Q.What is the difference between till and alluvium?

Think of a river valley as a young, energetic thing — narrow, steep-sided, rushing. Now imagine that same valley after a glacier, a slow-moving river of ice, has spent thousands of years grinding through it. The difference is like comparing a knife-cut to a gouge made by a bulldozer. That transformation is the heart of glacial valley morphology.

A normal river carves a V-shaped valley. The water flows downhill, cutting downward, and the sides slope in toward the stream. A glacier, however, doesn't just cut downward — it scrapes sideways, widening and deepening the whole trough. The result is a U-shaped valley: steep, almost vertical walls, a broad, flat floor, and a shape that looks like a giant letter U carved into the mountain.

Why the difference? A river is a fluid; it follows the path of least resistance. A glacier is a solid mass, hundreds of metres thick, moving under its own immense weight. It doesn't flow around obstacles — it bulldozes through them. As it moves, it plucks rocks from the valley walls and drags them along the floor like coarse sandpaper. This is called abrasion and plucking, and together they do the heavy lifting of reshaping the landscape.

The key idea is that a glacier doesn't just make a valley bigger — it makes it different in kind. A river valley is narrow and steep because water erodes mostly at the bottom. A glacial valley is wide and flat-bottomed because ice erodes the sides as much as the floor. That's why you can look at a valley and tell, just from its shape, whether it was carved by water or ice.

Here's what you'll actually see in a glacial valley, and each feature tells a story:

• The U-shape itself — the signature of glacial erosion. The flat floor is often filled with sediment or a lake, so the "flat" part may be hidden, but the steep, straight walls remain unmistakable.
• Hanging valleys — smaller tributary valleys that end abruptly high above the main valley floor. Before the glacier, these side streams met the main river at the same level. The main glacier carved its valley much deeper, so now the side valleys are left "hanging" — often with a waterfall where the stream drops down.
• Roche moutonnée — a rocky knob that looks like a sheep's back. One side is smooth and polished (where the ice flowed over it), the other side is rough and jagged (where the ice plucked rocks away as it moved past). It's a directional clue — it tells you which way the glacier was moving.
• Ribbon lakes — long, narrow lakes that form where the glacier gouged the floor especially deep, or where a ridge of rock (a rock bar) dammed the valley. The lake sits in the over-deepened part, like water filling a bathtub that's been dug deeper in one spot.