Observations on a Pen and ink drawing by Vincent Van Gogh 1853-1890

I was asked to find a pen and ink drawing of an outdoor natural scene by Vincent Van Gogh from the internet and make observations of the types of marks he employed. I have found The Olive Trees a pen and ink on paper downloaded from the Bridgeman Education Library.
Types of marks used:
1.  The foliage of the trees are a series of short lines in undulating columns each column going in different directionsgiving a feel of movement of wind blowing through the trees.  He has also indicated darker tones using a darker coloured ink.  The lighter strokes appear to be of a sepia colour and the darker strokes in a grey/black hue.
2.  Hatching is used in the field in the background towards the right and also small hatching strokes on a 'rocky' area in the foreground in the lighter coloured ink.  He has left larger areas of white paper here giving the impression of light.
3.  Short lines in the foreground to the left some rounded off to look like small rocks or mounds in the darker ink .  He has also used this technique under the trees on the left in a more concentrated fashion with the darker ink and the sepia colour gining depth and deeper shadow.
4.  He has used stipling in the foreground along with short marks and this gives a textural feel of short grasses and shrubland.
5.  He has also used zigzag lines to indicate foliage/plant shapes on the ground.
6.  Small hatching strokes on the bark of the trees.
7.  Stronger lines over distant rocks and softer lines on the foreground.

The use of the two different coloured inks the sepia and the grey/black give the picture a Watteau feel in a very Van Gogh way.
  I just love this picture, I have never looked in so much detail at a picture to discover how an artist created a piece of work it was very enlightening and exciting.