1. Byobuga Ura: A 60 m Sea Cliff That Retreats 50 to 100 cm a Year
Byobuga Ura is a sea cliff that stretches for about 10 km (6.2 miles) from Choshi City near Inubo Cape to Gyobu Cape in Asahi City, Chiba Prefecture. The drop is about 60 m (197 ft). On top of old rock layers called the Inubo Group, there are layers of Katori Formation and Kanto Loam [1]. According to national records, the erosion speed of this cliff was very fast, at 50 to 100 cm (20 to 39 inches) per year. However, this is a past description and does not mean it is still falling at the same speed today [1].
To understand these numbers, if a speed of 50 cm (20 inches) per year continued for 100 years, the cliff would move back 50 m (164 ft). If it were 100 cm (39 inches) per year, it would move back 100 m (328 ft). This calculation is just a rough guide assuming the speed stayed the same. In reality, the retreat changes depending on the year and location. In March 2016, Byobuga Ura was designated as a National Place of Scenic Beauty and Natural Monument [1]. So, what is wearing away the hard rock cliff?
2. How Waves Cut the Base of a Sea Cliff
A sea cliff is formed by the power of waves. First, waves carve a hollow into the rock near sea level. This hollow is called a notch. When the notch gets deep, the rock above loses support and collapses, leaving a steep, high cliff [2]. It is not that waves cut the top of the cliff; instead, the base is scooped out, and the top falls down.
On the sea side at the base of the cliff, a flat rock shelf called a wave-cut platform also forms. The shape of sea cliffs and wave-cut platforms is explained as being related to the strength of the waves and the resistance of the rock [2]. A report from the Geological Society of Japan shows an example where rock strength affected the shape. On Ebisu Island in Shimoda, Shizuoka Prefecture, volcanic breccia, which is stronger, had a higher wave-cut platform than sandstone [3]. Even on the same coast, the way it is eroded changes depending on the type of rock.
So far, we have looked at cliffs that move backward slowly because of waves. Can a cliff appear in just one night?
3. How the 1891 Nobi Earthquake Created a 6 m Cliff
The Mizutori Fault Cliff in Neodani, Gifu Prefecture, is a cliff with a 6 m (20 ft) drop that appeared during the Nobi Earthquake on October 28, 1891. It was designated as a Natural Monument on June 14, 1927, and later became a Special Natural Monument [4]. The designation document describes this fault that appeared on the surface as "extremely rare."
A fault is a surface along which rock has shifted. Among these, those that have moved repeatedly in recent geological times and are thought to continue moving in the future are called active faults. There are about 2,000 of them in Japan [5]. A fault cliff is a steep slope that continues in a nearly straight line, formed by the vertical shift in the fault. It is said that the cliff grows with each fault movement every thousand to several thousand years. However, because it is also eroded at the same time, collapsed soil and debris pile up at the base [6].
The height ranges from low cliffs under tens of meters to huge ones reaching hundreds or thousands of meters. However, those over hundreds of meters are large-scale examples. It is thought that most are usually a few meters to tens of meters high. We have seen cliffs that drop because of erosion and cliffs that grow because of shifting. Are there cliffs that remain just because of the difference in rock type?
4. How Soft Rock Wears Away and Leaves a Cliff Behind
Differences in rock hardness can also leave a cliff behind. In areas where hard layers and soft layers overlap and tilt slightly, the soft layers are worn away first, and the hard layers are left behind. This creates an asymmetric hill where one side is a steep cliff and the other is a gentle slope. This is called a cuesta [7].
An example where rock type determined the cliff's shape is the White Cliffs of Dover in the UK. They are explained as layers of chalk (calcium carbonate) formed about 100 million years ago. On the ancient sea floor, calcium shells from tiny algae called coccolithophores piled up and were pressed together. These layers then rose up to form the cliffs [8]. Byobuga Ura also has different layers, like the Katori Formation, sitting on top of the old Inubo Group layers. Cliffs are places that show the difference in rock types as layers of earth.
Are there cliffs formed by forces that are neither waves nor faults? Let us look near rivers.
5. How River Terraces Form Steps and Cliffs Beside Rivers
A landform where flat table-like surfaces and steep cliffs line up like steps along a river is called a river terrace. The flat surface is called a terrace surface, and the steep cliff is called a terrace cliff [9].
The way it forms is through the river cutting deeply into its bottom. Old river beds and floodplains are left behind at a higher position, and the steep slope in front of them becomes the terrace cliff [9]. If sea waves make sea cliffs, then river currents are making these cliffs. The number of steps in these terraces may be related to changes in the river's ability to cut downward.
6. How to Find Clues to a Cliff's Origin Near You
If you live near a river or the sea, try to find one cliff or step nearby. If the top is flat and the front is steep, it might be a river terrace. If it faces the sea and has a flat rock shelf at the base, it might be a sea cliff. A long, straight slope may be a clue to a fault. However, you cannot determine the cause just by looking at it.
Clues for research are in public records. Explanations for Byobuga Ura and the Mizutori Fault Cliff are available on the Cultural Heritage Online website. The Cabinet Office's disaster prevention information explains active faults from the basics. Do not approach the base or edge of cliffs; please observe them from a safe distance.