Palisades sill (Wikipedia)
USGS on the Palisades (NYC Regional Geology)
Geology of the NYC Region (Source of the above chapter)
This site provides some helpful interpretation of features visible along the Henry Hudson drive, including the lower contact of the sill, columnar jointing of sill basalt and an embedded xenolith (photo above). GPS coordinates are provided. More detailed discussion can be found in this report.
Dept of geomorphologyPeriodic mass-wasting events occur along the HHD which, after all, is carved into the side of a steep escarpment. The photo shows a landslide that occurred in the Spring of 2009 following heavy rains. This event echoed an even larger slide that had occurred a month or two earlier at the same site, that completely covered the road. By early September the raw hillside thus exposed was largely covered with new growth, and soon it will be difficult to recognize the location. One can easily imagine that all the slopes we see above us, as we pedal along, have slid at one time or another and are in some stage of plant succession, as the cycle continues.