use as a baseline, any soil that looks different from surrounding soils, soil

from different levels, and soil from shoes clothing, tools, vehicles, or other objects at the crime scene); 4)

Collect soil samples beyond the crime scene (from a few feet north, south, east and west of the crime scene, and

in a perimeter 20-25 feet from the crime scene); 5) Document, package, and label all samples according the

normal evidence-packaging guidelines.

Soil evidence collected at a crime scene, on an object, or on a victim or suspect is analyzed both

macroscopically and microscopically. Forensic analysts conduct many tests on the chemical and physical

properties of soil. Chemical testing is done on each sample to determine its pH, its chemical and mineral

compositions, and its reactions to different reagents. Soil samples are heated, and the wet and dry masses are

compared. The moisture content is calculated. Reactions of the soil components to magnets and UV light are

noted.

X-ray diffraction is a method of determining the mineral composition of soil. The soil is crushed and exposed to

X-rays, and the diffraction pattern image is produced and examined. Each mineral and chemical produces a

unique pattern. The pattern allows scientists to determine the unique mineral composition of the soil. Scientists

compare and contrast the patterns produced by evidence samples to patterns from samples from a known

location. Although it is not possible to say conclusively that an evidence sample came from a specific location,

it is possible to determine if a soil sample is or is not consistent with another sample. The more evidence that

can link a suspect, the greater the probability that the suspect is associated with the crime.

Although many cases have used soil samples as evidence, its reliability continues to be questioned by the

courts. The Organization for Scientific Area Committees (OSAC) has a special subcommittee to established

protocols and standards for working with geological materials.

Finding Gravesites

It’s difficult to convict a murderer if you can’t find the body. How and where does one look for a body or a

gravesite? Hilltops and rocky, dry areas are usually avoided for hasty burials due to the difficulty of transporting

the body. Locating a burial site depends on being able to distinguish natural from disturbed areas. Excess soil

may be mounded above a new grave because, with a body in the grave, it’s difficult to return all of the original

soil. Older gravesites tend to sink as the loose soil becomes compacted and the body decomposes. Sighting

possible gravesites is best done when the sun is low and shadows are easier to see. Gravesites are more easily

detected from the air or from a treetop or nearby hilltop because they may cast a shadow unusual for the terrain.

The deeper a body is buried, the more limited the invertebrate and other predator activity. A burial of three feet

or more will protect the body from temperature fluctuations. The closer to the surface, the greater the

availability of oxygen and the higher the bacterial, insect, and predator activity. Soils with higher moisture

content increase the likelihood of adipocere formation, which is a waxy substance sometimes produced in the

later stages of decomposition. Clay soil moistens bones, and they expand when they absorb moisture. Bones

contract when the soil around them is drier. Acidic areas such as peat bogs lead to tanning of the skin and can

preserve bodies for centuries. Both highly alkaline and highly acidic areas reduce decomposition due to reduced

microorganism activity (microorganisms are most active at a neutral pH). In anaerobic (low-oxygen)

environments, the gases cadaverine and putrescine are produced by the decomposition of body proteins. Their

stench can attract trained cadaver dogs to bodies in shallow graves.