<p>In the previous paper (ARAI et al., 1963), we made clear that the Atokura Klippen consist of the Atokura formation (Senonian) and the older rocks covered unconformably by it and that the Klippen were thrust up from the north. Then, searching for the root of the Klippen, we surveyed the Shimonita district to the north of the Atokura district, southern Gunma Prefecture, Central Japan. This paper is the result of a collaboration study on the root problem and a few subjects arose newly in the course of the study. In the Shimonita district, a characteristic zone of E-W trend, 3km wide, i.e., the Shimonita Tectonic Zone, is developed along the northern margin of the Kanto mountainland which is composed of the Paleozoic and Mesozoic formations as well as of the Sanbagawa crystalline schists. The characteristic rocks of the Shimonita Tectonic Zone, from the older to the younger, are as follows. The Nanjai formation is composed mainly of mudstone and sandstone, with thin beds of intercalating acid tuff. The rocks are suffered generally from shearing movement and also from contact metamorphism near quartz diorite mentioned below. Some of the hornfels are quite similar to those of the Atokura Klippen. In spite of the remarkable minor deformation, the formation, as a whole, gently undulating folds. Geologic age of this formation is unknown but is surely presenonian as mentioned later. Probably, it is early Cretaceous or Cenomanian-Turonian. The Name granite occupies the northern part of the tectonic zone. It is composed of medium to coarse-grained leucocratic granite rarely containing biotite. The effect of shearing movement is remarkable in this granite and it results, in some cases, in mylonite. Several small masses of this granite are found also as sheets in the Nanjai formation. Quartz diorite, which is also intruded into the Nanjai formation, is quite identical with the Kawaiyama quartz diorite, one of the constituents of the Atokura Klippen. The Kanohara conglomerate formation, that has long been regarded as the basal conglomerate of the Miocene, was proved to be much older. It is composed mainly of boulder conglomerate in which boulders attain a colossal dimension of 2m across. Most of the boulders are represented by granite porphyry and welded tuff, and rarely by granite. Matrix is represented generally by coarse sand of acid volcanics, but tuffaceous matrix and thin beds of acid tuff are also found in some parts. Granite porphyry, which is identical with the boulders of the Kanohara conglomerate formation, crops out in the southern part of the tectonic zone, and is intruded into the Nanjai formation. Moreover, the conglomerate formation covers unconformably the quartz diorite. Therefore, the Kanohara conglomerate formation is unquestionably in an unconformable relation to the Nanjai formation. An extinct formation of acid welded tuff, which had been thoroughly eroded out, is inferred from the boulders of the Kanohara conglomerate formation. The Kotsutateyama acid tuff formation covers the Kanohara conformably. Some of the rocks of this formation are also welded, but welding is less conspicuous than in the welded tuff boulders of the Kanohara conglomerate formation. The Kanohara and the Kotsutateyama formations seem to maintain a gentle geologic structure, excepting the narrow belts where they are steeply inclined along high-angled faults. They are more than 150 m and 100 m thick, respectively. Prior to and posterior to the Kanohara conglomerate formation, two cycles of acid volcanic activity are recognised in this tectonic zone, so far as the mapped area is concerned. These activities suggest the first stage of the tectonic development of this zone. The Miocene sediments with many Molluscan and Foraminiferan fossils rest on these older rocks with remarkable unconformity. The basal conglomerates are variable in thickness from 0 to 30m. A questionable instance is known where the</p><p>(View PDF for the rest of the abstract.)</p>