Unit 1 - Page 13 - 16

What is the primary cause of waterfall formation?

• Rising sea levels

• Sudden change in gradient along a river

• Accumulation of sediments

• Tidal action

Answer explanation

Why waterfalls form (b): A waterfall forms when a river flows over a sudden steep drop. For example, Niagara Falls formed where hard rock resists erosion but soft rock below gets worn away.

Which rock type is commonly resistant to weathering, contributing to waterfall formation?

• Limestone

• Sandstone

• Igneous rocks like Whin Sill

• Shale

Answer explanation

Resistant rocks in waterfalls (c): Hard rocks like igneous rocks (e.g., Whin Sill) don’t erode easily. That’s why waterfalls like High Force in England exist—because tough rocks protect the top.

What process involves the gradual collapse of the resistant rock at the top of a waterfall?

• Hydraulic action

• Undercutting

• Abrasion

• Deposition

Answer explanation

Undercutting (b): The water erodes the softer rock underneath the hard rock at the waterfall's edge. Over time, this makes the hard rock collapse. Think of peeling an onion layer by layer.

Gorges often form as waterfalls retreat due to:

• Erosion of underlying soft rock

• Deposition of sediments

• Weathering of resistant rock

• Tectonic activity

Answer explanation

Gorge formation (a): When a waterfall retreats (moves back), it leaves behind a narrow valley called a gorge. For instance, the retreat of Niagara Falls has created a gorge downstream.

What are levees?

• Natural embankments formed by repeated flooding

• Depositional features at river deltas

• Raised platforms in coastal areas

• Erosional landforms

Answer explanation

Levees (a): Levees are raised areas on the riverbanks made by sediment left during floods. Natural levees, like those along the Mississippi River, protect surrounding areas from flooding.

Which area is typically found behind levees?

• Floodplain

• River bluffs

• Backswamps

• Alluvial fan

Answer explanation

Backswamps (c): These are wet areas behind levees that don’t drain well, often turning into swamps. For example, the Atchafalaya Basin near the Mississippi Delta.

River bluffs are characterized as:

• Gently sloping areas along floodplains

• Steep edges marking the boundary of a floodplain

• Depositional beds in deltas

• Erosion-resistant plateaus

Answer explanation

River bluffs (b): These steep edges mark where the floodplain ends and higher land begins. They’re visible along rivers like the Missouri, where bluffs separate the valley from farmland.

Floodplains are primarily formed by:

• Erosion of riverbanks

• Deposition during floods

• Tidal action

• Glacial movement

Answer explanation

Floodplain formation (b): When rivers flood, they deposit sediment, creating flat, fertile areas called floodplains. These are ideal for farming, like the Nile’s fertile floodplains.

Which type of delta is characterized by a fan-shaped pattern?

• Bird’s foot delta

• Cuspate delta

• Arcuate delta

• Lacustrine delta

Answer explanation

Arcuate delta (c): A fan-shaped delta, like the Nile Delta, forms when the sediment spreads out evenly, creating a wide, curved shape.

What defines a cuspate delta?

• A triangular shape formed by gentle wave action

• A narrow, finger-like shape caused by strong river currents

• A circular shape influenced by tidal forces

• A delta that forms in lakes

Answer explanation

Cuspate delta (a): Shaped like an arrowhead, these deltas form where waves push sediment into a triangular shape. The Ebro Delta in Spain is an example.