glacial ice with a depth great enough for the ice to flow：

1. with a depth great enough for the ice to flow不是独立主格，而是介词短语作定语，修饰glacial ice。

2. 在这个介词短语中，great enough for the ice to flow是形容词短语作定语，修饰a depth，相当于一个定语从句。补充完整为：glacial ice with a depth which is great enough for the ice to flow

意思：有足够深度可让冰块流动的冰川。