1. Valley
2. Third Order
3. Erosion

(i) The important features of the pacific ocean:
(ii) The pacific ocean is the largest and deepest ocean on the earth.
(iii) It covers about one-third of the earth's total area and spreads for about 168.72 million sq.km
(iv) This oceans shape is roughly triangular with its apex in the north at the bering strait which connects the pacific ocean with the arctic ocean.
(v) The bering sea, the china sea and the Philippines sea, the sea of Japan and Tasman sea are some of the marginal seas of the pacific ocean.
(vi) Indonesia, philippines, japan, hawaii, newzealand are some of the islands located in this ocean.
(vii) The deepest point mariana trench is 10,994m and is located in the pacific ocean.
(viii) A chain of volcanoes is located around the pacific ocean called the pacific ring of fire.

Plains are highly populated.
Reason:
(i) Plains are a flat and relatively low-lying lands.
(ii) Plains are usually less than 200 metre above sea level.
(iii) Most plains are formed by rivers and their tributaries and distributaries.
(iv) These plains are used extensively for agriculture due to the availability of water and fertile soil.
(v) They are most suitable for human inhabitation.
(vi) Hence, they are the highly populated regions of the world.
(vii) The Indo-Gangetic plain in North India is one of the largest plains in the world.
(viii) The plains formed by river cauvery and vaigai are important plains found in tamil nadu.

Plateaus:
(i) Plateaus are the elevated portions of the earth that have flat surfaces bounded by steep slopes.
(ii) The elevation of plateaus may be a few hundred metre or several thousand metre.
(iii) Tibetan plateau is the highest plateau in the world.
(iv) So, it is called as the 'Roof of the world'.
(v) The flat lopped part of the plateau is called tableland.
(vi) The plateaus are generally rich in minerals.
(vii) The Chota Nagpur plateau is one of the mineral rich plateaus in India.
(viii) Therefore, rnining is one of the major aclivities of the people living here.
(ix) The deccan plateau in peninsular india is of volcanic origin. Dharmapuri plateau, coimbatore plateau, madurai plateau are found in tamil nadu.