Eric Hazelitt, age 15, of Harrisburg,Penn., for his question:

WHAT IS A FALL LINE?

A fall line is a boundary between an upland area and a coastal plain. Streams that come out of upland regions at s fall line will drop over rapids or waterfalls to the softer or less rocky soils of the coastal plain.

The low Atlantic coastal plain used to be the bed of a shallow ocean. It is made up of layers of sand and clay left by the water.

The Piedmont Plateau rising to the west was formed from hard rock squeezed together and folded up into mountains. These were later worn down to the level plateau. When the streams rushing toward the ocean over the hard rocks of the plateau reached the coastal plain, they quickly wore down the softer material and formed falls or rapids.

For this reason, the boundary line between the coastal plain and the Piedmont Plateau is called the fall line. Fall lines in other parts of the world are similar to this one in general characteristics.