Re: Question....where is the extra dirt coming from?
Weathering and erosion of mountains creates "dirt" which is washed by rain/rivers to the lower areas and it collects and compacts into soil and then compacts more (over time) into rock, i.e. sandstone = compacted sand, limestone = (mostly) compacted calcium carbonate/shells.
Also decaying vegetation becomes "dirt" and soil eventually.
Plate tectonics pushes continental plates together and they create new mountains so its all a big circle.
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