Tory Craig, age 11, of Niagara Falls, Ontario, Canada, for his question:

How did Niagara Falls begin?

In 1969, U.S. Army engineers closed off the American side of Niagara Falls. They studied the cliff to learn how best to make it more secure. Residents in the region know that rock slides from the brinks cause the falls to march steadily back up the river. This threatens generators installed to use the force of the falling water. It also makes us wonder how long the falls have been marching back up the river and how come the thundering water started plunging over the cliffs.

Geologists estimate that Niagara Falls began falling sometime between 20,000 and 27,000 years ago. They can be more precise about how they were formed, for the evidence is scored deep into the earth's crust. The geological story unites the mighty falls, the Great Lakes and their connecting rivers. Together they form a single waterway that was remodeled by ice age glaciers.

Earth formations surrounding the lakes indicate what the region was like before the massive, moving glaciers left their scars. In pre ice age days, a series of streams cut modest channels through the hard surface. They flowed northeastward, down a long gentle slope to the sea. Later, the advancing glaciers paralled their paths. They gouged out the old river valleys and dug a chain of five elongated basins. When at last they retreated, their melting waters filled the basins to form the Great Lakes and their connecting rivers. The tilt of the continent guided the runoff down the same long, gentle slope.

Along most of the route, the water ran down the slope in easy stages. But there was a sizeable step down from Lake Erie to Lake Ontario. The brimming water from Lake Erie spilled along the shallow Niagara River. But in its path stood a mighty escarpment    a steep cliff of softish shales and sandstones, topped with a layer of hard dolomite limestone. When the water first plunged over this precipice, it was seven miles farther down the river, near Lake Ontario.

Then, as now, the thundering cataract swirled in a turbulent whirlpool at the foot of the falls. Its force eroded the softer rocks on the face of the cliff, and as they washed away they undermined the hard layer on top. From time to time, the unsupported ledge collapsed and rock slides plummeted down to the river bed. Each time, the cliff and the falls shifted a step back up the river. Meantime, the river below the falls dug itself a deep, steep sided gorge    seven miles long.

The major Horseshoe Falls steps back a little bit each year. At its present rate, it could have traveled seven miles up the river in 20,000 years. Perhaps we shall find a way to stop or delay its rock slides. Otherwise, eventually the mighty waterway may erase all its old glacial scars. The falls may wear away their steep cliff, the lakes may fill with sediments. Perhaps after another 20,000 years, the old waterway may become a series of streams, much as it was before the glaciers remodeled the region.