Never made it to the Gunks when I lived in the Old Dominion, but Matt's right, both the Gunks and Seneca are Tuscarora sandstone, a hard quartzite rock of the Tuscarora Formation that runs through PA, MD, VA, and WV. Of this I am absolutely sure for Seneca. The difference is that the bedding angle is nearly vertical at Seneca because of upthrust, whereas at the Gunks, it's horizontally bedded. The rock is nearly bright white and hard as shit...
The NRG is composed of a sandstone called Nuttall (sp??). I'm reasonably sure of this. It's a tan rock, and kinda "blobby-looking" in exposed formations. It cuts a bit easier than the Gunks/Seneca rock, but it is still a bitch to drill...