Reading: Diagram Label Completion: Question 4

Limestone caves form when slightly acidic rainwater, having absorbed carbon dioxide from the soil as it seeps downward, dissolves the calcium carbonate in the surrounding rock over thousands of years, gradually enlarging cracks into passages and eventually into large underground chambers.

Water often enters the cave system through a vertical shaft called a sinkhole, a depression on the surface formed where the roof of a passage below has collapsed or where surface water has steadily dissolved a channel straight down through the rock. Rainwater seeping through the cave roof carries dissolved minerals with it, and as each droplet reaches the ceiling of a chamber, a small amount of water evaporates before the droplet falls, leaving behind a tiny deposit of calcium carbonate; over centuries, countless droplets falling in the same spot build up a tapering, icicle-like formation hanging from the ceiling, known as a stalactite.

Droplets that fall from a stalactite before evaporating completely often land in the same spot on the cave floor below, where the remaining minerals are deposited in a similar way, building upward instead of downward to form a stalagmite. Where a stalactite and a stalagmite growing directly beneath it eventually meet after many thousands of years, they fuse into a single continuous formation running from floor to ceiling called a column.

Beneath the cave's dry upper passages, water that has percolated all the way through the rock collects in the pore spaces and fractures of the limestone, forming an underground reservoir; the upper surface of this saturated zone is known as the water table, and any passage that lies below it stays permanently flooded. Where a passage remains above this level, water draining through it instead gathers into a flowing underground river that carves the passage floor even wider as it makes its way toward a distant resurgence, the point where the water finally re-emerges at the surface.

The diagram below shows a cross-section through a limestone hillside containing a cave system, running from the surface downward to the underground river, and from the cave entrance on the left toward the distant hillside resurgence on the right.

Label the diagram below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1. the vertical opening at the surface through which water enters the cave system
2. the icicle-like formation hanging down from the chamber ceiling
3. the upward-growing formation on the cave floor, directly beneath it
4. the single formation running from floor to ceiling where the two meet
5. the upper surface of the water-saturated zone beneath the dry upper passages
6. the flowing body of water below that level, which carves the passage floor
7. the point where the underground water finally re-emerges at the surface