The "North Rock" designation includes both the hill itself and the surrounding highlands. In the map included within the game's manual, the area is called Northstar Rock, an anagram of Rockstar North, the developers of the game. In-game, the location is just called North Rock.
North Rock itself is a hill made from gray stone that rises up slowly, and is covered with smaller destructible boulders, which can rarely lead to the stalagmite glitch. The hill is surrounded by trails but there is no direct path up to the peak.
The rest of the area consists of highlands around the rock, which descend for a bit higher, then flatten out in the northwest. This area is dense with redwood trees. On one of the trails, there is a cabin nearby, that shares textures with The Panopticon. This may suggest that the area was used for logging in the past.
North Rock is not home to many myths, because it is located so close to Los Santos. Nevertheless, the area was popular shortly after the game's release, and more recently for brand new players, because the areas of Flint County had yet to be unlocked. As a result, North Rock was one of the first locations that Bigfoot and Leatherface were reported in. Unfortunately, North Rock does not contain any unique mysteries of its own, and all myths are ones from other areas of the wilderness.