It is well acknowledged that when land connections were established between Southeast Asia and India, a wide range of plants migrated from the former to the latter. The existence of several taxa in the Siwalik palaeobasins during the Palaeogene and the Himalayan uplift, as evidenced by the plant mega and micro fossil records, served as proof of this. In addition to trace fossils like burrows, footprints and trails, the Siwalik Group of rocks has a wide range of plant fossils, including impressions of leaves, stems and wood trunks. These fossils have different morphologies, sizes and shapes. They were mostly stored in the sandy mudstone, LSS, and MSS mudstone. Although they are not as regularly preserved, wood fossils can be discovered in sandstone from LSS and MSS. These fossils can be found in the exposed Siwalik rocks of the Balasan, Tista, Sevok, Lish and Gish rivers, among other places. A comprehensive account of the fossils found in the research area is given in this chapter. Morphology of Plant Leaves: There are at least five or six different types of plant leaf fossils or impressions in the Siwalik Group in the West Bengali Darjeeling sub-Himalaya. To better comprehend the paleogeography of Darjeeling and Sikkim, this chapter offers morphological descriptions of plant fossils. The finding of plant fossils establishes a marine deltaic to fluvial depositional environment and refutes the continent-interior alluvial fan-fluvial explanation for the sedimentary sequence.