Abstract The Spiti Valley is located in the Trans-Himalayan terrain of India, from where non-geometrical microliths have been discovered. While the Siwalik Hills have been subjected to extensive prehistoric surveys, this is the first evidence of lithic tools discovered in the Trans-Himalayan region of Himachal Pradesh, India. Due to its topographical and intense climatic features, the Trans-Himalayan region has generally been regarded as a barrier since prehistoric times. However, Dzamathang cannot be considered as an isolated site in the Trans-Himalayas. In fact, in the adjoining areas of Ladakh, Tibet and Nepal, similar lithics have been reported from several sites and been assigned to the Middle and Upper Palaeolithic. The discovery of this site suggests that the Trans-Himalayan zone may have acted as a possible route rather than a barrier during human migration. Large numbers of lithics have been recovered in the Dzamathang area of the Spiti Valley. Based on the collection of the artefacts from the surface, this paper tries to understand the geological and geographical setting of the area, particularly concerning prehistoric settlements in the Trans-Himalayas. This assemblage consists of assorted artefacts, which include a unifacial chopper, microlithic cores, flakes, blades, bladelets, burin and a large amount of debitage fragments. The majority of artefacts are on quartzite or quartzarenite. Future surveys will be targeted at recovering primary context sites for excavations and absolute dating.

Recent recognition of the existence of a deep fault of regional dimension along the northern boundary of the Great Himalayan (Himadri) lithotectonic subprovince is a repudiation of the time-honored notion of the transition from high-grade metamorphics of the basement complex forming the Great Himalaya into the Phanerozoic fossiliferous sedimentary succession of the Tethyan domain. This plane of dislocation has attenuated and truncated basal Tethyan units (e.g., by the Malari Fault in Kumaun); it has disharmonically deformed or backfolded the lower Paleozoic formations (as discernible north of Nanda Devi in Kumaun and north of Kanjiroba-Annapurna in western Nepal); it has split the lithologic succession into a schuppen zone (as done by the Chomolungma/Main North Himalayan Thrust in the Sagarmatha [Everest] region in northeastern Nepal and by the Trans-Axial Fault in northwestern Sikkim); and it has caused considerable deformation, including mylonitization of the basement metamorphics, migmatites, and mid-Tertiary (28 to 18 Ma) granites that occur as concordant bodies and cross-cutting dikes. The Trans-Himadri Fault (T-HF) may represent the southernmost of the old normal faults in the continental-margin basin in the frontal part of the northward advancing Indian plate. This fault (which becomes a low-angle thrust in northeastern Nepal) was reactivated following the blocking of movements along the Indus-Tsangpo Suture Zone (ITSZ) and slowing down of sliding along the Main Central Thrust (MCT). The T-HF may therefore be accommodating a part of the convergence of the Indian and Asian plates. The domal structural architecture north of Annapurna, the pronounced upwarps of the metamorphic basement immediately to the south of the ITSZ in the Mansarovar region, and the faulted crystalline basement blocks and slices thrust northward in the Tso Morari area in Ladakh bear testimony to the blocking of movements and the consequent upwarping of the Indian crustal plate at its leading edge. The fall and then rise of the Moho in the Kashmir-Nanga Parbat-Pamir section and its abrupt deepening from 55 km under the Sagarmatha to 70 km a few tens of kilometers north, through to the Tibetan country, must be viewed in the context of postcollision, perhaps neotectonic, movements along the deep T-HF.

The Salt Range and its Trans-Indus extension bridges the reentrant between the outer ranges of the northwestern Himalaya and the Sulaiman Mountain arc. Upper Proterozoic to Recent successions occur in the range, which makes up the southern thrust front of the orogen. There are two regional features of particular interest. The first is the occurrence of thick saliferous deposits of Eocambrian age, overlying Precambrian basement in the Potwar Plateau and thrust southward in the Salt Range over the alluvial Cenozoic. Thick, saliferous deposits also occur within the Eocene sequence of Kohat. These incompetent formations played a significant role in determining structure. The second feature is the presence of four major unconformities: between the marine Eocambrian to Cambrian sequence and the glacial, Lower Permian conglomerates, and below the Paleocene, the Miocene, and the late Pliocene–Pleistocene formations. Metamorphic rocks, linking with the Precambrian crystalline basement of northwestern India, crop out only in the Kirana Hills some 80 km south of the Salt Range. Within the Salt Range and related areas, unmetamorphosed sedimentary rocks compose the exposed succession, mainly shallow-water marine, until mid-Tertiary time, and lacustrine and fluvial from Miocene time onward. Prior to Quaternary time, only epeirogenic forces affected the region, accompanied occasionally by local warping. In contrast, during Quaternary time, the effects of the Himalayan orogeny extended southward. Accentuated by movement within the Eocambrian saliferous formation, the Salt Range developed as a complex anticlinorium, emplaced southward along a major thrust, which has recently been determined by seismic reflection measurements to involve a décollement of at least 20 km. Complex fold and fault structures resulted elsewhere within the region.

Heavy mineral and paleocurrent direction data suggest that the ancestral Indus is an analog of the modern Indus River (Pakistan). During the past 18 m.y., the Indus has maintained a relatively stationary outlet position along the Himalayan Front. Sandstones from three sections of Siwalik strata have been sampled and their heavy mineral suites analyzed. These stratigraphic sections cover some 200 km in an east-west direction in the Potwar Plateau area. In the western and central Potwar Plateau sections, blue-green hornblende makes a conspicuous first appearance at 11 Ma, the boundary between the Chinji and Nagri Formations. Above this boundary, blue-green hornblende dominates the heavy mineral assemblage of Siwalik sand. In the Upper Siwalik beds the heavy mineral suite contains as much as 75 percent blue-green hornblende. Unroofing of the Kohistan Arc terrane is the most likely explanation for this detrital hornblende. In marked contrast, the eastern Potwar region does not show the same abundance of blue-green hornblende. Significantly, the modern rivers of the eastern Potwar do not carry abundant blue-green hornblende either; only the Indus River does. Paleocurrent measurements taken in the Chinji Village section near the center of the Potwar Plateau indicate a northwest-to-southeast flow direction, whereas those in the Trans-Indus 120 km west of Chinji Village indicate a northeast-to-southwest flow direction. These data indicate that the Siwalik sequence of northern Pakistan is configured as a large-scale alluvial fan with the ancestral Indus shifting course back and forth across the Potwar Plateau region with a frequency of 10 4 to 10 5 yr/cycle. River sinuosity varied systematically from side to side of this fan, with minimum sinuosity attained along a north-south axis. As indicated by the absence of blue-green hornblende, the ancestral Indus did not reach the eastern Potwar Plateau (Kotal Kund area, 100 km east of Chinji) during the past 11 m.y.