Depending on the period in which the rocks are formed, they are grouped under various heads. The oldest rocks are called as Archaen rocks, whose existence is traced back to some 4.5 million years ago. Similarly, some of the other periods to which rocks are credited depending on when they were formed are: low and upper Pre-Cambrian periods, early Cambrian period, Gondwana period, Deccan traps and the most recent Alluvium deposits.