ON THE RETREAT OF THE SEA CLIFFS ALONG THE PACIFIC COAST AT HARANOMACHI, JAPAN

Abstract

In order to investigate the cause of local variation in sea cliff retreat, the author surveyed the sea cliffs along the Pacific coast of Ohmika, Haranomachi City in Fukushima Prefecture, Japan (Fig. 1). Throughout this coast of 3 kilometers in length, we find sea cliffs with a height of 5 to 25 meters. Most of them are 20 meters high cutting a marine terrace, but some of them at the northern part are found on a side of another lower fluvial terrace, which is about 5 meters high. An obtuse angled cape divides this coast into two: northern and southern parts (Fig. 2). Judging from the old (1912) and the recent (1959) maps of this area, it is possible to recognize a clear difference of the land area lost between the northern and southern parts (Fig. 4). The southern part showed more loss than the northern one, though the former has a wider beach in general. Moreover, there is a difference in type between the northern and southern cliff profiles (Fig. 3). The northern cliffs are almost everywhere characterized by a vertical bluff just like a wall showing a smooth face. Deep sea-caves at their foot and narrow stepped platforms in the middle of the cliff profiles also can be seen at some places in thenorthern part. On the other hand, the southern cliffs are rough and vertical at the lower part and also quite steep, but not vertical, at the upper. There are no sea-caves but shallow notches at their foot. Wave-cut benches could be seen at the foot of the cliffs in the southern part. As regards to the fallen debris at the foot of these cliffs in the southern part, a large amount of all sizes having fresh faces and sharp edges are to be found. On the contrary, in northern part, there are less debris but most of them are larger and well-rounded. As for the geology of this coast, they consist generally of sand and gravel deposits at the upper, a sandy mudstone layer at middle and lower (Fg.5). At the southern part, this sandy mudstone layer is divided into two by an inserted unconsolidated stratum of sand, and this sandy stratum locates just at the foot of the southern cliffs, The middle and lower parts of the northern cliffs consist of only homogeneous mudstones. To know one of the resisting powers of these strata against wave attack, the author measured hardness for every stratum on the southern and northern cliffs by using a penetrometer, an instrument for measuring soil consistency (Tab.2). The strikes and intervals of the joints in the mudstone layers are also important factors for destruction of the cliffs by storm waves. In the southern part, the strikes are oblique to the present shoreline. On the contrary, the strikes of the northern cliffs are parallel to the shoreline. Other factors concerned with destruction of these cliffs such as wave, wind (Fig.7) and shallow water topography (Fig.6) were also examined, but the author considered that they are not so much concerned with the difference of cliff retreat as geological factors in this coast are. Finally it was proven that the main cause of local variation in cliff retreat, and cliff profiles between the northern and southern parts on this coast depend mainly upon the geological factors, such as differences in stratigraphy, joints in mudstone layers and hardness of strata on this coast.