FIG. 15.

Comparative morphologies of polygonal oscillation cracks from the 3.48 Ga Dresser Formation, the 2.9 Ga Pongola Supergroup, and El Bibane, Tunisia (modern). The frequency distributions of the polygon diameter/gas hole diameter are similar in all three cases. Dresser Formation polygons are 10–20 cm wide, 2.9 Ga Pongola Supergroup examples are 20–50 cm wide, and examples from the modern sabkha of Tunisia are 15–50 cm wide. In the Dresser Formation, the gas escape holes have a diameter of 1–3 cm, and the comparable younger structures show diameters of 3–10 cm, occasionally up to 15 cm. Examples for such structures are shown in Fig. 14. Color images available online at www.liebertonline.com/ast