A deep valley characterized by a step-like slope is known as a canyon. A canyon is formed by the erosive action of water over a long period of time, typically through a process known as downcutting. As the water flows through the valley, it gradually erodes the rock, creating a steep-walled and narrow channel.

Option 1: A U-shaped valley is not the correct answer. A U-shaped valley is formed by glacial activity, where a glacier scrapes away the sides and bottom of the valley, resulting in a wide and U-shaped channel.

Option 2: A blind valley is also not the correct answer. A blind valley is a valley that does not have a through-flowing stream or river because it ends abruptly without an outlet.

Option 3: A gorge is a narrow and steep-sided valley that is typically created by the erosive action of a river. While similar in some ways to a canyon, a gorge is usually shorter in length and located closer to a water source.

Therefore, the correct answer is option 4, canyon, as it best describes a deep valley with step-like slopes created by water erosion.