A substratum of something is a layer that lies under the surface of another layer, or a feature that is less obvious than other features.
例句
...its deep substratum of chalk...深厚的白垩地层His recreation of the city is credible, with a substratum of fact to bolster the fiction.他对该城历史的再现是可信的，虚构的小说情节隐念着一些事实作为支撑。