As a result, rivers from both the northern Eurasian land mass (called Angara) and the southern Indian land mass (called Gondwana) started depositing large amounts of sediments into the shallow sea that was the Tethys. There were marine animals called ammonites living in the sea at the time.

The two land masses, the Eurasian and the Indian sub-continent, moved closer and closer. The Indian plate was moving north at the rate of about 15 cm per year (6 inches per year). The initial mountain-building process of the Himalayas started about seventy million years ago (or the Upper Cretaceous period) when the two land masses (or plates) began to collide with each other. As a result, the already shallow seabed rapidly folded and was raised into longitudinal ridges and valleys.

The Spiti river, originating from the foot of a glacial peak marked K III on old maps, flows approximately160km in a southeasterly direction up to its confluence with the Pare Chu at Sumdo (district border between Spiti and Kinnaur) before merging into the Satluj at Khab further downstream. It has carved out a unique storehouse of Shale and the rock faces are veritable storehouses of the geological history of the Himalayas, dating back 500 million years.

Ichnofossil assemblage recorded from the Spiti region suggests that the availability of nutrients strongly influenced their distribution, as well as abundance. The assemblage was also influenced by the presence of oxygen in both water columns as well as in unconsolidated sediments.

The typical Cambrian - Precambrian boundary marker, Trichophycus pedum has been recorded from the Kunzam La, Pin and Parahio sections. The Parahio formation of the Parahio valley in the Spiti region, in the Tethyan Himalaya of India is the best biostratigraphically resolved section of Cambrian strata in the entire Himalaya.