The sedimentary rock is called the Tapeats Sandstone. It is
typical of the kind of sand that might accumulate along a
shoreline, although in this case, the beach existed 525 million
years ago. It would have been a strange environment to those of
us in the world today...there was no life on land, just barren
rock and sediment. There was life in the sea, but few species
that would be familiar to us. The fossils most commonly found
are trilobites, arthropods that are vaguely similar to pillbugs
and horseshoe crabs. There were sponges, and some brachiopods
(lampshells), and many strange and wonderful soft-bodied
creatures who left little in the way of a fossil record (google
'Burgess Shale' if you want to know what they looked like). Then
the land slowly sank, and sea transgressed across the landscape.
The beach sands of the Tapeats buried all but the highest of the
hills, which projected as modest islands. They weren't
completely inundated until several tens of millions of years
later. That's what I found incred