Eli5 Where does the dirt come from?

Question

When looking at a geological timescale, typically ‘the deeper you dig, the older stuff gets’, right?
So, where does this buildup of new sediment come from?
I understand we’re talking about very large timeframes here, but I still dont really get it.

In: 19

Answer 1

Rocky mountain range used to be about 2 miles higher. All that erosion drained down range that you can see it happening right before your eyes forming canyons, flat top mountains, arches, plains( no big rocks plains) and finally drained all that silt to Gulf of Mexico.