Elevations were an interesting concept back then. The elevation was
tied to the airport, and existed for a certain radius around it. When you
crossed that line, the airport details became visible and the ground level
jumped to match the airport; no gradual transition. This wasn't a visible
effect, it simply changed how soon you would contact the ground.

i guess that explains the strange effect i ran into: i took off from
lake tahoe (5000 ft MSL) and flew down towards oakland (sea level). i
was cruising along, thinking about nothing evil when suddenly the
altimeter and VSI went wild. they kept swinging up and down,
contradicting each other (one said i was climbing while the other
claimed i was descending). another flight produced exactly the same
result...