Final answer:
The flat glaciated plain north of St. Petersburg is part of the Eastern European Plain and its inhabitants share cultural similarities with the neighboring Finns.
Step-by-step explanation:
The flat glaciated plain north of St. Petersburg is part of the Eastern European Plain, also known as the Russian Plain. This region is agriculturally productive and historically significant as most of Russia's population live here. The inhabitants of this area, particularly those closer to Finland, are similar in language and culture to the neighboring Finns due to historical interactions and geographical proximity. The Eastern European Plain features a flat terrain and was formed by the retreating glaciers from the last Ice Age, contributing to its current topography and soil richness ideal for farming.