Valley
For the next month, Sam is on the constant move, he cleared almost one-third of the island. Then only he realized that it is almost impossible to clear the whole island in three months. Because it is hard to find the formations on the island. It is just that Sam was initially lucky enough to enter a place with many untouched formations.
But most of the formations which are easier to find are almost emptied by previous visits and due to Sam's extra advantage in numbers which made it easier for him to explore more area in the same time and resulted in him find a few unexplored parts.
The whole time, he acted near the borders of the island and a little deeper, he didn't go too much inside to explore this place.
He wanted to deal with the place step by step.
As for his friends, he didn't see them all this while, they must have been far inside.