fossils are in sedimentary rocks. Sedimentary stratums
offer us a registry more or least exact of geological
time. The age of fossils found in a stratum is similar
to the age of that stratum.
Granitics rocks, not sedimentary, don't have fossils.
Calcareous rocks are the ones that have more fossils.
When these rocks are half-decomposed, fossils can be collected
Yellow sand from Mioceno can have fossils very well preserved.
In Spain, the Triassic has not fossils. It has rocks and
sand of a color like rose or red wine. Nevertheless, in
Countries like Germany, fossils from that period are abounts
and very good.
Quaternary era, characterized by deposits of clay, doesn't
have fossils. Neither deposits of gravel have.
Where to look for, then?
In quarries. In calcareous rock or marl mountains. In
torrent's bottoms where fossils could have been left by
the water. In the sides of new roads, slopes, etc...
The experience is the best friend, as you can know where
to find them just having a look to a new area, or where
is better not to lose your time.
Learn how to recognize the rocks. This is the first step..