Firstly, the formation of fossils is not part of evolutionary biology; it just *uses* the fossils which form as evidence.
Fossilisation is geology (or paleontology).
Secondly, bones do not rot - they r made of a mineral (a form of calcium phosphate), & neither do shells (calcium carbonate).
Fossilisation is a rare occurrence, & only happens when the remains of an organism r buried (in sediment or similar). If these conditions exist in a single place, then it is more likely that multiple organisms will be fossilised in the same spot (for example, at the bottom of a sediment-thick lake), so *of course* u find many fossils all together in one place.
The laying-down of layers of rock is a well-understood geological process (and its study is called ''stratigraphy'') - the deeper u go, the further ago the rock was deposited. And we know (from other dating methods like radioisotope dating - but not carbon dating, which does not stretch back far enough) the ages of the deposits.