The geologic province of the western foothills is the site of a Late Cenozoic sedimentary basin west of the Central Range. Recent paleontological studies indicate that sedimentation in this basin began in Oligocene time and continued into the early Pleistocene. Major orogeny began in early Pleistocene time, and all the sedimentary units in this western basin have been folded and faulted to form the geologic structures of the western foothills. The western foothills are composed of a series of mountains and rolling hills flanking the western margin of the Central Range, but the boundary between the two is topographically not well-defined. The geological boundary, however, is marked by the Chuchih fault, a tectonic line that separates the western foothills from the upthrust argillite-slate series of the Central Range to the east.

Quaternary deposits in the western foothills are represented largely by Pleistocene mudstone, siltstone, shale, sandstone and conglomerate, including both marine and continental fades. A large part of the reef limestone lenses or layers in southern Taiwan have been dated as Pleistocene in recent studies. The Pleistocene deposits are exposed mainly in the hills and lowlands in the western part of the western foothills. Recent paleontologic studies have placed the boundary between Pliocene and Pleistocene in the middle part of the Cholan Formation, which was considered to be mainly Pliocene in older literature. Recent palynological studies have also established Quaternary stratigraphy and zonation in a number of sections in Taiwan.

The Toukoshan Formation is the oldest Pleistocene unit in western Taiwan, and is distributed extensively in the western foothills. The next younger Pleistocene unit is the Tananwan Formation, which is believed to rest unconformably on the Toukoshan Formation and other older units. All the Pleistocene units younger than the Tananwan Formation are generally flat-lying, and are unconformable with respect to the older strata (CENTRAL GEOLOGICAL SURVEY, MOEA, n.d.).

The other Quaternary deposits are mainly surficial veneers and valley or basin fillings consisting of fluvial, lacustrine, estuarine, or marine sediments. They show varied landforms, such as coastal or river terraces and alluvial plains. Terrace deposits are extensive in the rolling hills and coastal terraces of western Taiwan and the Hengchun Peninsula. A uniform sequence of terrace development is recognized in western Taiwan, remarkably dissected into several terrace levels. Terrace deposits are also present in several inland basins, the Eastern Longitudinal Valley, and on some offshore islands (CENTRAL GEOLOGICAL SURVEY, MOEA, n.d.).