Terraces are step-like structures formed along the banks of the rivers.The decrease in the level of river due to its erosional activity is called Degradation and the terrace formed is called degradational terrace.The increase in the level of the river due to accumulation of materials on its bed is called aggradation. Due to seasonal oscillations of the sea-level or fluctuations of climate or tectonic movements of the ground the river undergo a series of such degradation an aggradations thus forming a succession of terraces.Generally the highest terrace will be the oldest since it is first formed and the subsequent terraces will be younger or more recent as they are formed later.
According to their modes of origin three kinds of terraces are seen
Tectonic terrace which are formed due to sudden tectonic movement of the earth's super crust bringing about a subsidence or an uplift of part of the river's course like that of a waterfall thus forming a terrace.
Thallassostatic Terraces are formed due to fluctuations in the sea level.These fluctuations of the sea-level are called eustatic.
Climatic terraces are formed due to various climatic fluctuations.Generally during wet climatic conditions degradational terraces are formed and during dry climatic conditions aggradational terraces are formed.