A Study of the Terrace Topography of the Musashino Upland Along the River Tama. (Part 2)

Abstract

On the longitudinal profile of the terraces of the Musashino Diluvial Upland along the River Tama (Fig. 2), the author fixed his eyes upon the phenomena that the Musashino terrace surface is parallel to the Tama river-bed and the Tachikawa terrace surface crosses both of them, that is, in the west of this region the Tachikawa terrace surface covers the Musashino terrace surface and forms a storied composite fan, in the central part it becomes a fluvial terrace below the level of the Musashino terrace, and in the east it is buried under the Tama river-bed. Then, he thought that these phenomena represent the earth movents of the upland as follows: By the time of formation of the Tachikawa terrace surface after the formation of the Musashino terrace surface, the Musashino Upland, taking an immovable center in the neighborhood of the intersection of the Musashino terrace surface and the Tachikawa terrce surface, tilted downward toward upstream. To the west of the intersection, therefore, the Tachikawa terrace gravels rested on the Musashino terrace and formed there the storied composite fan, while to the east of the intersection the river forming the Musashino terrace surface rejuvenated and formed the Tachikawa terrace surface below the level of Musashino terrace surface, so that their relative height was increased downstream. Up to the present after the formation of the Tachihawa terrace surface, the Musashino Upland has tilted upward toward upstream, and so the Tachikawa terrace surface is steeper than the present river-bed. However, it is required to calculate the quantity of the tilting of each terrace surface quantitatively in order to study that the terrace topography was formed by whether eustatic movement or earth movement. To solve this problem, the author thought out the following method so as to know a Distance-Altitude Curve of the original river-bed (a graded profile of the river when the terrace surface was a river-bed). On a graded river, it is assumed that H is the altitude of the river-bed at a locality, x is the distance from the starting point to the same locality, and γ is the constant of integration. Transforming the theoretical formula by S. Shulits, the Distance-Altitude Curve of the river-bed can be expressed as: H=Aekx+γ H>0, A>0, k<0………………………………………………(1)in which e is the base of natural logalithm. Differentiating this equation by x, the Distance-Gradient Curve which represents the relation between the gradient and the distance becomes: S=σekx S<0, σ<0, ………………………………………………………………(2) On the other hand, the Distance-Grain Size Curve which represents the relation between the median diameter G, read from the cumulative curve drawn by the weight percentages of size composition with class units in phi terms, of the fluvial deposits on a graded river and the distance x from the starting point can be shown after H. Sternberg as: G=aebx G>0, a>0, b<0………………………………………………………(3) Transcribing the equation (3), and incorporating it into the equation (2), we obtain an equation for the Grain Size-Gradient Curve which represents the correlation between the gradient S and the median diameter G as follows: