The focal point of this work at first appears to be the fisherman in the center foreground of the painting. His fishing pole points diagonally across the river toward a boat colored with a dab of red paint. Further up the riverbank one sees the eponymous sandpit and sand barge, and to the left of those a village which likely is Valmondois. Together the sandpit and barge create a triangle with the fisherman and the boat, suggesting that at the heart of this tranquil scene is the juxtaposition of labor and leisure. It has been noted by scholars that, at the time Daubigny would have painted this work, the Oise River valley was growing industrially and thus losing its bucolic charm. In response to this, the artist frequently removed these signs of labor so as to present instead a peaceful landscape. Here, however, he has not so much as removed the elements of industry but minimized them so that the viewer focuses on the fisherman and a life of leisure in the countryside in spite of this change.
I have an article on this painting and two other works from the Columbia art collection coming out soon in Nineteenth-Century Art Worldwide, a free, peer-reviewed e-journal. When it is released I will put a link to the article where you can read more about this important work in the collection.