LAKE AGASSIZ'S STRANDLINE RECORD ADJACENT TO THE SOUTH OUTLET

Strandlines of glacial Lake Agassiz on the La Mars 7.5’ Topographic Quadrangle (North and South Dakota) illustrate a more complex record of shoreline history for the southernmost end of the lake than previously thought. Past research resulted in the description of four distinct strandlines: Herman (323 m), Norcross (317 m), Tintah (311 m) and Campbell (299 m) first described and named for towns east and southeast of the south outlet channel. Correlation of these strandlines northward was based on their development and elevation relationships from the gently sloping (0.0037) southeastern corner of the lake basin. The La Mars quadrangle is immediately northwest of the lake’s south outlet channel and strandlines are found at every elevation between 328 and 300 m. In the La Mars study area, with a 1.5 m (5’) contour interval, strandlines are formed on the relatively steeper (0.012) north and east-facing slope of a moraine upland. Other than small spits, the strandlines consist of narrow ridges 50 – 150 m wide and usually less than 3 m high. The ridges extend for up to 2 km, or as short discontinuous ridges (0.2 – 0.5 km). Vibracores and bucket auger cores from lagoons behind spits, and on or between shoreline ridges, indicate that very little littoral sediment was deposited along this slope except in the Norcross lagoon, and Herman – Norcross strandplain in the lee of the upland. In addition, oblique aerial photographs taken during a drought period indicate only thin deposits of coarse-grained sediment.

Compared to the southeast corner of the lake, with its four main strandlines, strandlines are found at every elevation in the La Mars study area. Spits are found at 1.5 to 3 m intervals between 314 and 323 m elevation, which may represent slowly varying water levels rather than the four relatively stable levels generally assumed in the past. While it has been suggested that some of the strandlines are off-shore bars, the paucity of coarse sediment forming broad offshore zones suggests a different origin. Instead the spacing of the strandlines ridges suggests that many of the strandlines may be beach ridges that developed in response to short-lived high lake levels controlled by meltwater influx and climate.