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Abstract:
Sinks, disappearing streams, hanging valleys, and natural bridges add a karst-like element to the miniature mountain topography represented in badlands. The Chinle Formation [Triassic] of the Petrified Forest in Arizona largely consists of compact, montmorillonitic and illitic claystones. Sinks in it result from disaggregation of swelling clay minerals rather than solution which affects limestone in true karsts. Ravines whose bottoms are pierced by sinks may develop into hanging valleys because their channels, robbed of surface flow downstream from these swallow holes, cannot keep pace with downcutting in the master drainage to which they are tributary. Growth of the sinks soon creates a disappearing stream that continues to deepen the upstream segment of a ravine. Thus the abandoned downstream segment beyond the sinks, no longer eroded by the stream, develops into a transverse barrier. Where the abandoned channel was initially short, the barrier may be eventually narrowed by weathering and slope erosion to form a natural bridge. Other bridges consist of jumbled material. that has collapsed from steep valley walls, undercut by small stream meanders