REMOVE

1. hills
2. trees
3. hats
4. coats (garments)
5. sky
6. trains (vehicle groupings)

WHAT

1. Visual Works (hierarchy n … (1)
2. automobiles (1)
3. banks (landforms) (1)
4. black-and-white film (1)
5. boys (1)

More 

WHERE

1. New Windsor, Maryland (1)

WHO

1. Campbell, Sandra L. Still (1)
2. Clyfford Still; Still, Cl … (1)
3. Sandra L. Still Campbell; … (1)
4. Still, Clyfford (1)

More 

WHEN

1. 1969 (1)