西江的河流阶地与洪冲积阶地

西江第一级阶地主要由Q3沉积构成，三水青歧以上属于可被洪水淹没的半埋藏阶地，与Q4河漫滩共同组成冲积平原，青歧以下属于西江三角洲下面的埋藏阶地。第二级阶地主要由Q2末期的冲积物组成，少量可延续到Q3初期；第三级阶地多由Q2中期的冲积物组成；第四级阶地仅在封开发现，是目前广东最高的河流阶地，由Q1末期或Q2初期的冲积物构成。西江两旁有老和新的两级洪冲积阶地，老阶地高出山前倾斜平原，主要由Q2洪冲积物组成，新阶地在山前倾斜平原呈埋藏和半埋藏状态，主要由Q3洪冲积物组成，与少量的Q4洪冲积物共同组成该山前倾斜平原，其成因与西江第一级半埋藏阶地和Q4河漫滩共同组成的冲积平原相似。

THE RIVER TERRACES AND THE PLUVIAL-ALLUVIAL TERRACES ALONG THE XIJIANG RIVER

The first stage terrace of the Xijiang velley is mainly made of Q_3 sediment. Upstream at Qingqi of Sanshui the terrace is semi-buried and may be flooded, and together with Q_4 flood land it composes an alluvial plain, while downstream at Qingqi the terrace is buried below the Xijing Delta. The second stage terrace is mainly made of the alluvium of late Q_2, some alluvium may be continued to early Q_3. The third stage terrace is mostly made of the alluvium of Q_2 at middle phase. The fourth stage terrace is made of the alluvium of last phase Q_1 or early Q_2, which is only found at Fengkai, and is the highest river terrace in Guangdong at present. There are new and old pluvial-alluvial terraces of two stages along both sides of the Xijiang River. The old terrace is well above the slope plain in front of mountain and is mainly made of Q_2 pluvial-alluvial matter, while the new terrace is buried and semi-buried in the slope plain in front of mountain and is mainly made of Q_3 pluvial-alluvial matter, with some Q_4 pluvial-alluvial matter it forms the slope plain.