The rates of increase in weathered zones and decrease in rock strength

在年代确定的侵蚀阶地作用下，基岩的风化速率可归纳为：(1)建立了高风化带、中度风化带、轻度风化带和弱风化带厚度增长速率的经验方程。风化带厚度(Z)与风化时间(t)的关系表示为Z=alpha ^<beta>，其中alpha和beta为常数。结果表明：(1)各风化带厚度的增长速率（dZ/dt）不是恒定的，而是随时间(t)逐渐减速；(2)风化等级越低的风化带，其风化速率越快。(2)对比泥岩和砂岩的风化速率（dZ/dt）表明，泥岩的风化速率大于砂岩，且风化带厚度增加。泥岩的风化速率随时间的推移而减慢，最终被砂岩所超越。(3)建立了基岩表面任意深度强度衰减速率的经验方程。泥岩和砂岩从风化开始到强度迅速下降，强度变化不大（砂岩t=7.8d^<1.2>，泥岩t=4.2d^<1.4>）。(4)对比泥岩和砂岩的强度衰减速率（dR/dt），在基岩表面附近，泥岩的风化速率大于砂岩。另一方面，在距离地表30cm以上的深度点，砂岩的风化速率大于泥岩。

The weathering rates of bedrock under dated erosional terraces were concluded as follows.(1) The empirical equations were established to express the rates of increase in the thickness of each of four weathered zones, i.e.highly, moderately, slightly and faintly weathered zone. The relationships between the thickness of weathered zone (Z) and the weathering time (t) are expressed as Z=alphat^<beta>, Whera alpha and beta are constants. It is concluded that (1) the rates of increase in thickness of each weathered zone (dZ/dt) are not constant, but decelerate with time (t) gradually, and (2) the zones with lower weathering grades have a faster weathering rate.(2) Comparism with mudstone and sandstone in the weathering rates (dZ/dt) shows that at first mudstone have larger weathering rates which increase in thickness of weathered zones than sandstone. The weathering rates of mudstone are decelerate with time and finally are passed by sandstone.(3) The empirical equation that shows the rate of decrease in strength at arbitrary depth from the bedrock surface are established. Mudstone and sandstone have no change in strength from the start of weathering to the point where decrease in strength rapidly (t=7.8d^<1.2> in sandstone, t=4.2d^<1.4> in mudstone).(4) Compare with mudstone and sandstone in the rate of decrease in strength (dR/dt), at the point near the bedrock surface, mudstone have larger than sandstone in weathering rates. On the order hand, at the deep point where deeper than 30cm from the surface, sandstone have lager than mudstone in weathering rates.