Chained to the Rocks and shocklands
A permanent with the Mountain land type. Sacred Foundry can be enchanted with Chained to the Rocks.



"Mountain" could mean mean two things:


An object whose name is "Mountain". e.g. Mountain
An object which has the land (sub)type "Mountain". e.g. Mountain and Sacred Foundry


When a card refers to something that can be a name or a type, it always refers to the type[1]. As such, Chained to the Rocks looks for objects with the land type "Mountain".

For example,


A Mountain does have that type. It can be enchanted by Chained to the Rocks.
A Mountain enchanted by Contaminated Ground does not have that type. It is a Land, it is a Basic, and it is a Swamp, but it is not a Mountain (despite having that for name). It cannot be enchanted by Chained to the Rocks.
A Sacred Foundry does have that type. It can be enchanted by Chained to the Rocks.
A Boros Guildgate does not have that type. It is a Land, and it is a Gate, but it is not a Mountain. It cannot be enchanted by Chained to the Rocks.





For references to permanents, this is covered by 109.2.


  109.2. If a spell or ability uses a description of an object that includes a card type or subtype, but doesn’t include the word “card,” “spell,” “source,” or “scheme,” it means a permanent of that card type or subtype on the battlefield.


The same is understood to apply for references to non-permanents, though this is not explicitly stated in the rules.