Because the geological structure is relatively soft, the lateral erosion force of the rivers is powerful and the range of disintegration is expansive, so that river bluffs of greater magnitude have developed here.
天祥位於大沙溪及立霧溪匯流處，由於地質結構較軟，河川切割時側蝕力量大，崩塌範圍廣，乃形成規模較大的河階。