The results of five field seasons of work on the Siwalik sediments of Northern Pakistan have greatly expanded our knowledge of these Miocene sediments and their vertebrate faunas. We have measured six long stratigraphic sections on the north limb of the Soan synclinorium near the town of Khaur. These columns, the longest of which is over 3,000 meters, provide the stratigraphic framework for our paleontological studies and give a detailed description of the lithological sequences in the Khaur region. We have concluded that the formational units of previous workers are poorly defined and of little practical value for biostratigraphy or chronostratigraphy. We recognize three major lithological facies: a blue-gray sand facies; a buff sand facies; and a silt/clay facies. The results of intensive paleomagnetic sampling allow a provisional correlation to the La Brecque magnetic time scale. The paleomagnetic sampling has also defined a series of isochrons, one of which we have followed laterally along a 30 km-long belt of outcrop. Certain lithological horizons may also be reliable chronostratigraphic markers. We hypothesize that the two sand facies correspond to two separate river systems which co-existed for millions of years. The characteristics of the blue-gray sandstones suggest a large braided river carrying sediment derived from a freshly weathered terrain. The buff sandstones display characteristics of both meandering and braided channels. The sediments of this river system were derived from an area of intense weathering. The silt/ clay facies represent levee and floodplain deposition. The pattern of interfingering of the two sandstone facies, with broad overlap on a scale of at least 30-40 km, indicates periodic fluctuations in the dominance of one or the other river system. These fluctuations are seen as the result of periodic, extensive influxes of the blue-gray system. Increased production of sand in the source area might have been the result of climatic or tectonic changes. Fossils are usually found only in the buff sands or their laterally equivalent fine-grained floodplain and levee deposits. We recognize three types of fossil localities based on the characters of the fossil assemblages and the sediments. Localities in channel-related deposits were formed as composite events averaged overtime and space, and therefore provide information suitable for paleoecological reconstructions. On the basis of appearances of key species we are provisionally defining a series of eleven biostratigraphic zones. These span the sequence from the Lower Siwaliks to the base of the Upper Siwaliks. The faunas of the three lowest zones show similarities to the Asteracian faunas of Europe and to the East African middle Miocene faunas. Zones 4 through 8 appear to be a period of faunal endemism although there are some resemblances to European and Asian faunas. Correlations of these middle zones are to Eurasian localities dated between 10 and 8 million years (m. y.). Beginning in Zone 9 the faunal endemism is disturbed by a series of immigrations and emigrations. Most of the interchanges seem to be with Africa. Correlations suggest ages of 8 to 6 m. y. for Zones 9 and 10. Paleomagnetic evidence places the base of Zone 11 at 5.1 m. y. Particularly important among the many thousands of fossils we have found are over one hundred new specimens of the hominoids Ramapithecus, Gigantopithecus, and Sivapithecus. Among the new finds are post-cranial elements which can be attributed to these three hominoids. The bulk of the hominoid collection comes from a stratigraphic level provisionally dated at 9 m. y. From the geological evidence we infer the large river systems were not stable through time. River floodplains were well drained with few lakes or ponds except in cut-off channels. Most of the time the shifting, braided channels of the buff river system were dominant, creating a mosaic of habitats by constant destruction and renewal of plant successions resulting in a vegetational mosaic of grassland, bush, and woodland. There is little detectable change in the trophic structure of the herbivores from Zone 1 through Zone 8. The faunal change in Zone 9 does suggest an underlying habitat change.