So what is this showing? There are two distinct rock units here. On the right are interlayered gray shales and sandstone and the beds are tilted such that they are almost vertical. On the left are beds of an orange-brown rock called dolostone which are tilted about 45 degrees from horizontal. There is a distinct contact between these two units running from the lower-left to the upper-right of this picture.
Shale is made primarily from quartz and various clay minerals (lithified mud). Sandstone is made from quartz (SiO2). Dolostone is basically limestone (calcium carbonate or CaCO3) with some magnesium tossed in -- (Ca,Mg)(CO3)2. They're very different types of rocks.
Geologists call the unit on the right (the older one, since it's below the other), the Austin Glen Formation and they call the unit on the left (the younger one), the Rondout Formation. It's actually very easy to distinguish them from each other in the field (they look more different in person than they do in the image).
This contact, where the Rondout Formation lies on top of the Austin Glen Formation and they both have different orientations is not just seen here. You can drive to other locations in the area to see this as well. Geologists call this type of contact an angular unconformity.
Fossil evidence in the Austin Glen indicates it's from a period of time called the Middle Ordovician Period (let's say 465 Ma). The Rondout Formation is from the Late Silurian Period (let's say 420 Ma). That means that there's some 45 million years missing in that unconformity! Missing rock strata indicates from that from around 465 Ma to 420 Ma, this area was undergoing a time of erosion - there were no sediments accumulating to later form rock layers.
A very basic principle in geology, first formulated in the mid-1600s by Nicolaus Steno in Italy, is that of original horizontality. What this means is that sediments originally accumulate in horizontal layers which then lithify into horizontally-bedded sedimentary rocks. If the layers are tilted, such as the one's above, then some force later tilted them. Another of Steno's principles is superposition - the basic idea that in layered sedimentary rocks, the oldest layers will be at the bottom. Pretty simple.
So what does the outcrop above tell us? An entire sequence of events...
1. Around 465 Ma, the Austin Glen Formation was laid down as marine muds and with periodic influxes of sands in horizontal beds in a deep basin (the fossils tell us this). They gradually accumulated and were buried. Over time they lithified into shales and sandstones.
2. Sometime between 465 and 420 Ma, the Austin Glen rocks were tilted by some type of event. Looking at numerous outcrops reveals that these layers were actually folded and cut through by faults due to horizontal shortening in a present-day east-west direction.
3. Around 420 Ma, the Roundout Formation was deposited as horizontal sediments in a shallow, tidal-flat environment in a hot, arid climate. Eventually these sediments were buried and lithified into beds of dolostone. At this time, the angular unconformity existed between the Rondout and Austin Glen Formations but it too would have been horizontal.
4. At some time younger than 420 Ma, Everything was folded and faulted yet again by another shortening event (again in a present-day east-west direction).
In other words, what this outcrop represents is two distinct tectonic events. We know, from studying metamorphic and igneous rocks to the east, that these were huge mountain-building events. To form each of the rock layers, tilt them twice, erode them flat, etc. will take very long spans of time - in this case tens of millions of years. Some time later, I may discuss these but the post is getting long enough as it is.
One last thing I'll mention is that in 1788, a Scottish physician and amateur geologist named James Hutton saw an outcrop at Siccar Point on the coast of southwestern Scotland. At Siccar Point, tilted strata of red sandstone dated around 345 Ma overlie almost vertical strata of graywacke (a type of sandstone) dated around 425 Ma (80 million years of missing time). It's almost exactly analagous to the Taconic unconformity in Catskill.
Siccar Point from Wikipedia
Back in Hutton's day, they didn't know the ages of these rocks, but Hutton did recognize that such an outcrop showed an immense span of time. To create such an outcrop through naturalistic processes would have taken far, far longer than the traditional 6,000 year Biblical age of the Earth. It was the beginning of modern geology.
I could keep going for pages but I guess I'll stop here and pick this up another time.