Endogenic and exogenic forces

The correct answer is A: potholes. A pothole is a rounded or cylindrical hollow formed in the bedrock of a river channel. First, a small depression or crack exists in the river bed. During fast flow, pebbles and other hard fragments become trapped in it. Swirling water makes these stones rotate, and they repeatedly strike, scrape, and grind the bedrock. This process, called abrasion or corrasion, gradually enlarges and deepens the depression until it becomes a rounded hole. A, potholes, directly describes this fluvial erosional feature. B, a spit, is a narrow ridge of sand or shingle deposited along a coast by longshore drift, not a hole in a river bed. C, a barchan, is a crescent-shaped sand dune formed by wind in dry regions. D, a moraine, is debris transported and deposited by a glacier. Memory cue: rotating stones drill potholes in river beds.

Direct answer: A, sea cave. A sea cave is a hollow chamber formed when waves erode a weak zone in coastal rock. Waves repeatedly attack cracks, joints, and softer layers, especially near the base of a cliff. Water pressure forces into openings, while sand and pebbles carried by the waves act like abrasive tools. Gradually, the opening becomes larger and extends inward, producing a cave. A is correct because it is the coastal hollow made by wave erosion. B is wrong because an oxbow lake forms when a river cuts through the narrow neck of a meander and abandons the old loop. C is wrong because a barchan is a crescent-shaped sand dune formed by wind, usually where sand supply is limited. D is wrong because a moraine is a deposit of rock debris transported or left behind by a glacier. If a sea cave is eroded through a headland, it may become a sea arch; this sequence helps distinguish the terms.

The direct answer is D: ventifact. A ventifact is an individual stone or rock that has been shaped, polished, grooved, or given flat faces by wind-driven sand and dust. In dry regions, strong winds carry abrasive particles. When these particles repeatedly strike an exposed rock, they slowly wear away its surface. This process is called wind abrasion or aeolian abrasion. The rock may develop sharp edges, smooth polished faces, or several angular facets. A, a spit, is a narrow ridge of sand or gravel deposited along a coast by longshore transport, so it is a depositional coastal feature rather than one wind-shaped stone. B, a gorge, is a deep and narrow valley, commonly cut by a river. C, a horn is a sharp mountain peak formed by glacial erosion around a mountain. D is correct because it names a single stone modified by wind. Memory cue: “venti” means wind-related; a ventifact is a wind-shaped artifact of rock.

A is correct. A glacier can carry clay, sand, pebbles and large boulders together. When the ice melts or retreats, it deposits this mixed load without sorting the particles by size, so the moraine is generally unsorted.

B is correct. A moving glacier can drag rock fragments across the bedrock beneath it. These fragments scrape the surface and may leave long grooves or striations, often aligned with the direction of ice movement. Deltas, dunes, and coastal bars form through water or wind processes, not glacial abrasion.