Which of the following statements with reference to Glacial trough is/are correct ? 1. Alpine glaciers strip valleys of their soil, regolith and sediment to form glacial troughs. 2. When the sea level is below the floor of a glacial trough, the seawater enters as the ice front recedes, creating a fiord. Select the answer using the code given below :

Correct — C, Both 1 and 2. Statement 1 is straightforward glacial geomorphology. A valley glacier in a mountain range does not flow over its bed so much as grind into it: the ice picks up rock by plucking, drags that debris along its base, and abrades the floor and walls with it, stripping away soil, regolith and loose sediment down to bedrock. A river cuts a narrow V; ice widens the whole cross-section into the U-shaped trough, broad-floored with steep sides and truncated spurs. Statement 2 is the one the paper has damaged, and the card marks it rather than repeating it. A fiord is a glacial trough that the sea has drowned, and for the sea to get in, the floor of the trough has to lie below sea level — NCERT puts it as 'very deep glacial troughs filled with sea water and making up shorelines'. The booklet prints the relationship the other way round, saying the sea enters when sea level is below the trough floor, which is the one arrangement in which no water can enter. Taken as the textbook sentence it was copied from, the statement is true and both statements stand, which gives (c); taken word for word, statement 2 fails and the answer would be (a). Learn the condition in its correct form: floor below sea level, sea floods in as the ice front retreats.

• (a)1 only — This is what a strictly literal reading of the printed statement 2 gives, and a student who spots the inversion has reasoned well. It is not the answer carried here because the sentence is a garbled copy of a standard textbook line rather than a deliberate error, and a key would follow the intent. Keep the reasoning; the geography behind it is right.
• (b)2 only — Throws out statement 1, which is uncontroversial. Alpine glaciers are erosive agents precisely because they strip a valley of its loose cover and then work on the rock beneath.
• (d)Neither 1 nor 2 — Would require denying that glaciers erode valleys at all. The U-shaped trough, the hanging valley and the truncated spur are the standard evidence that they do.

A glacial trough is the valley a glacier leaves behind — U-shaped in cross-section, with a broad floor, steep walls, truncated spurs where old ridge ends were sliced off, and hanging valleys where smaller tributary glaciers could not cut as deep. Ice erodes by plucking rock from the bed and by abrading the bed with the debris it carries. Where such a trough reaches the coast in high latitudes and its floor lies below sea level, the sea floods it once the ice retreats, and the drowned trough is a fiord.

The mechanism worth carrying away is why a glacier can cut below sea level when a river cannot. A river is graded to the sea: as it approaches sea level its gradient flattens and it stops incising. Ice has no such limit, because it is driven by the weight and slope of the ice surface rather than by the level of the water body it ends in, so a thick valley glacier keeps deepening its bed well past the shoreline. That is why fiords are so deep — the Sognefjord in Norway reaches more than 1,300 metres, far below the sea outside — and why a shallow sill of rock or moraine is often left at the mouth where the ice thinned and lost its cutting power. One printed slip has to be flagged: the booklet reverses the relation between sea level and the trough floor in statement 2. Reproduced here as printed, corrected here in the teaching.

• NCERT: 'Glaciated valleys are trough-like and U-shaped with broad floors and relatively smooth, and steep sides.'
• NCERT: 'Very deep glacial troughs filled with sea water and making up shorelines (in high latitudes) are called fjords/fiords.'
• A fiord forms where the trough floor lies below sea level; the sea moves in as the ice front withdraws.
• Fiord coasts are concentrated in Norway, Greenland, Chile, New Zealand, British Columbia and Alaska — all formerly glaciated high-latitude coasts.
• Cirques, aretes, horns and hanging valleys are the other erosional forms of the same ice; the Matterhorn and Everest are horns cut back by radiating cirques.

• Confusing a fiord with a ria. A ria is a drowned river valley with a V-shaped, tapering profile; a fiord is a drowned glacial trough, deep and steep-sided.
• Assuming ice cannot cut below sea level. A glacier is not graded to the sea, which is exactly why fiords are so deep.
• Reading a statement past its wording. This item's second statement inverts the condition, and noticing that is worth more than the mark.

Usually as a landform-to-agent match or a two-statement item on glacial erosion, and sometimes as a single-line definition question asking what a fiord is.

• practice — not a real PYQ

  A fiord is best described as

  • (a)a drowned river valley on a coast of subsidence
  • (b)a deep glacial trough invaded by the sea
  • (c)a coastal lagoon closed off by a sand bar
  • (d)a submarine canyon cut by turbidity currents

  Answer(b) a deep glacial trough invaded by the sea — the trough floor lies below sea level, so the sea floods it once the ice has gone.

• practice — not a real PYQ

  Truncated spurs and hanging valleys are landforms produced by the erosive work of

  • (a)running water
  • (b)wind
  • (c)moving ice
  • (d)groundwater

  Answer(c) moving ice — a valley glacier slices off the projecting spurs and deepens the main trough faster than its tributaries, leaving them hanging above it.