The geologic province of the western foothills is the site of a Late Cenozoic sedimentary basin west of the
Central Range. Recent paleontologic 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 geologic 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.
The western foothills gradually merge westward into the tablelands and coastal plains bordering the Taiwan
Strait. The Penghu Islands in the Taiwan Strait and the Hengchun Peninsula at the southern tip of the island
of Taiwan are also included in this geologic province. Since the Restitution of Taiwan, this geologic
province has been extensively studied and mapped, because it is the most important one economically,
including all the leading coal and oil fields of Taiwan. These deposits of fossil fuels may also continue
westward under the western coastal plains and offshore areas. Therefore the western foothills make up the
most well-known geologic province of Taiwan, and pertinent geologic data are much more abundant than those
of the other provinces.