A hint as this isn't a cave ending in the normal pot, cave, swallet, etc.
First word describes building material such as concrete or brick that has a vitrified surface (also the word is a synonym for something of poor quality). The second word is a synonym - think base of a valley, or the part of your body that you sit on! Horribly cryptic and not the most well known of caves.
The reference to concrete threw me. As a civil engineer, I've never heard of concrete with a vitrified surface (brick, yes), but clinker is an intermediate step in the manufacture of cement, which is one constituent of concrete.