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Reading passage Grade 7 English Language Arts

Sinkholes and Springs: The Limestone Under Ossett

A grade 7 informational passage on karst country, where water races through cracks in limestone instead of filtering slowly, with literal and inferential questions.

Objectives

  1. Students will explain how water moves differently through limestone than through sand and gravel.
  2. Students will cite textual evidence to support an inference about why a spill can reach a spring quickly.
  3. Students will define karst, sinkhole, and spring using the passage.

Vocabulary

Sinkhole
A sinkhole is a hole or depression in the ground that forms, sometimes suddenly, when underground rock dissolves or a cave roof collapses. Sinkholes are common in areas with karst topography, where rock such as limestone slowly dissolves underground.
Spring
A spring is a place where groundwater flows out of the ground on its own. Springs form where the water table meets the land surface, often on hillsides or at the bottom of slopes.

Also: karst, limestone, conduit, dye trace

Teacher overview

Groundwater is usually described as water creeping slowly through sand and gravel, which is accurate for much of the country but wrong for limestone country. This passage introduces karst, where rain has dissolved open channels in the rock and water can travel for miles in a day. Set in the fictional town of Ossett, the text uses a dye test and a collapsed pasture to show why a spill far from town can reach a spring within hours. It pairs well with a unit on groundwater or watersheds, and it gives students a clear chain of evidence to cite.

Sinkholes and Springs: The Limestone Under Ossett — Grade 7 English Language Arts
Name: ____________________________ Date: ______________

Passage

Under the fields around the fictional town of Ossett lies a thick layer of limestone. Limestone is a rock made mostly of the shells and skeletons of sea creatures that lived here when this valley was ocean floor. It is hard enough to build with, and it also dissolves slowly when the water touching it is even slightly acidic.

Rain picks up a small amount of carbon dioxide as it falls and as it soaks through soil. That makes the water slightly acidic. Weak acid does almost nothing to granite, but it dissolves limestone. Over thousands of years, rain has widened the natural cracks in the rock under Ossett into passages. Some are the width of a pencil. A few are large enough to walk through.

Geologists call this kind of landscape karst. It is common across many parts of the United States, and most of it looks ordinary from the road. A few surface features give it away. Streams disappear into holes in their own beds and are not seen again. Round ponds sit in fields with no creek feeding them. Water bursts out of a hillside in a steady flow and becomes a creek in a single step.

That last feature is a spring, and Ossett's water comes from one. Ossett Spring rises at the base of a wooded ridge, and the town has drawn from it since the first mill was built there. The spring runs clear most of the year. After a heavy rain, it runs brown for a day or two, and it can carry twigs and bits of leaf.

That detail matters more than it looks. In sand and gravel, water travels slowly, sometimes only a few inches a day, and the sand strains out particles as the water passes. Sand and gravel do part of the cleaning. In karst, water moves through open passages instead, and the passages act like buried pipes. Geologists call them conduits. Water in a conduit can travel a mile in a day, or faster. Very little is strained out along the way. Whatever enters the ground can arrive at the spring almost unchanged.

Ossett has a measurement of that speed. Some years ago, a farm pasture six miles north of the spring began to sag, and then a section of it collapsed into a hole the width of a truck. A collapse like that is a sinkhole, and it opens a direct door into the rock below. Because the farmer worried that his pond drained toward town, the county hired a team to run a dye trace. The team poured a harmless green dye into the sinkhole and placed cloth detectors in every spring and well within ten miles. The dye reached Ossett Spring in about thirty hours.

Thirty hours is not much protection. If the dye had been fuel from an overturned truck, the town would have had a day and a little more to act. That is why Ossett now does things a flatland town might skip. The utility keeps a list of every sinkhole in the recharge area. It asks farmers to fence livestock away from the ones that stay open. It watches the weather, because a hard rain is the moment when the spring runs dirtiest. When the turbidity rises, operators can switch to a storage tank and let the spring settle.

None of this means karst water is bad water. Ossett Spring has supplied the town for generations, and the treatment plant is built for the conditions it faces. It does mean that the ground under Ossett offers less protection than the ground under a town built on sand. The people who manage that spring have to think about the whole area that drains into it, not just the ridge where the water comes out.

Questions

  1. According to the passage, what makes rainwater able to dissolve limestone?
  2. How does the passage define karst, and what are two signs of it on the surface?
  3. What happens to Ossett Spring after a heavy rain?
  4. Explain the difference between how water moves through sand and gravel and how it moves through a conduit in limestone.
  5. Why does the passage say that the muddy water after a rain "matters more than it looks"? Cite at least two details.
  6. What is a sinkhole, and why does the passage call it a direct door into the rock below?
  7. The dye reached Ossett Spring in about thirty hours. What does that result suggest about the town's ability to respond to a spill? Support your answer with evidence.
  8. Based on the whole passage, why must Ossett's utility pay attention to land six miles away from the spring?

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: Carbon dioxide picked up as rain falls and soaks through soil, which makes the water slightly acidic. The passage notes that this weak acid barely affects granite but does dissolve limestone.
  2. Answer: Karst is the landscape formed where rain has dissolved cracks in limestone into passages. Accept any two surface signs from the passage: streams that vanish into holes in their own beds, round ponds with no creek feeding them, or water bursting from a hillside as a spring.
  3. Answer: It runs brown for a day or two and can carry twigs and bits of leaf. The passage contrasts this with the clear water the spring produces most of the year.
  4. Answer: In sand and gravel, water moves slowly, sometimes only inches a day, and the sand strains out particles. In a limestone conduit, water moves through an open passage like a buried pipe, can travel a mile a day or faster, and very little is strained out.
  5. Answer: Answers should cite that the brown water and floating twigs show material is arriving at the spring unchanged, and that conduits strain out very little compared with sand and gravel. Together those details indicate that whatever enters the ground upstream can reach the spring quickly.
  6. Answer: A sinkhole is a collapse of the ground surface into the rock below, like the pasture that sagged and then fell in. The passage calls it a direct door because material entering it goes straight into the passages rather than filtering through soil.
  7. Answer: It suggests the town would have only about a day and a little more to react to a spill six miles upstream. Students should cite the dye result and the passage's comparison with fuel from an overturned truck.
  8. Answer: Because water that enters the ground miles away travels through conduits and arrives at the spring quickly and largely unchanged. Accept answers citing the dye trace, the sinkhole list, or the request that farmers fence livestock away from open sinkholes.

Differentiation

Support

Provide a two-column chart labeled "sand and gravel" and "limestone conduit" with the rows already headed speed, straining, and travel distance, and let students fill it in before answering question 4. Allow questions 1 through 3 to be answered with copied sentences.

Extension

Ask students to write a one-page set of rules that Ossett could ask nearby landowners to follow, drawing every rule from a detail in the passage, and to add a sentence explaining the evidence behind each rule. Rules with no support in the text should be crossed out.

ELL supports

Pre-teach "limestone," "dissolve," "sinkhole," "spring," and "conduit" with photographs and a demonstration in which a piece of chalk sits in vinegar overnight. Provide the frame "In sand, water ___, but in limestone, water ___" for question 4.

Extensions

The United States Geological Survey publishes maps showing where karst occurs. Students can find whether their own county sits on karst, then write a short paragraph predicting what that would mean for a local well, using the reasoning in the passage rather than outside research.