how niagara falls is formed

Niagara Falls comprises three distinct cataracts.The tallest are the American and Bridal Veil falls on the American side,separated by tiny Luna Island and plunging over jagged rocks in a 180ft drop; the broad Horseshoe Falls which curve their way over to Canada are probably the most impressive.They date back a mere twelve thousand years,when the retreat of melting glaciers allowed water trapped in Lake Erie to gush north to Lake Ontario.Back then the falls were seven miles downriver,but constant erosion has cut them back to their present site.The falls are colorfully lit up at night,and many say they're most beautiful in winter,when the grounds are covered in snow and the waters turn to ice.
Ten miles east of Niagara Falls,the town of LOCKPORT takes its name from the series of locks that raise and lower boats some 65ft at the western end of the Erie Canal.You can see the impressive flight of locks from the Pine Street Bridge,or up close on canal boat tours (May–Nov daily at 12.30 & 3pm,also 10am on Sat; $9; 716/693-3260).

尼加瓜拉大瀑布是怎么形成的?