Residue left as insiiu after weathering followed by transportation gives rise to residual deposits in due course. These are the insoluble products of rock weathering, which still mantle the original rocks from which they have been derived.
The residual deposits are formed because of the accumulation of insoluble products of rock weathering, when the other constituents are removed by the transporting agency. As a result there is a decrease in volume, affected almost entirely by surficial chemical weathering.
Mode of formation:
For the formation of residual deposits, the following conditions are required:
(i) Humid-tropical climate.
(ii) Presence of rocks, where some of the constituents are soluble and a majority of constituents should be insoluble.
(iii) The relief must not be very great.
(iv) Long continued crustal-stability is essential.
(v) There should be conditions for good drainage.
Under conditions of poor drainage clay minerals like kaolinite, illite, montmorillonite etc. are formed. If there are well defined wet and dry seasons and fairly good drainage, the clay minerals are decomposed to form laterite. But with an evenly distributed rainfall through out the year, iron-oxide tends to be separated in the solution because of aeration leaving behind mainly aluminium hydroxide, which is called ‘bauxite’.
Thus the examples of residual deposits are clay, laterite and bauxite.
Besides, Terra Rossa forms a good example of residual deposits, which is the insoluble residue of clay and other mineral matter left behind after solution of the limestone.
Residual deposits sometimes contain valuable ore deposits and the Process is termed as residual concentration.