<P> The concentration of rock debris in the ice and the rate at which sediment is delivered to the tunnel by melting and from upstream transport determines the amount of sediment in an esker . The sediment generally consists of coarse - grained, water - laid sand and gravel, although gravelly loam may be found where the rock debris is rich in clay . This sediment is stratified and sorted, and usually consists of pebble / cobble - sized material with occasional boulders . Bedding may be irregular but is almost always present, and cross-bedding is common . </P> <P> Eskers are critical to the ecology of Northern Canada . Several plants that grow on eskers, including bear root and cranberries, are important food for bears and migrating waterfowl; animals from grizzly bears to tundra wolves to ground squirrels can burrow into the eskers to survive the long winters . </P> <P> In Sweden Uppsalaåsen stretches for 250 km (160 mi) and passes through Uppsala city . </P> <P> Great Esker Park runs along the Back River in Weymouth, Massachusetts and is home to the highest esker in North America (90 ft). </P>

What does this curved ridge (esker) record