Transportation by underground water takes place in solution. Some­times they are carried to sea or lakes through percolation and sometimes they are added to the stream water.

Deposition

The dissolved materials, travelling in solution are subjected to precipi­tation and consequent deposition due to the following factors:-

(i) Evaporation.

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(ii) Loss of Co2

(iii) Chemical reaction.

(iv) Change in physico-chemical condition (i.e. temperature and pressure).

Important Depositional Features

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1. Stalactites and Stalagmites

Particularly in caverns lying above the water table, water may be seen dripping from the straight line of a crack in the ceiling of the cave. These droplets lose some of its dissolved gas by evaporation and deposit small granules of calcium carbonate at the point of the evaporated drop.

In due course, with subsequent deposition of calcium carbonate (by the process as stated above) it grows downwards as an icicle-like pendant. These deposits hanging from the roof taper towards the floor and are called Stalactites.

The water that drops from the end of the stalactite falls to the floor of the cave immediately beneath it and with continued evaporation and deposition dome shaped or conical deposits of calcium carbonate start growing upwards. Such deposits are called Stalagmites.

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Stalactites and stalagmites together constitute what is known as drip-stone.

2. Sinter and Travertine

Ground water emerging on the surface as springs or seepages form loose superficial deposits of silica or calcium carbonate.

The silica deposits thus formed at or near the exit of such springs are known as siliceous sinters whereas those formed of calcium carbonate are called travertine or calc-sinter.

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3. Geode

Deposits resulting from the ground water often forms incrus­tations on the walls of the cave or fissure, below the water-table. Successive layers of amorphous silica thus formed differ in their colour and texture, and are known as agate.

But, when crystallization is perfect, well developed crystals of quartz etc. are found tapering towards the centre of the cavity as the teeth of a comb. Such deposits are known as Geode-which are partially or completely filled cavities.

4. Concretion

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These are spherical, nodular, discoid or lens-like forms and are due to concentrated solution effects of groundwater on rock substances which are rather difficultly soluble.

These are deposits formed around some nucleus which differ in chemical and mineral composition from the enclosing substance. In clays and shales the concretions are mainly calcareous and in sandstones they may be ferruginous or calcareous.

5. Replacement deposits

By the process of replacement, ground water dissolves matter present in any particular substance while depos­iting an equal volume of material it contains. Thus it is some sort of a substitution on a volume-for-volume basis and the new material pre­serves the most minute texture of the one replaced.

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Shells of fossils are replaced in this way; even petrified wood is also a result of such replacement.