Survey Notes: Soil and Water in Tamarack Gap
1Tamarack Gap is a town of about 3,000 people in a steep mountain valley in the western United States. Small earthquakes rattle the valley a few times a year, and a fault, a long break in the rock where huge blocks slip past each other, runs along the east side of the valley floor. In 2019, a landslide came down the west side of the valley and broke the pipe that carried the town's water from Spring S1. Now the town council wants a safer water source and land for a community farm. Geologist Mr. Anselm Takeda asked two seventh graders, Odette Kariuki and Bruno Ferraz, to help him survey the valley.
2Soil forms where frost, plant roots and water break bedrock into small grains, and dead plants rot and mix in. On these slopes, bedrock turns into soil at about 0.1 mm a year. Gravity also moves soil downhill, slowly as it creeps a little each year and suddenly in landslides. During floods, the river spreads mud and sand over the valley floor. The team dug five soil pits and measured how thick the soil was (Table 1). Pit C is in the bare scar the 2019 landslide left behind, and Pit D is in the pile of soil and broken rock the landslide dropped at the foot of the slope. The soil in Pit E is made of many thin layers of silt and sand.
3The valley sides are made of two rock layers. The upper layer is sandstone, which has connected spaces between its sand grains, so water moves through it easily. Under it lies shale, made of tightly packed clay, which water can barely move through. The boundary between the two layers is at about 1,450 m above sea level on both sides of the valley. On the west side, the layers tilt gently down toward the valley. On the east side, they tilt gently down away from the valley. Rain and snowmelt soak into the sandstone on the high ridges. Along the fault, earthquakes over many thousands of years have ground the shale into broken, crushed rock. The team measured the flow of every spring in the valley (Table 2). Apart from Spring S4, no water comes out of the shale anywhere in the valley.