Mississippi Valley
During the last glacial era, large walls of ice dropped sediment, which is rocks and dirt, onto the land.  This became sedimentary rock.  This rock is softer than the other two main types of rock.  Sedimentary rock forms in layers.  When the Mississippi River began to flow, it washed away parts of the rock.  So there are places in the northern valley with tall bluffs, places where the river water cut some of the rock away.  People can see the layers in the bluffs.  Some of them are 400 feet tall.  In the places where the ground was made of harder sedimentary rock, for example limestone and dolomite, the cliffs are steep.  In the places where the ground was made of softer sedimentary rock, for example shale and sandstone, the cliffs are gentle.