Looking at this painting, you can realize a key truth about van Gogh's work. The grass, the shading on the rocks, the branches of the trees... all of these strokes are made with the same size and type of brush. Probably a round brush or a rounded filbert.

The most striking features are the blue-black horizontal lines outlining the rocks of the piece, which may have been laid on with a knife.

The sky's brushstrokes could be made with the same brush as the grass, just pressed hard against the canvas, creating a wider stroke.

The small strokes of ocher tall grass could be the same brush, just with lighter pressure applied.

Two things that can be concluded:

1. Van Gogh was limited in tools. He probably made this work with three or less brushes.

2. The limitation of tools forced him to succeed at expression and color rather than realism.

3. Composition is of utmost importance.

He also seems to have made The Rocks in one sitting session following these rules:

1. The rocks themselves and the sky were painted first.

2. The tree and grass were painted second.

3. The rocks were painted grey-white, and then dappled shadows in blue were added on top.

A few segments of the painting break these rules and you can see a struggle beginning to form, but fortunately van Gogh doesn't struggle too much. The painting coheres due to the complexity of greys and sophisticated colors in the sky and rocks, and it runs the risk of falling apart in the simplicity of the greens and later-applied brushstrokes on the ground.

The Rocks gets so close to failing, yet it succeeds. This painting convinces you that such a place exists, and that you could step on the rocks or through the grass under an overcast sky.