A floodplain sometimes has distinct layers from soils deposited in flooding. Gray or gleyed colors are typically indicative of wetlands, saturated for significant periods, and anoxic. It suggests a high water table, and a soil that does not perk well. This material sometimes has a sulphuric smell when exposed. You are right, black soils are high in organics, and tend to hold alot of water. Sometimes, during a flood, there can be areas of alluvial deposits that have buried organics in a layer. I think this is called a histic epipedon. Floodplains can have fairly complex deposit histories, and those channels that migrate across the valley through time can have buried channels, remnants of the past. Geologists and geomorphologists are typically involved with sampling and describing these histories.

Iron is an element that adds red to brownish colors, depending on the materials. Sands often have a tan color. I have seen a few light blue colors that were indicative of limestone. Parent materials are important in soil coloration. I worked with soil scientists for many years. Soil scientists use a Munsell soil color chart to describe the different colors and hues of soil, and they use the colors as they work to classify soils as to series, using combination of structure (sand, silt, clay, rocks, oganics, etc.), layers (horizons), chemistry such as pH, nutrients, hydrology, topographic position, plant indicators, etc.

Civil or structural engineers use various tests to evaluate suitability for building foundations. Floodplains are not necessarily uniform in composition, so tests specific to site are often needed.

The clay (better to say “mud”) deposits of modern floodplains can take on different colors in relation: 1) to their content (the dark color is generally indicative of a high content in organic fraction), 2) to the characteristics of the circulating fluids (in reducing environments gray and blues prevail, while ocher and red prevail in oxidizing environments), 3) not least, to the efficiency of surface drainage that in turn control the circulation of fluids (in areas with higher drainage efficiency yellow/ocher colors tend to prevail).

Of course, all these characters are deeply conditioned by climate and peculiar features can be locally present. From a geological point of view, these characteristics are found as well in the fossil record of the alluvial plains, in which the colors of the fine deposits can be indicators of facies and their distribution - together with all the sedimentological characteristics of the deposits – can be related to the stratigraphic position within the valley infill.

Different clay material weather along a floodplain or not display colour variation due to chemical composition of the parent rock as we as the prevalent climatic and geochemical processes leading to their formation.

The colour variation is mainly caused by qualitative and/or quantitative variation in mineral contents and humus (i.e. dead parts of plant and animals). For example, shades of orange and red colouration is caused by ferrugenous contents whereas high organic material provides grey to blackish grey colour.

Dear Mr. Bhowmick,

Colors in rocks and minerals need chromophores. The most common ones are bivalent Fe which tend to give green shades, whereas trivalent Fe tend to yield brown to red colors. Other coloration effects are based upon Cu contents (green) or Pb (green, brown, blue). It is the question which only you can answer based upon the knowledge of the amount of clay minerals and non-clay minerals. A pure nontronitic clay/-stone or mud/ stone (depending on the content of silt-sized material) and the texture as well as structure can despite its high content of trivalent Fe accommodated in its lattice take on green shades, while saponite may cause a brighter tint. Kaolinite-group minerals can be snow-white. On the other hand the claystones enriched in kaolinite and/or dickite may turn into a black coal tonstein due to the abundance of coalified material or blackband/ clayband when Fe minerals are present. As long as no detailed information is presented on the minerals associated with the clay minerals it is a matter of conjecture.

H.G.Dill

aerobic and anaerobic conditions produce colour changes ie. reducing and oxidising conditions.