* Erosion: Glaciers, unlike rivers, can erode both vertically and horizontally. This means they can carve deep, U-shaped valleys with steep sides, unlike the more gentle, V-shaped valleys carved by rivers.
* Overdeepening: Glaciers often carve out their valley bottoms to a deeper level than the original river valley. This creates a "trough" shape with a relatively flat floor, making the valley appear straighter.
* Abrasion: Glaciers carry rocks and debris embedded in their ice, which grind against the valley walls. This abrasive action effectively smooths out curves and irregularities, contributing to a straighter appearance.
* Plucking: As a glacier melts, water can seep into cracks in the rock. When it freezes, it expands, breaking off chunks of rock and further smoothing the valley walls.
* Lateral erosion: Glaciers also erode laterally, widening the valley floor, further enhancing the straight appearance.
It's important to note that:
* Pre-existing topography: The original shape of the river valley plays a role. If the valley was already relatively straight, the glacial action will further emphasize this characteristic.
* Not always straight: Glacial valleys aren't always perfectly straight. Some may have meanders, curves, or tributaries, especially in areas where glaciers were less powerful or where pre-existing topography was more complex.
In conclusion, while glaciers don't actively "straighten" valleys, their powerful erosive forces often create a straightened appearance by deepening, widening, and smoothing out pre-existing river valleys.
