Rates of waterfall recession, and major factors that influence the rate, were studied using waterfalls in Boso Peninsula, Japan. The mean rate of waterfall recession was estimated by determining the age and original location. The principal factor in determining the rate of waterfall recession is the ratio of the erosive force of stream to the bedrock resistance. This is expressed in terms of measurable variables, which include the discharge (drainage area and precipitation), the width and height of the waterfall, and the unconfined compressive strength of the bedrock. An empirical equation connecting the force/resistance ratios and the rates of waterfall recession is derived.

Tafone-like depressions have developed on the Aoshima sandstone blocks used for a masonry bridge pier in the coastal spray zone. A thin layer of partial granular disintegration was found on the surface in depressions. To evaluate quantitatively the strength of the thin weathered layer, the hardness was measured at the surface of the sandstone blocks using both an Equotip hardness tester and an L-type Schmidt hammer. Comparison of the two testing results indicates that the Equotip hardness value is more sensitive in evaluating the strength of a thin layer of weathered surface rock than the Schmidt hardness value. By applying two methods, i.e. both the repeated impact method and the single impact method, the Equotip tester can evaluate the strengths of fresh internal and weathered surficial portions of rocks having a thin weathering layer. Comparison of the two strengths enables evaluation of strength reduction due to weathering. Figure 2. Measurement ranges of Equotip hardness tester, Schmidt hammer, needle-type penetrometer (model SH-70, Maruto, Tokyo) and cone penetrometer (model YH62, Yamanaka, Tokyo).Figure 7. (a) Aoshima sandstone block used for calibration of Equotip rebound value of weathered rock surface (area A) and fresh surface (area B): the split surface was cut by a diamond saw. (b) Sectional view of the edge of split surface.Sandstone is mainly composed of quartz grains having a diameter of 0·1-0·2 mm. Micro-undulation due to protruding sand grains (white arrows) and cracks (black arrows) seems to be formed by granular disintegration due to salt weathering. The photograph shows that the weathering layer has a thickness of about 0·5-0·7 mm. Figure 8. Rebound values of Equotip (L-value) on weathered surface and split surface of Aoshima sandstone block.impact method (10 individual impacts on different points). Figure 8 shows the results. The rebound values obtained, from both the weathered and unweathered parts of the sample, increase as the number of impacts increases. It should be noted here that the first impact values of the weathered and unweathered portions are clearly different (443 for the former and 551 for the latter), but L max , the mean of the largest three values during the measurement, shows similar values (716 for the former and 720 for the latter). The value of L max thus defined designates the hardness of the unweathered interior of the rock. A new technique for non-destructive field measurement of rock-surface strength Figure 10. Relationship between estimated L-value of fresh rock (L intact -value) and measured L surface -value.value of L surface , which results in a higher δ -value. When considering the strength reduction due to weathering, therefore, the minimum value in the data cluster of each zone is of significance. Figure 9 shows that δ = 0·67 for the spray zone and 0·76 for the intertidal zone. The results indicate that the strength of a block in the spray zone has been reduced to 67% of the strength of the unweathered fresh part, and in the intertidal case it has been render...

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