Complex sentence: The ground at the intended location proved to be too unstable to support a structure as heavy as the planned obelisk; at that site, WNW from the Monument, there now stands a small monolith called the Jefferson Pier.
Split into simpler sentences: The ground at the intended location proved to be too unstable to support a structure as heavy as the planned obelisk. At that originally intended site, which is WNW from the current monument, there now stands a small monolith called the Jefferson Pier.