Park at the Red Rock parking lot (marked in the picture) off Exit 14 on 128.
Hike WEST on Rt. 133 until you see the trailhead on the North side of the road. It is marked with a little white Essex Greenbelt sign.
Follow this main trail (drawn in ORANGE on the map) into the woods.
There is a NEW TRAIL (drawn in BLUE on the map) that you can now take, which is marked by these little white Greenbelt signs and you get on it by turning right off of the main trail within about 100 feet of the trailhead. This will lead you directly to the Cavalry boulder area.
Otherwise, continue on the original trail (ORANGE) to the NEXT obvious right turn to continue towards the Cavalry Boulder.
DO NOT miss one of the above two right turns or you will be walking the wrong way (in a big loop around the top of the rocky hillside west of the cavalry). This is shown by the DOTTED orange line in the map.
The trail drawn in GREEN will get you from Cavalry, through the Valley, to the Alcove, then if you continue on it you will be hiking into the vast woods of the Extensions Area.