Air mass movements are also influenced by mountain barriers. The rich, deep soil of the prairie was formed from sediment washed down from the Rocky Mountains, mixed with rubble from glaciers, and windblown sand, silt, and clay. Forest regions generally contain about 20-50 tons of topsoil per acre. On the other hand, an acre of tallgrass prairie contains as much as 250 tons of topsoil. The tough prairie sod is a great conserver of soil and water.
Sod acts like a giant sponge catching and holding rainwater.
As a result, the amount of water runoff from the prairie is small compared to land where there is no strong network of roots to absorb the rain. The roots also bind the soil tightly to the earth, protecting it against erosion. Prairie sod is so dense that settlers once used it like bricks to build houses. A square yard of soil just 4 inches deep may contain roots that would stretch for 20 miles if they were placed end to end. Grasses are the basic fabric of the prairie since they are uniquely adapted for life in the open.