ORIGIN OF BEACH RIDGES AND SWALES

Sandy beach ridges occur in four main categories: (1) Swash-built, (2) Settling lag, (3) Eolian, (4) Storm surge. Ridges in the first two cl asses are geometrically regular, only a few tens of centimeters above adjacent swales, and commonly in ridge sets and systems (tens to hundr eds of ridges each). Individual sets (of 5-25 ridges) tend to stand 0. 5 m to 2 m above (or below) adjacent sets. Ridges in the third and fou rth classes de, not have these characteristics, do not occur in sets, and are generally not suitable for detailed historical studies. No one of these ridges is the same as the storm-built berm, which is almost never preserved. Swash-built sandy beach ridges have diagnostic (1) ma p spacing, (2) accretion rate, (3) periodicity, (4) cross-bedding and (5) granulometry. The last two indicate fair-weather waves on a sandy beach, in contrast with settling-lag ridges (Postma style), which have the same external geometry but which were deposited without important wave work; and in contrast with storm-built berms, generally seen on eroding coasts, but rarely or never in beach ridge plains. The grain-s ize kurtosis of beach sand is an excellent index to near-shore wave en ergy density. Changes in kurtosis at ridge set boundaries mark changes in long-term wave energy density, hence in sea level. The latter can be deduced also from set height differences and from spacing differenc es. Periodicity is typically 30-60 years, map spacing 25-50 m and accr etion rate fairly close to 1 m/yr. A few ridge sets, with intervals of 3-7, 10-12, or 18-19 years, have smaller spacings. This is not what c an be done, if each ridge is built by one storm. If each ridge had bee n built by one storm, wide beach ridge plains (like one in Denmark, da ting from about 11,700 yr B.P.), would have been formed in the last 30 -50 years, at accretion rates of hundreds of meters per year. Instead, each swash-type sandy beach ridge was made by a sea-level rise-and-fa ll couplet (amplitude, 5-30 cm). The swale marks the lower position. T his mechanism reflects the fact that the transverse profile, from beac h to sea, is gently concave upward, with maximum curvature close to sh ore; this is a shape which is out of adjustment with the shoaling wave system shortly after a small sea-level change. Settling-lag ridges we re built without waves, hence the ridge-and-swale sequence in this cas e cannot be attributed to waves, either fair-weather or storm. These r idges show the same historical pattern as do swash-built ridges; this fact suggests that both were controlled by the same mechanism (sea-lev el change).