That first picture was a BIG help the Cave is right where that Red circle is. So I climbed the top of the mountain and did a slip and slide down to the circle. Found the Cave. BTW If you see that water on your map go a little to the right of it before you start your slide.
Go to the Forgotten Mountain node keeper. Head east up the mountain on foot all the way to the top, you will have to do some substantial climbing. The cave will be about halfway down on the northern face, look out below for the particularly jagged vertical rocks sticking out above the entrance.