In case you did not know what you were looking at, those strips of rock are actually old lava flows. If you could image the old mountain being all around those strip, when the volcano was erupting, there was lava flow oozing up between cracks in the mountain and running down the mountain side. As everything cooled and over the millions of years, the rest of the mountain just weathered/eroded away and all that was left is the solidified lava flow that came up between those fisures in the old mountain. That rock is harder than the mountain itself, that is why as it eroded, the mountain disappeared first, leaving the old lava flow exposed. You can see the same thing in Colorado.