What you are looking at is an area that was known as Tabaristan, most of which is the modern-day Iranian province of Mandaran.

The region is very different from the rest of Iran, from which it is largely separated by the Elburz Mountains, which run almost from Azerbaijan to Turkmenistan. The area north of the mountains has a very high rainfall, and (historically at least) was home to dense forests, while the areas south of the mountains are much more arid and steppe-like.

The region of Tabaristan proved hard to conquer compared to the rest of the former Sassanid Empire, and although it occasionally submitted to Arab armies, it was largely independent under local dynasties, which more often than not were Zoroastrian. The region would eventually become Islamized, but it was a process taking several centuries. Even today people in the region speak Iranian languages such as Mazandarani and Gilaki, which are from the Northwestern Branch of Iranian languages, rather than the Southwestern Branch that produced Farsi.