The Neogene rocks are exposed in the Siwalik Hills and other foot hill comprising the Outer Himalaya. Siwalik group is a thick succession of medium to coarse sandstone, conglomerate and clay. Base is not exposed.
- Nahan formation exposed in the South-eastern Himachal overlies the Kasauli Formation with a gradational contact. This unit is generally regarded as equivalent of the lowermost stratigraphic unit of the Siwalik Group.
- The sedimentation seems to have begun in Early Miocene and continued until the Early Pleistocene of Quaternary period. The sediments of the Siwalik Group were derived from the rising mountains in the north.
The Siwalik Group is a rich store house of fossils. Among them the vertebrates are more dominant than invertebrates and plant fossils. A general coarsening of sediments upward in the sequence indicate an increase in the energy of the fluviatile basins. The whole succession of the group shows a frequent repetition in the lithology, as consequence, the characteristics of rocks are not useful to classify the group. The Group is consists of three subgroups : Lower Siwalik, Middle Siwalik, Upper Siwalik
- Staurolite is the dominant mineral in the heavy mineral components of the Lower Siwalik sub-group, whereas Kyanite and Hornblende are the dominant heavy minerals of the Middle and Upper Siwalik sub-group.
- Warm and humid climate seems to have been prevailed during the deposition.
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