苗栗丘陵區之階地至今仍殘存者，概分三義、銅苗及通霄等三區。三義區主要為 高位紅壤階地，其階面高度 590 ∼ 350m，對比為 LH、LT1、LT2 等三段。銅苗區的階地分 布於後龍溪及西湖溪兩側， 其中高位紅壤階地的階面高度 300 ∼ 150m，分為 LT3 ∼ LT5 三段，而非紅壤的低位階地分為 FT1 ∼ FT3 三段；通霄區高位階地分布於南勢溪上游，階 面高度 220 ∼ 70m，分為 LT4、LT5 二段，低位階地分布於苑裡溪至大安溪之間。 階地的 紅壤礫層一般厚約數公尺至十數公尺，上覆於頭嵙山層，其與火炎山相礫層不易分出界面， 但與香山相砂頁岩層之界面較為清楚。 紅壤粒徑中數（ Md φ）大部份介於 6.5 φ∼ 7.0 φ，各階地紅壤粒度並無規律變化。階地受新期斷層、背斜、向斜構造之影響，部份階面產 生變形。三義區階地向東及向北傾動，八甲一帶的階地呈向斜變位。通霄東南之階地於倒梯 崎則成反斜崖現象。 至於三義至銅鑼的階地西側陡崖有 100 ∼ 330m 的落差，狀似斷層崖 ，再經侵蝕而成，但其斷層面可能發生在火炎山相巨厚的礫層中，故不易直接找到變位證據 。依本區階地高度、紅壤粒度、礫石覆瓦的特徵，三義南段階地及通霄階地似為大安溪階地 的殘面，而三義以北的階地由後龍溪逐次堆積而成。古後龍溪由南轉向北，階地可分為九階 ，紅壤六階，非紅壤三階，西湖溪為其舊流路。

　　　　　River terraces are well developed in Miaoli Hills. Nine surfaces are recognized, including six lateritic surfaces and three non-lateritic surfaces, and are correlated as LH, LT and FT, respectively. Terrace surfaces older than LT5 have been extensively deformed by recent tectonic events. For examples, terraces in Sanyi area have been tilted northward due to the movement of Sanyi Fault, and the surfaces along the Tunglo Syncline have been warped. According to the altitude and characteristics of lateritic soil and gravels, the southern part of the terraces in Miaoli Hills was formed by Taan River, whereas the northern part was formed by Houlung River, which shifted from south to north since late Pleistocene.