REMOVE

1. trees
2. automobiles
3. hills
4. platforms (general)

WHAT

1. Visual Works (hierarchy n … (1)
2. boys (1)
3. exterior views (1)
4. farmers (1)

More 

WHERE

1. Alberta, Canada (1)

WHO

1. Clyfford Still; Still, Cl … (1)
2. Still, Clyfford (1)

WHEN

1. 1920 (1)
2. 1945 (1)
3. between circa 1929 and ci … (1)