Yeah, what Jason said. The dents are due to dehydration, and can be caused by low humidity. You may want to try a larger water bowl, putting the water bowl over the warm side, or even decreasing the amount of air flow in the tank. Also, you can try misting the tank, although that is a very temporary solution.

Be sure to measure the humidity with a digital hygrometer, and keep the levels around 50-60%. That should fix the problems right up!