Geol 33_Environmental Geomorphology Soils

Geol 33 Environmental Geomorphology

J Bret Bennington

Soils

The definition of the word "soil" varies from profession to profession. In general though, soil is used to refer to the weathered, often unconsolidated material that overlies bedrock (although the exact boundary between the two may itself be difficult to define precisely).

Soils and surficial deposits may be classified into two groups:

Residual - those that form in-situ by mechanical and chemical weathering.

Transported - those that have been moved into place by wind, water, ice, or gravity.

Residual Soils

Saprolite - material that retains the texture of the parent rock but which is "rotten" because certain susceptible minerals have weathered. Saprolites commonly form in humid environments underlain by coarse-grained igneous and metamorphic rock. Feldspar and pyroxene crystals weather into clays, which expand and disrupt the fabric of the rock. Although banding and other mineral patterns from the rock are still visible the soft saprolite can be excavated with a hammer or shovel.

Saprolites are often very thick - for example upland areas of the Piedmont province have up to 30 m of saprolite covering the unweathered bedrock. Usually the boundary between saprolite and bedrock is uneven and gradational, which can cause problems when trying to seat a foundation on this material.

Terra Rosa- this reddish, clayey soil develops from weathering limestone. Although limestone is weathered by dissolution and leaves no solid residue, almost all limestones contain some proportion of dispersed clay and / or thin beds of clay and chert. This insoluble material accumulates along with iron oxides to form a red soil.

Soils in carbonate regions vary greatly in thickness due to their common accumulation in karst depressions and sinkholes.

Clayey soils - most clayey soils develop on deposits of shale. Shale weathers quickly as it is exposed to cycles of wetting and drying that cause the component clay minerals to alternately contract and expand, disrupting the coherence of the rock.

Thick, shale-derived soils are almost always problematic substrates for development. Shale itself is very unstable on slopes. Weathered shales and clays are prone to changes in volume during drying and wetting. In fact, expansive clay soils are one of the costliest geological engineering problems, resulting in over $300 million in homeowner expense annually.

Transported Soils

Colluvium - this is soil that is being transported slowly downslope due to gravitational creep. Typically, colluvium consists of jumbled blocks of bedrock surrounded by a weathered clayey matrix. In humid areas colluvium can be very thick - up to 100 m.

Structures can be (and often are) constructed on colluvium because the rate of movement is slow enough to cause little direct damage over the life of a structure (less than 100 years). However, excavations and changes in slope or water table position can accelerate creep and lead to landslide failure. Often the effects of creep can be observed in leaning tombstones or utility poles on hillsides, which are slowly rotated by the increased rate of creep at the surface of the colluvium.

Alluvium - includes all sediment deposited by streams. Alluvium is usually stratified gravel, sand, silt and clay, although in mountainous regions alluvium typically consists of boulders.

During floods, streams scour their channels and move cobbles and boulders. As the flood wanes the cobbles and boulders are redeposited in the stream channel. Flood waters that overtop the banks spread out into the surrounding floodplain, carrying fine sand, silt, and clay. The sand is deposited first and is later mantled by the silt and clay as floodwaters receed. This creates blankets of fertile soil underlain by permeable sand that can function as a floodplain aquifer supplying subirrigation water.

Glacial drift - all materials deposited by glaciers. Includes till (nonstratified) and stratified drift.

Till is a poorly sorted collection of boulders, cobbles, pebbles, sand, silt and clay. It can be plastered down by advancing ice (lodgement till) into a tightly compacted mass with oriented clasts, or it can be a looser mass of material formed at the margin of the melting ice (ablation till).

Stratified drift is deposited by streams of melting water flowing off the front of the glacier. As such it is a special type of alluvium. Unlike till, stratified drift is well-sorted and contains few large boulders.

Lacustrine and marine deposits - fine grained sediments originally deposited in lakes or embayments of the sea. These deposits often provide poor support for foundations and can create engineering and construction problems if inadvertently built upon.

Eolian deposits - windblown sand (sand dunes) and windblown silt and rock flour (loess). Because dunes are subject to continual movement they are rarely chosen as sites for construction. Dunes that become vegetated will stabilize, but the vegetative cover is easily disturbed, which can allow the dunes to remobilize. Sand dunes will only form if there is a local source of abundant sand.

Loess is more widespread than sand. In the midwest loess mantles millions of square miles forming thick, fertile soils. Much of this loess was disgorged as rockflour from the melting front of the Wisconsin ice sheet and transported by winds blowing off of the icefront. Loess deposits also form downwind from deserts as prevailing winds transport silt far beyond the limit of sand migration.

Soil Horizons

Soils change gradationally downward away from the surface and form characteristic zones or horizons. The two most active horizons are the A and B horizons. Generally, material is lost by leaching from the A horizon and this same material moves downward and precipitates in the B horizon.

Soil Classification

There are several soil classification systems that have been developed and applied.

Currently in use is a complicated taxonomy of soils developed by the US Soil Conservation Service, which groups soils into 11 orders, 55 suborders, 230 great groups, and 1240 subgroups. Known as the SCS System or the Seventh Approximation, this system is highly descriptive and useful because it is based on observable physical and chemical properties in soil profiles.

An older system developed by the US government in 1938 is considered obsolete by soil scientists, but is still presented in some geology textbooks (including the one we are currently using).

Commonly encountered soil terms

Podzol - an obsolete term used to describe soils typical of humid, temperate climates such as found the northeastern US. In a podzol the A horizon is enriched in humus but depleted in soluble minerals and clays. These materials precipitate and flocculate in the B horizon, which becomes blocky and clayey.

Laterite - this is an accumulation of iron minerals that develops in the B horizon of tropical soils developed over intensely weathered iron- and aluminum-rich rock such as basalt. The laterite is usually brick-red and can be composed of up to 80% iron oxide in the form of hematite or goethite.

Caliche - In arid climates with thin soils the evaporation of soil water before it reaches the water table results in the precipitation of carbonate around the grains of the B horizon. The accumulation of carbonate can result in nodule of carbonate called caliche or a cemented, hard layer called calcrete. Water can also be drawn upward from the water table by capillary action. Near the surface this water also evaporites, causing additional precipitation of carbonate.