American Falls &amp; Bridal Veil Falls were formed by water of the Niagara River cascading over an escarpment of Silurian-aged Lockport Group dolostones.  This erosion-resistant stratigraphic interval forms an escarpment throughout much of eastern North American (America &amp; Canada), from the Niagara area to Wisconsin.  Dolostone is a crystalline-textured chemical sedimentary rock composed of dolomite (CaMg(CO3)2 - calcium magnesium carbonate).  Most dolostones form by the diagenetic chemical addition of Mg to pre-existing limestones.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/19500738044/">American Falls &amp; Bridal Veil Falls (Niagara Falls, northwest of Buffalo, New York State, USA) 3</a>