Hill Valley of the Past was a display showing Hill Valley as it was during the Pleistocene era. It was developed by Lamont's House of Ermine for the Hill Valley Science Exposition of 1931. It, along with the Hill Valley of the Future exhibit, was displayed outside the front of Hill Valley High School.
The exhibit included models of a:
- A caveman with fur hanging around his neck, while sinking into a tar pit.
- A tree
- A dinosaur
Unlike Hill Valley of the Future, this exhibit only had one button and it provided description only. It pointed out how the present (at the time) was free of worries compared to the past, including examples such as:
- The past having a Tyrannosaurus which was much more dangerous than a wolf.
- Drowning in debt won't as bad a drowning in a tar pit.
Upon looking at the diorama, Marty joking referred to the dinosaur as a Tannenosaurus. This may or may not be a reference to the The Animated Series episode "Forward to the Past" which included a dinosaur credited as Tannensaurus.