Let $R$ be a regular local ring, $S$ be a normal extension of $R$ that is finite over $R$ and is unramified at height $1$ primes of $R$. Griffith's article normal extensions of regular local ring (Comment before Theorem 1.6) states that the purity theorem, which says that $S$ is already unramified over $R$, can be easily deduced from the existence of big Cohen macaulay modules.
However, I can't quite see the issue. Any help will be appreciated.
