Introduction to Landforms and Geology of Japan


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Yoro Mountains (a dissected fault scarp)

Yoro Mountains

Photo 1  Yoro Mountains

The Yoro Mountains are a small mountain range about 26 km long and 10 km wide with the highest peak of 860 m, formed by faulting on the west side of the Nobi Plain in central Honshu. The mountain range has an asymmetrical cross section with the steep eastern slop and gentle western slope, which is a tilted block. The Yoro Mountains are characterized by the eastern slope of the fault scarp.


Fig. 1  Location (left)
Fig. 2  Panoramic view of  the Yoro Mountain (viewed from the southeast) (right)

Fault scarp

Fault scarps are manifestations of vertical displacement caused by faulting (see "Types of fault motion"). A good example is a fault scarp produced by a single earthquake in Neodani (see Neodani Fault Museum). The eastern slope of the Yoro Mountains was created by multiple faulting and eroded over a period of one million years or so. The fault scarp looks like an illustration of dissected fault scarp found in some textbooks of geomorphology like Figure 3.

Model of fault scarp

Fig. 3  The dissection of a fault scarp
This figure shows sequential forms in the dissection of a fault scarp (A: initial form). The Yoro Mountains correspond to "D". 
Modified from Cotton 1922.

Triangular planar surfaces are always drawn in figures of dissected fault scarps. These are one of the major characteristics of fault scarp, called triangular facets or terminal facets. In Photo 2, a series of triangular facets is found on the linear front of the Yoro Mountains. A fault scarp becomes higher as faulting intermittently occurs for long periods over several hundred thousand years at the same place, the height of which may reach several hundred meters. Dissection begins on the surface of the scarp immediately after it is produced and streams flowing down almost perpendicular to the fault line (consequent streams) incise the scarp to make many spurs. Each triangular facet is found the terminal of spur; therefore it is regarded as a remnant of the fault scarp. However, the inclined angle of facet is lower than that of the fault surface due to denudation. As intermittent faulting raises the scarp, triangular facets are part of the scarp that resisted erosion and developed with additional fault surfaces, rather than the mere remnants of the primary fault scarp.

Triangular facets

Photo 2  Triangular facets

The fault scarp is a simple form that has a constant length and angle of slope. In such simple geomorphic settings, it is known that spacing between valley mouths of main streams is in proportion to the length of the drainage basin (or relative height of the fault scarp) (Figure 4). Therefore, the spacing is nearly even on the fault scarp with a constant height and triangular facets are formed at the terminals of spurs. The Yoro Mountains clearly exhibit these features (Figure 5, Photo 2).

Length of drainage basin and spacing between valley mouths

Fig. 4  Length of a drainage basin and spacing between valley mouths (left)
S is in proportion to L in simple geological settings
Fig. 5  Topographic map near the spot where Photo 2 was taken (right)
Red triangle, valley mouth; dashed-dotted line (at left), main ridge

Alluvial fan

A fan-shaped depositional body (segment of a cone) is produced in front of the mouth of main stream, which is called an alluvial fan (Figure 3). Sediment abraded from mountains is transported through a stream and deposited on a broad area (plain) of mountain fronts because the flow slows down. The stream bed on the depositional body becomes higher as sediment is continually provided, and the flow channel moves to a lower area. This process is repeated to form an alluvial fan. The Yoro Mountains developed alluvial fans in their fronts. Photo 3 shows a good example of alluvial fan in Hannyadani.

Alluvial fan

Photo 3  Alluvial fan in Hannyadani

Water in streams on alluvial fans often drains away into under the ground and disappears because alluvial fans are generally composed of coarse-grained materials. Such streams are called wadi-like streams. However, torrential rains may cause flooding. In Japan, the course of stream on alluvial fans is often held with embankments to prevent flooding. Embankments were constructed along a stream on the alluvial fan in Hannyadani. As a result, the stream bed rose to be higher than in the surrounding area because a large amount of sediment from the mountains deposited on it, which is called a raised-bed river. At a grade separation in Photo 4, the stream is the overpass and a road and a railroad are the underpass. Raised-bed rivers are found other regions such as the Kyoto Basin and the Kofu Basin.

Raised-bed river

Photo 4  Raised-bed river


Faulting creating the Yoro Mountains

Reverse faulting created the Yoro Mountains over a period of about one million years. The Yoro faults run along the boundary between the Yoro Mountains and the Nobi Plain, striking north-northwest. This fault zone is part of a long fault system extending from Tsuruga Bay in the Sea of Japan to Ise Bay to the south of the mountains, about 140 km long.

The Nobi Plain is a tectonic basin, about 1800 km3, 45 km from north to south and 40 km from east to west in width. The lower courses of revers flowing into the plain concentrate in the western area because heights in the plain are lower westward. For example, the lower reaches of the Kiso, Nagara, and Ibi Rivers, three major rivers that deposited massive sediment in the basin, are in the western margin of the Nobi Plain (near the boundary between the Yoro Mountains and the plain) (Figure 6, Photo 5). The down-to-the-west tilting is referred to as the tectonic tilting of the Nobi Plain, causing subsiding in western area and uplifting in the eastern area. The Yoro faults are related to the tectonic tilting of the Nobi Plain. This tilting began about five million years ago and became more active one million years ago. Mesozoic and Paleozoic rocks constituting the Yoro Mountains are found about 2000 m below ground at the Yoro faults; that is, the thickness of deposits filling the basin is 2000 m or more at the end of the plain (Figure 7). The tectonic tilting is still ongoing. The Nobi Plain is subsiding at 1 to 3 mm/year (in the south part of the plain) and the Yoro Mountains is rising with occasional earthquakes.

Nobi Plain

  Fig. 6  Nobi Plain

Kiso, Nagara, and Ibi Rivers

Photo 5  Lower courses of three major rivers
Kiso, Nagara, and Ibi Rivers in order from the left. Flooding frequently occurred in the past because three large rivers concentrate in this area. The photo was taken near the south end of the Yoro Mountains (at Observation Tower in Kiso Sansen Park Center).

Cross section of Nobi Plain

Fig. 7  Simplified cross section of Nobi Plain
Tokai Group consists of non-marine sediments that accumulated in the end of Miocene to the Early Pleistocene.


Yororailway (Yoro Line) runs along the Yoro Mountains, which is available from Kuwana (JR Kansai Line or Kintetsu Nagoya Line) or Ogaki (JR Tokaido Line). I took photos in this topic in the south part of the Yoro Mountains from Tado Station to Ishizu Station. You can get good views of the Yoro Mountains and Nobi Plain at Observation Tower in Kiso Sansen Park Center. It is 3.4 km from Tado Station to the park.

Yororailway (in Japanese)
Kiso Sansen Park (in English)



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