Based on the description provided, the soil that Robin is holding is most likely composed of a high proportion of silt or fine particles, which gives it a soft texture similar to flour. The characteristics mentioned—being damp, soft, not sticky, and feeling like flour—suggest that the soil may also have good drainage properties, indicating it is not clayey or overly sandy.
Therefore, Robin's soil is most likely a silty soil, which is known for its smooth texture and ability to retain moisture while still draining well.