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Anomalous Beach Ridges of Sangamon Age
William F. von Drehle (1)
Six large-amplitude ridges parallel the mainland of Gulf County, Florida. The lateral extent of the ridges is about 35 km and their widths range from 300 to 1000 m. Their height above present sea level is between 3 and 6 m. Swales between the ridges are from 1.0 to 4.0 m. above sea level and have a maximum width of 200 m. The ridges are flat topped and show very little evidence of eolian activity. Their regularity is disturbed in places by erosion or human activities.
A portion of the Intracoastal Waterway system, the Gulf County Canal, cuts through the ridges, exposing them in nearly a dip section. Internal bedding is well preserved in many places in all the ridges. It is typically low angle, parallel bedding, dipping primarily seaward.
Although the ridges vary in thickness, each one can be divided into three vertical zones. The lower zone occurs in the basal one-quarter to one-third of the ridges and consists of seaward dipping laminae. Dip angles average less than 5° and typically no more than 5 or 6 laminae share the same dip angle without change. The middle zone extends upward to within 0.7 to 1.5 m from the soil horizon. It is composed primarily of seaward dipping laminae but contains up to 10% landward dipping laminae. Foreslope angles average 5° to 9° and backslope angles average 3° to 4°. Laminae sets showing a common dip occur up to 1 m in thickness. The upper zone extends to the soil horizon. Bedding in this zone is obscured, probably as a result of the action of roots and ground water. Virtually all bedding appears to be of beach origin, mostly foreshore type but with minor bachshore features. Eolian and current type bedding are absent. Laminations within the ridges range from less than 2 mm to as much as 10 cm in thickness.
A statistical analysis of 91 samples from various places within the ridges showed that systematic changes occur between the three established zonations within the ridges. Average mean grain size, standard deviation, and skewness decreased from the lower zone to the upper one whereas the kurtosis increased. These trends correspond with profiles taken up the beach face on some Florida Panhandle beaches.
The composition of the ridges is greater than 99% quartz sand which is typically white but varies locally to yellow or brown. Heavy mineral and clay content varies but their combined total never exceeds 0.5% by weight of the total sample. Humate masses are abundant locally and appear to be formed as roots and other organic matter in the ridges decayed. The ridges are capped by a soil profile up to 2 m thick, which is typically brown but locally yellow.
Internal bedding and sediment parameters strongly suggest that these ridges are beach deposits. External morphology of the ridges resembles that of beach ridges except for the extreme breadth of these. Air photo analysis indicates as many as six smaller ridges have coalesced in places to form each of the larger ridges and field evidence supports this. The Gulf County ridges are therefore suggested as being a type of beach ridge. The difference between these and the classical beach ridges is probably due to the fact that the processes which terminate one ridge to begin another were largely inoperative in this case. A large cuspate foreland of Holocene age, St. Joseph Spit, now protects the southernmost half of the ridge system from wave attack, thus allowing a probable minimum Sangamon age for these ridges. The abundance of humate concentrations supports this conclusion.
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