Wind and water.
Wind moves sand and dirt. Seas, rivers and lakes flood and leave behind a layer of dirt. Sometimes large fires or volcano eruptions lay down a layer of ash. That effect over 65 million years means they get buried quite deep. During that time, the loose layers of sand, dust and dirt also compress and petrify, becoming stone.
Of course, all that dirt and sand has to come from somewhere, so not every skeleton gets preserved. The ones that aren’t preserved eventually get worn down by the elements.
So, in short, not every dinosaur gets buried. But the deeper they’re buried, the better they’re preserved.
>Did rock form over them
Yes, that’s why geologists can date rocks, sediments get constantly deposited in seas / lakes / rivers / lowlands etc., and form layers one over another.
The reason we can find them is because eventually those rocks get uplifted by tectonic activity (stuff moves around) and are now hills or mountains, which start eroding instead (depositingthe sediments in seas / lakes… You get it). As those rocks erode, we can find fossils hidden in them.
As a small factor, there is also material our planet picks up from space. Estimates are that we acquire about 5200 tons of space dust per year. Multiply that by 65 million years, and that adds roughly 338 billion tons of dust. Reality is, that’s probably less than an inch or two over the whole earth, but it still adds to the erosion materials.
It’s not just dinosaurs. Our modern world is quite literally built upon the previous one. You know the stories about digging up artifacts and entrances to old cities? Many generations worth of history only to end up as the literal foundation for just another regular city and we don’t even think much about it in our daily lives
But our time in the world is miniscule compared to how long ago the dinosaur’s ages were, millions of years of wind and dirt will bury you deep.
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