I love the historical contrast between old and new in this shot. Digital SLR vs Daguerreotype; and new vs old location of the Niagara Horseshoe falls. There is such a huge volume of water that rushes over the Niagara Falls (up to 6 million cubic feet per minute) means that the stone cliff is worn back more than 30cm every year. In the old photo it can be clearly seen how much further forward the Falls were 170 years ago. Notice that the left hand side edge of the Falls had water rushing over it in 1840, but now no longer does. Fascinating on so many levels.