Fjords, long and narrow inlets with steep cliffs, are typically formed by glacial erosion. Glacial erosion refers to the process by which glaciers, large bodies of ice, carve out the landscape as they move and flow. Fjords are commonly found in areas that have experienced glaciation, such as Norway, Alaska, and New Zealand.

Here is a detailed explanation of how fjords are formed through glacial erosion:

1. Glacial Formation:
- Glaciers form in high-altitude areas where snow accumulates and compacts over time.
- The weight of the snow causes it to transform into ice, creating a glacier.
- Glaciers are constantly moving downhill due to their own weight and gravity.

2. U-shaped Valleys:
- As glaciers move, they erode the landscape beneath them, especially in areas with softer rock.
- Glaciers carve out U-shaped valleys as they grind against the bedrock, removing large amounts of material.
- The erosion is more pronounced at the base of the glacier due to the immense pressure and weight.

3. Tributary Glaciers:
- As the main glacier moves down the valley, smaller tributary glaciers join it from the sides.
- These tributary glaciers also erode the surrounding rock and contribute to the widening and deepening of the valley.

4. Glacial Erosion:
- Glaciers erode the landscape through various processes, including plucking and abrasion.
- Plucking occurs when the glacier freezes onto rocks and pulls them out as it moves.
- Abrasion refers to the grinding action of the glacier against the bedrock, wearing it down.

5. Formation of Fjords:
- As the glacier continues to erode the valley, it creates a deep and narrow U-shaped trough.
- When the glacier retreats or melts, the trough is filled with seawater, forming a fjord.
- The steep cliffs on the sides of the fjord are the result of the glacier's vertical erosion.

In conclusion, fjords are typically formed by glacial erosion. The movement and melting of glaciers carve out deep U-shaped valleys, which are later filled with seawater to create the distinctive fjord landscape. This process takes place over thousands of years and is influenced by factors such as climate, geology, and topography.