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Shells, Lime and Marble: The Rock Cycle in a Farm Valley

Model how limestone moves through the rock cycle and what energy drives each step.

Answer Key

The creek in a dry year

Read the passage and study Table 1. Then answer the questions.

What the Dry Creek Showed

1Cedar Flats is a farm town in a wide valley of wheat and alfalfa fields. Last year was the driest anyone there could remember: from January through June, the valley got only about a quarter of its usual rain. Cedar Creek shrank to a trickle, and in places its bed dried out, showing gray bedrock that had been underwater for decades. Seventh grader Wren Castellanos, whose family farms beside the creek, noticed that the bedrock was full of small fossil seashells.

2Wren took her questions to Siddharth Rao, a geologist with the state geological survey. He explained that the bedrock is limestone, a sedimentary rock made mostly of shells, the skeletons of sea animals and lime mud that piled up on a shallow sea floor that covered the region long ago. Buried under more layers, the material was pressed and cemented into rock. Layers like these usually form nearly flat. At a quarry on the valley floor, workers crush the same limestone into a powder called ag lime. Farmers spread it on their fields to make the soil less acidic, and over several years, rain and soil water slowly dissolve it.

3In the Ridge Hills east of the valley, an old quarry cuts into white marble. Mr. Rao said the marble began as the same kind of limestone. Long ago, as huge pieces of Earth's crust pushed together, some of the limestone was buried several kilometers deep. There, heat from Earth's interior and great pressure changed it into marble without melting it. The same pushing slowly raised the hills, while rain, frost and flowing water wore away the rock above. Along the creek near the hills, Wren saw that the limestone layers lean steeply instead of lying flat.

4The county measures how much sediment (sand, silt and mud) Cedar Creek carries past the Route 9 bridge each month. Wren compared the dry year with a normal year (Table 1).

Table 1. Rain and creek sediment, January to June
MonthNormal rain (mm)Dry-year rain (mm)Normal sediment (t)Dry-year sediment (t)
Jan903040060
Feb802035040
Mar701530020
Apr501015010
May305605
Jun100100
t = metric tons (1 t = 1,000 kg).
1.

From January through June, how many fewer metric tons of sediment passed the bridge in the dry year than in a normal year? 1135 t

2.
Which explanation of the dry-year sediment data fits best?
  • A. Farmers spread more ag lime, which glued the sediment to the creek bed.
  • B. The dry soil hardened into limestone, so less loose sediment remained.
  • C. Less rain meant less flowing water to wear away rock and carry sediment.
  • D. Heat from Earth's interior weakened during the dry year and slowed erosion.
3.
Cedar Flats is hundreds of kilometers from any ocean. Which explanation of the fossil seashells in the creek bed fits the passage best?
  • A. The rock formed from shells on an ancient sea floor, now dry land.
  • B. Birds carried shells inland long ago and dropped them into the creek.
  • C. Floods carried the shells upstream from the ocean into the valley.
  • D. The shells grew inside the rock after the creek dried out last year.

Modeling the rock cycle

Use paragraphs 2 and 3 to build and use a model of the rock cycle in the valley.

4.
Complete the model with words from the bank. Two words are not used.
Word bank: cemented · heat · melting · uplifted · weathered · sunlight · cooled

Shells and lime mud piled up on a shallow sea floor, were buried, and were cemented into limestone. Deep underground, heat from Earth's interior and great pressure changed some limestone into marble without melting it. Pushing forces slowly uplifted the marble into hills, where rain and frost weathered the rock above it and then the marble itself.

5.
Circle the three processes in the valley that are driven mainly by energy from the Sun, through weather and the water cycle.

Rain dissolving ag lime in a fieldLimestone changing into marble deep undergroundCedar Creek carrying sand past the bridgeThe Ridge Hills slowly risingFrost cracking marble on a hilltop

6.
Suppose uplift and erosion together brought the marble toward the surface at an average of 1 mm per year. (1 km = 1,000,000 mm)

About how many years would it take to bring marble up from 3 km deep to the surface? 3000000 years

7.
What is most likely to happen over a very long time to the ag lime that farmers spread on their fields?
  • A. It melts in the summer sun and soaks into the bedrock as magma.
  • B. It is used up by the soil and stops existing after a few years.
  • C. It turns back into marble once it has lain on the field long enough.
  • D. Its dissolved bits travel with water and may settle as new sediment.
8.
Which arrow belongs in a model of the rock cycle in this valley?
  • A. Marble → (weathering and erosion) → sediment
  • B. Marble → (cementing in the sea) → magma
  • C. Sediment → (melting deep underground) → limestone
  • D. Limestone → (cooling at the surface) → marble
9.
Near the hills, the limestone layers lean steeply. Which explanation fits the passage best?
  • A. The creek wore the layers into a leaning shape as it flowed past.
  • B. Pushing forces in Earth's crust tilted the layers after they formed.
  • C. The shells and mud settled on a steep slope, so the layers formed tilted.
  • D. The dry year shrank the creek, and the layers leaned as they dried out.
10.
A farmer says, "This dry year stopped the rock cycle in our valley." Do you agree? Make a claim, use evidence from Table 1 and the passage, and explain your reasoning with a model of the rock cycle and the energy sources that drive it.

Claim: I disagree. The dry year slowed some surface parts of the rock cycle, but it did not stop the cycle. Evidence: From January to June the creek carried only 135 t of sediment instead of 1,270 t, so erosion slowed a lot, but it still carried some sediment, such as 60 t in January. Rain still fell, about 80 mm, so ag lime and rock kept dissolving slowly. The marble formed deep underground from heat from Earth's interior and pressure, and the hills are being raised by pushing forces in the crust, which do not depend on rain. Reasoning: Weathering and erosion are driven by the Sun's energy through weather and the water cycle, so less rain means less flowing water to move sediment. Burial, the change to marble and uplift are driven by energy from Earth's interior, which keeps working in wet and dry years. Because the parts are driven by different energy sources, the cycle keeps going, just more slowly at the surface in a dry year.