Notchの形態と成因について

Notches are undoubtedly supposed to be one of the most familiar features in coral reef regions, but few studies on topography of notches have been carried out in the past.<BR>In the central part of Ryukyu Islands -Yoron Island, the southern coast of Okinoerabu Island, and a group of Iheya-Izena Islands -, the classification of notch profiles and the origin of notches are discussed in this paper. In these islands 139 notches were surveyed precisely, and the following results were obtained : <BR>(1) Each part of a notch is termed as Figure 1.<BR>(2) The notch profiles are classified roughly into six types, namely, A : V-shaped, B : cave-like, C : U-shaped, D : bow-shaped, F : stem-shaped, and F : stern-shaped. According to the features of the roof slope and the base slope of the notch, these types are subdivided into (a) symmetrical, (b) with a long roof slope, (c) with a long base slope, (d) without an roof slope, (e) without a base slope, and (f) deformed. Moreover, the position of the retreat point in the notch profile is taking into consideration for the classification. The doubled notches are also distinguished. In case that a retreat point is situated at the upper part of the notch profile, · mark is put at the right of upper corner of alphabetical mark. If the point is located at the lower part, mark will be put below. In case that there is a small nip at the seaward end of a foot slope, - (dash) mark are added. If the doubled notches are recognized, the upper is written “above” and the lower “below”, and both are tied by a parenthesis.<BR>(3) As the agencies of notch formation, the solvent action and the mechanical action would be taken up. The latter implies the wave attack, and the wearing by sand and gravel. It is difficult to explain the origin of the notch by only one agency. These two agencies are superposed complicatedly.<BR>(4) Generally speaking, the stronger the waves are, the smaller the depth of notches becomes (=D type). Conversely, the weaker the waves are, the larger the depth of notches becomes (=B-A type).<BR>(5) On the average, the retreat point is situated ca. 1 m above the base point, and the roof point is situated ca. 2 m above the base point. However, as the height of the roof accords with the upper limit where a spray of sea water falls on, it stands higher in the open sea coasts. Geological structures also affect the notch types.<BR>(6) In Yoron Island and its neighboring islands, the period of the formation of main notches had been over. Because, it is evident by the facts that there begin to form the nips and the lower notches at the end of a foot slope of the main notches, and that there begin to form the small lapies on the foot slope of main notches.<BR>(7) If the depth of notches is deepened at the rate of 1 m per 2000 years, it can be deduced that the sea level was stagnated at the stand 1 m higher than the present sea level between B. P. ca. 1000 and B. P. ca. 5000 years.<BR>(8) The notches as well as the benches and the beach rocks will be able to use as the indicator which shows the post-glacial sea level changes.