这些山丘是冰川作用形成的。
很明显冰川作用就发生在相对不远的过去，因为漂流物很软，像新鲜的沉积物。
It was apparent that the glaciation occurred in the relatively recent past because the drift was soft, like freshly deposited sediment.
然而，使所有的黑土形成冰川作用的问题是，在前寒武纪沉积物中广泛分布有黑土。
The problem with ascribing all tillites to glaciation, however, is that tillite deposits are widespread in Precambrian sediments.
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