According to the exhibit at the Delta Museum, the Peach Bottom Slate predates dinosaurs by several hundred million years.  A time-line indicates that while Pangaea broke up into several continents 250 million years ago, Peach Bottom Slate formed over 600 million years ago.

The Maryland Geological Survey refers to several studies by Knopf and James(MGS) and offers an in-depth discussion on using relations to granite bodies, as well as plant fossils.  Yet, it questions that perhaps Cardiff and the Peach Bottom Formations are the final deposits of sediment in a geosyncline that started to develop and was mainly filled during the late PreCambrian time, but continued to fill with sediments into the early Paleozoic.  Although the age of the Peach Bottom slate hs been debated for many years, a precise age is still unknown.  The minerals that make-up the slate were deposited in thin layers by settling to ancient seas and river beds.

There are 3 kinds of slate:  mica, clay and igneous.

Mica slate is the type of slate from which roof tiles are made.  It was formed from clay containing silts originally deposited underwater in horizontal beds or seas.  These clay sediments compacted over many millions of years under pressure from the sedimentary deposits above then so that the final clay contents became very small through metamorphosis.  Some beds were subjected to horizontal geologic forces that commanded the bed to fold or heave, vertically or turn upside-down in the Earth.  The geologic movement to undego fundamental changes in its chemical composition eventually to become what we now call slate, primarily consisting of mica in the form of fine flakes in parallel order.