Transport processes and depositional environments

Water

Small particles settle through water very slowly. They therefore tend to remain in suspen-sion, enabling them to be transported much further by rivers than larger particles. The larg-est particles are carried—if at all—by being washed along the bed of a river, rather than in suspension. Pebbles, gravels and coarse sands tend to be deposited on the bed of the river along most of its course. As the river changes its course due to the downstream migration of meanders (bends), or erodes a deeper channel in a process known as rejuvenation (fol-lowing, e.g. a fall in sea level), the coarse material is left behind to form a terrace. Silts and fine particles may also be deposited on either side of the river following a flood, because the floodwater is comparatively still. A soil deposited along the flood plain of a river is known as alluvium, or an alluvial deposit.

A river tends to flow more rapidly in its upper reaches than in its lower course. For example, the Amazon has a gradient of about 1 in 70000 in its lowest reaches, compared with gradients as high as 1 in 100 in many of the upper streams (Robinson, 1977). This means that particles which were carried in suspension in the upper reaches of a river begin to be deposited downstream as the flow velocity falls. At the mouth of the river, sediment

Table 1.1 Simplified geological classification of soils and rocks in terms of eras and periods of time (from Robinson, 1977)

Origins and classification of soils 9 builds up on the river bed, and constant dredging is usually required if shipping channels are to remain navigable. Sediment is also carried into the sea and deposited: if it is not removed by the tide, a build-up of sediment known as a delta is formed, gradually extend-ing seaward from the coast.

The structure of a typical deltaic deposit is illustrated in Figure 1.4. The bottomset beds are made up of the finer particles, which have been carried furthest in suspension beyond the delta slope before settling out. The foreset beds are made up of coarser material, which has carried along the river bed before coming to rest on the advancing face of the delta. The topset beds are deposited on top of the foreset beds, in much the same way as the alluvial deposits further upstream. Deltaic deposits generally comprise clays and silts, with some sands and organic matter.

Figure 1.4 Structure of a deltaic deposit (after Robinson, 1977).

Wind

Approximately one-third of the Earth’s land surface is classed as arid or semi-arid.

Although it is likely that the original weathering processes took place when the climate was more humid than it is now, the primary transport process for soils in desert regions is the wind. Sand dunes gradually migrate in the direction of the wind. Fine particles may be carried for hundreds of kilometres as wind-borne dust. Dust may eventually arrive at a more humid area where it is washed out of the atmosphere by rain. It then settles and accumulates as a non-stratified, lightly cemented material known as loess. The cementing is due to the presence of calcium carbonate deposits, from decayed vegetable matter. If the soil becomes saturated with water, the light cementitious bonds are destroyed, and the structure of loess collapses. Extensive deposits of loess are found in north-western China.

A soil which has been laid down by the wind is known as an aeolian deposit.

Ice

Ice sheets and valley glaciers are particularly efficient at both eroding rock and transporting the resulting debris. Material may be carried along on top of, within, and underneath an ice sheet or glacier as it advances. The effectiveness of ice as a mechanism of transportation does not (unlike water and wind) depend on particle size. It follows that deposits which

10 Soil

have been laid down directly by ice action (known as moraines) are generally not sorted, and so encompass a large range of particle size. A mound deposited at the end of a glacier is termed a terminal moraine, while the sheet deposit below the glacier is known as a ground moraine (Figure 1.5). Unsorted glacial moraine is known as glacial till or boulder clay. The particles found in glacial tills are generally fairly angular, in contrast to the more rounded particles associated with typical water-borne deposits.

Ice and water

Material from on top of or within a melting glacier or ice sheet might be carried away by the meltwater before finally coming to rest. This would result in a degree of sorting according to particle size, with the finer materials being carried further from the end of the glacier. Soils which have been transported, sorted and deposited in this way are described as fluvio-glacial materials. The outwash from an ice sheet can cover a considerable area, forming an extensive out-wash plain of fluvio-glacial material (Figure 1.5).

Figure 1.5 Depositional mechanisms associated with glaciers and ice sheets.

In some cases, the till may be carried by the meltwater into a lake formed by water trapped near the end of the retreating glacier or ice sheet. The larger particles then settle relatively quickly, forming a well-defined layer on the bottom of the lake. The smaller particles settle more slowly, but eventually form an overlying layer of finer material. With the next influx of meltwater, the process is repeated. Eventually, a soil deposit builds up which consists of alternating layers of fine and coarse material, each perhaps only a few millimetres thick (Figure 3.15). This layered or varved structure can have a significant effect on the engineering behaviour of the soil, as discussed in section 3.6.

Material transported by ice, and deposited either directly or sorted and re-laid by out-wash streams, is known as drift. The principal depositional mechanisms associated with glaciers and ice sheets are summarized in Figure 1.5.

In this section we have discussed the breakdown of rocks into soils. We should note in passing that this is only one-half of the geological cycle. As soils become buried by the deposition of further material on top, they can be converted back into rocks (sedimentary or metamorphic) by the application of increased pressure, and perhaps chemical changes.

They might also be converted into igneous rocks, by means of tectonic activity. However,

Origins and classifi cation of soils 11 this book is concerned with soils rather than rocks, and a discussion of the formation of rocks is beyond its scope.

1.4 Soil mineralogy