What is readily apparent in the HRSP examples is the very large (~1 km) subsidence feature evidenced by the downwarped reflections within the Hawthorn Group. Discontinuities in the horizontal reflections (yellow vertical lines) may represent stress fracturing associated with the downwarping. Meisburger and Field (1976) identify this large subsidence feature as a pronounced fold. Popenoe and others (1984) identified the top of the Eocene on the downward flexure of the fold to be between -47 m (-150 ft) in the undisturbed section to about -75 m (-240 ft) at the deepest part. Karst-related dissolution at depth and subsequent near-surface subsidence might be another explanation, rather than a structural fold. The area highlighted by a green background (A-A’ and B-B’) appears to contain offlap and cross-bedded reflections that may represent fill when the depression was exposed.

The downwarped reflections of the Hawthorn Group are truncated at about 22 m depth, shown by the blue dashed line in the example profiles. This surface, and a second one near surface (red dashed line), may represent erosional surfaces related to sea level low stands. The area highlighted by a red background shows a second depression with offlap- and cross-bedded fill. This feature may represent an area of resumed subsidence following the first sea level cycle. It may also be an incised fluvial channel with fill occupying the topographic low created by the original subsidence event. These sequences of truncation surfaces and fill may be remnants of the last two sea level cycles, the parallel reflections overlying these sequences being the most recent marine deposition.