Salt, Stone and Wells: A Field Trip Across the Marlow Basin

Field Notes From the Marlow Basin

1Ms. Thandiwe Mabuza's seventh-grade class spent a day on a geology field trip across the Marlow Basin, a wide region in the central United States where the rock layers dip gently down toward the middle, like the inside of a shallow bowl. The first stop was a limestone quarry at the edge of the basin. Limestone forms from shells, skeletons and lime mud that pile up on a sea floor. With the quarry's permission, students collected fossil corals and brachiopods, shelled animals that lived attached to the sea floor. Today, most reef corals live in warm, shallow, sunny seawater. The quarry sells its limestone to make cement and gravel.

2The second stop was a museum at an old salt mine near the town of Easterly, close to the middle of the basin. The guide, geologist Dr. Ruben Esterhuizen, explained that rock salt and gypsum form when seawater evaporates and leaves behind the minerals that were dissolved in it. In a hot, dry climate, a shallow sea that is nearly cut off from the ocean can dry up and refill with seawater again and again, and layer after layer of salt builds up. He showed students Table 1, which comes from wells drilled across the basin. Today the Marlow Basin is hundreds of kilometers from any ocean.

3The last stop was the water department in the town of Cobb Mill, which gets its water from wells. Water from rain and melting snow soaks into the ground and fills the spaces in rock. Sandstone has many connected spaces between its rounded grains. Shale is made of tiny, flat bits of clay packed tightly together, so its spaces are very small and poorly connected. Limestone has few spaces of its own, but water can move through its cracks. Water with more than 1,000 mg/L of dissolved salt is called saline and is not used for drinking. The town wants a new well that gives fresh water and at least 100 L/min, so the water department showed students its tests of four wells (Table 2).