Superposition and cross cutting are both things that the Grand Canyon experience. The tectonic plates move, causing it to split. That's why it is the way it is today. 
     Superposition is the deposition of one geological rock on another. Cross cutting is when the rock splits, just like when humans cut rock to make, for example, jewelry or sculptures. Take limestone for example. I watched an episode of "Dirty Jobs" when they were working in a limestone mine to cut slabs of rock for things like counter-tops. 

     Quizlet is a website where you input the vocabulary words and their definitions, then play games to help you learn them. We use Quizlet for every section in science. 
     I think Quizlet is a helpful website to use as long as you actually use it to study. The vocabulary words are so hard to learn to spell, but there's one game called "Speller" that will say the word to you, (the definition is given to the right) and if you get it wrong, they'll spell it, then have you try again. Then, there's a Beat-The-Clock kind of game. I don't like competition, so I stick with the spelling game.
     I do use Quizlet to study. I'll try a little every now and then, then I'll spend about 2 hours on there the night before the test to really get those words sunken in. I don't know of any other sites like Quizlet that could help you the way Quizlet does. Quizlet is a really great site to use when studying. I even created a set for my 9-year-old sister who has a lot of trouble spelling. Although it'll take her longer to get them, all due to her attention span of a mouse, I have faith in her that she will.

     There were several contributions to Darwin's understanding of evolution and natural selection.