In Episode 21 ("Ragtag"), we see a flashback from 15 years ago, when a young Agent
Grant Ward is taken to the Wyoming woods by his Hydra mentor John Garrett
Garrett leaves him there, alone, for six months, to see if he can survive.
At the time, Ward believes that he is being recruited by S.H.I.E.L.D.
Ward survives and prospers in the wild. So Garrett teaches him other skills,
such as shooting, and tells him about Hydra - a secret agency within S.H.I.E.L.D.
These wilderness scenes were shot at Disney's Golden Oak Ranch,
at 19802 Placerita Canyon Road, in Newhall, CA.
(Which makes sense, since Disney owns both ABC and Marvel.)
Newhall is in the hills north of the San Fernando Valley, about five miles southeast of
Six Flags Magic Mountain, and about 30 miles northwest of downtown Los Angeles.
They've shot several S.H.I.E.L.D. scenes at the ranch, including scenes set
in Wrigley, Pennsylvania, Batesville Utah, Canada and Colorado.
They shot these scenes at two locations inside the Disney ranch:
Pine Lake - just northwest of the ranch's main entrance - and
Piney Woods - up near the north border of the center of the ranch.
(For the wide shot, however, they CGI'd in a few mountains in the background.)
Here is a Google StreetView of the main gates to Golden Oak Ranch.
Below are photos of the ranch's Pine Lake and Piney Woods: