Riverweed

Paper: 553mm high x 807mm wide

5 x 9 matrix of rectangles,  From right to left and back to right, count rectangles and fill with Y shape any that is a prime number. 4 groups of 5 x 9 rectangles are grouped to make a square and each group is shifted inwards one unit. As  ‘Birds Turning’, different y shape and the four groups are arranged as a rectangle.