The dark area to the left on the top image is a small ledge. It turns black because you are not getting a return from the transducer after the ledge drops off. Imagine you are shining a flashlight in a dark room. Your sonar is only going to show you what the light directly hits. That will help you imagine what it is you are seeing on side image. In the bottom image, it looks like maybe a large log or some other hard structure.