What characterizes V-shaped valleys in geological formations?

V-shaped valleys are primarily characterized by their formation through the erosion caused by river systems. A fundamental process in this context is downcutting, where a river erodes the landscape, deepening its valley over time. This erosion creates steep, V-shaped walls as the river cuts into the bedrock and surrounding material.

As the stream continues to downcut, the existing valley walls may become unstable and collapse, further contributing to the V-shaped profile of the valley. This dynamic interaction between the river and the geology of the area is a key feature of V-shaped valleys, illustrating how the shape is a direct result of both the erosive action of flowing water and the geological processes affecting stability.

In contrast, the other choices describe characteristics that do not apply to V-shaped valleys. For example, the presence of alternating layers of hard and soft rock may influence landscape formation but does not specifically create the V shape typical of river-cut valleys. Similarly, glacier movements are responsible for U-shaped valleys, not V-shaped formations. Lastly, broad, flat valleys are typically indicative of different geological processes, such as those found in floodplains rather than the steep walls characteristic of V-shaped valleys.