For the study of the various stratigraphic units of India, references are usually made to the tripartite physiographic divisions of India. The peninsula is made up primarily of very ancient rocks of Archaean and Pre-cambrian age, as well as Deccan traps and the sedimentary formations of the Post-cambrian age. The lndo-gangetic alluvial plains, which is the second physiographic unit of India, belongs to the Quaternary era. The extra-peninsula made up primarily of sedimentary formations ranging in age from cambrian to Pleistocene.
Recently, over 2500 million years age, has been more frequently used as the limit of the Archean era. The term ‘Archaean’ was introduced in the Indian Stratigraphy by J.D. Dana in 1872 to designate the geological formations older than Cambrian. According to Pitchamuthu (1965), the Archaean rocks of Peninsular India may be divided into two principal types, i.e., the Schistose (Dharwars) and the Gneissose (Peninsular gneiss).
The stratigraphic relationship between the schistose and gneissos rocks of the Peninsular India has been the subject of much discussion; while Bruce Foote, Oldham, Holland and Middlemiss believe that the gneissic rocks are forming the basement over which the sediments belonging to Dharwar Super-group are deposited; Smeeth, Fermor believe that Dharwar super group constitutes the oldest geological formations in India. The basement problem is; still controversial.
From radiometric age dating the age of the Archaean rocks are estimated to be above 2500 million years.
They cover 2/3rd of the peninsula. The areas occupied by the most ancient gneisses and schists are referred to as shields as they have remained virtually undisturbed and unaffected by changes. Peninsular India is a shield area.
South India, Madhya Pradesh, Rajasthan, Singhbhum, Eastern Ghat ranges are some of the important type areas of Archaean rocks.
The Archaean gneisses and schists etc. are exposed along the entire length of the Himalayas from Kashmir to Burma, known as the ‘Crystalline Axis’.
(i) In Kashmir-Hazara Area Salkhala series.
(ii) In Kumaon-spiti area Vaikrita system.
(Hi) In Garhwal area Jaunsar series.
(iv) In Simla area Jutogh and Chail series.
(v) (a) In Nepal and Sikkim the
Schistose rocks (Archaean) Daling series.
(b) Gneissose rocks (Archaean) Darjeeling series.
(v) Bhutan Buxa series.