Land: Rocks & Soils

Land is that part of the earth's surface composed of various types of solids and not occupied by large bodies of water.   These solids are soil, rock, clay and sand.   Bedrock, the rock under soil provides a firm foundation for buildings and bridges.   Cut or uncut rock, e.g., granite, marble, and limestone, is used for building materials.   Small rocks, gravel, are used in roadbeds to keep them free from water saturation.   Clay is fired to make bricks for buildings and residences.   Since ancient times, it was used to make pottery vessels for cooking and storage.   Sand composed mostly of quartz (not all sand is made of quartz) is used to make glass.   It is also combined with water and cement to make concrete, another important building material.

Rock

As used here, rock refers to non-metal mineral matter, either hard (consolidated), like granite and marble, or soft (clay) or unconsolidated (sand).   Rocks usually have a definite structure, but natural glass (obsidian) is an exception to this rule.   Technically, soil is also "rock", but it is discussed separately below because of its primary use in farming.   Rocks are important for the construction of buildings, roads, canals, bridges, tunnels, etc., that are subject to heavy loads (forces).   Granites and marbles are reknown for their beauty as well as their strength in construction. ^{Roberts 7}

Rocks are classified into 3 groups according to origin:

1. Igneous rocks are rocks formed by the cooling and solidification of molten material from the earth's interior.   The lavas and other material from volcanic eruptions forms dense rocks, like granite.
2. Sedimentary rocks are small rocks carried by wind, water, and glaciers to deposits that were subjected to high pressures to form less dense rock, like sandstone and limestone.   Sedimentary rocks are classified by geologists according to particle size:

a. boulder - greater than 10 inches in diameter

b. cobble - 2.5 to 10 inches in diameter

c.pebble - .15 to 2.5 inches in diameter

d. granule - .07 to .15 inches in diameter

e. sand - .00025 to .07 inches in diameter

f. silt - .00015 to .00025 inches in diamter

g. clay - less than .00015 inches in diameter ^{Roberts 17}

3. Metamorphic rocks are rocks that formerly were igneous or sedimentary, but the composition or texture of which was changed under great pressure and heat, which caused them to melt and reformulate in compositon.   Example: slate, formed from shale and marble. ^{Roberts 25}

Sand

Sand is small, loose (unconsolidated) rock grains.   Water movement exerts enough force to disintegrate rock into sand.   When composed primarily of the mineral quartz, sand becomes the prime ingredient in the production of glass and concrete. ^{Roberts 17}

Clay

Clay is very fine-grained, loose (unconsolidated) rock composed of hydrated silicates of aluminum that is plastic when wet.   Impurities give it color.   For example, red clay has considerable iron compounds that color it red.   Clay is the prime ingredient of ceramics, such as earthenware, stoneware, porcelain, and brick.   In addition to its applications in home and building products, clay has many manufacturing applications, such as filters in oil and paper production.

Chert is a very hard rock made up of consolidated grains of silicon dioxide [SiO₂].   It forms irregular modules in other forms of rock and resists weathering.   The flint from which natives made arrow and spear heads, knives, scapers, and axes is chert. ^{Roberts 24}

Soil

Soil is the earth material over bedrock consisting of various mixtures of sand, silt, clay, and rocks.   Soil is divided into a thin layer of topsoil, the remainder being subsoil.   Topsoil contains important organic material, called humus, minerals, oxygen, and water, which are essential to shallow-rooted plant growth, such as grasses and crops.   The topsoil changes gradually to subsoil, which contains less moisture, oxygen and plant nutrients.   Subsoils contain many useful raw materials, such as water aquifers, ores, coal, and petroleum.

Topsoil serves 3 functions to plants:

1. A foundation to hold plants in place;
2. A protective covering to roots;
3. A source of plant nutrients.

Land animals depend on land plants, and plants depend on soil, so soil is essential to all land animal life.   Soil needs continual care to be useful.   Neglect of topsoil used for farming has caused loss of soil, depletion of soil nutrients and consequent loss of crop productivity.   This was caused by one-crop planting, over-grazing by cattle, and other poor management techniques.   Soil neglect occurred throughout American history because of the huge availability of land that, for a long time, seemed limitless.   For many farmers, it was cheaper to deplete the soil, sell the farm, and move to more fertile land.   A few immigrant farmers, especially those from Germany who settled in southeastern Pennsylvania and along the Mohawk River in New York, were exceptional in their care of soil.   The Hatch Act of 1887 enhanced soil management to some extent by providing funds to agricultural agencies to teach farmers how to care for the soil on their farms.   However, poor soil management continued until the problem was brought home sharply with the advent of dust bowl regions in the 1930s.   Since that time, government-sponsored soil conservation programs have been taken more seriously and are now an integral part of improving farm productivity and the environment. ^{Considine 2607}