Note: This page is about a Mystcraft feature. For the Thaumcraft structures see Eldritch Monolith.
Obelisks are solid obsidian structures that are comprised of a 2x9x2 pillar centered on top of a 4x2x4 base.
They are only generated in ages with the "Obelisks" symbol, which is a secondary modifier found in the "Terrain Features" tab of the notebook page. Obelisks that are generated partially on air generate 4x1x4 layers down until they hit a solid block (assumption, please verify) or the bottom of the age. Obelisks can also generate on the bottom of an age, as long as there are no blocks in the way.