Here's why:
* Downward Erosion: The stream's energy is focused on cutting vertically downwards, eroding the bedrock and soil below.
* Base Level: This is the lowest point to which a stream can erode. It could be sea level, a lake, or another stream.
* V-Shape Formation: As the stream cuts down, it forms a narrow, steep-sided valley with a V-shaped cross-section. The sides of the valley are relatively straight and converge towards the stream's bottom.
Important Note: While a V-shape is the most common form for a valley primarily shaped by downward erosion, other factors like rock type, climate, and time can influence the valley's shape. Over time, weathering and mass wasting can widen the valley, making it less V-shaped.
