What was at issue in the Clark-Van Til controversy?

In the 1940s, Cornelius Van Til and other professors at Westminster Theological Seminary first tried to block Gordon Clark's entry into the Orthodox Presbyterian Church, then later put him on trial for removal from the denomination. **What theological issues were at stake in the controversy, why was it such a serious issue that Clark was removed from the OPC, and who are today's Clarkians and Van Tillians?** For the last question, I'm not looking for a list but lines of demarcation.