The polygonal fractures formed as frozen ground experienced thermal contraction induced by repeated cooling events. Owing to the equatorial location of the Fountain Formation, we suggest that diurnal, rather than seasonal, temperature variations provided the repeated cooling mechanism. Alternative causes of polygonal fracturing, such as desiccation of clay-rich sediments or thermal contraction of evaporite minerals, are untenable because the hosting strata contain minimal clay (< 14 % ) and are framework supported, indicating that there was insufficient space for either clay or evaporite minerals. A thermal contraction origin for these features implies that the equatorial Fountain Formation experienced at least two episodes of remarkably cold conditions. Furthermore, using maximum reasonable stream gradients (~ 0.02) between the polygonally fractured surfaces and the shoreline (gauged from shallow-marine deposits of the Denver basin), the fractures formed at relatively low elevation (≤ 1800 m).