The rocks that make up Joppenbergh mountain were deposited during the Late Silurian to Early Devonian ~425-400 million years ago. During this time, the area covering NYS today was covered by a broad, shallow inland sea influenced by fluctuating sea levels. It is in this environment that the limestones and dolostones of Joppenbergh formed. Like most marine environments, this shallow inland sea was full of life. Fossils from this time period can be found all throughout Joppenbergh mountain if you look hard enough.
This image shows the depositional environment of the rocks making up Joppenbergh. Note the Rondout Formation is the unit containing the cement beds, and the New Scotland formation is the unit making up the anticline outcrop and glacial erratics (see map)
If you look on the large rock near the start of the Trestle on the Wallkill Valley Rail trail, Crinoids can be found. Crinoids typically consist of a stem anchoring them to the seafloor, and arms that lie atop the stem almost like a strange flower. Crinoids were abundant during this time period, and all throughout the Paleozoic. Some still even exist today but in different forms.
Brachiopods were also abundant during this time and can be found throughout Joppenbergh mountain primarily at the top of the lookout.