GEOLOGICAL SETTING
The geology of the Monette area along the St. Francis River is that of a complex river valley. It is further complicated by the New Madrid earthquake of 1812 and perhaps some earlier disturbances of the same nature.
On a substratum of undifferentiated plio-miocene deposits, the cross-sectional profile of the valley (Fisk 1944, Plate 15, Sheet 1) shows an elevation of 100 to 125 feet above the present mean sea level as the base of the alluvial deposit. The top of this extensive graveliferous alluvium, 225 feet above sea level, was laid down by the Ohio River as its channel pattern changed through the centuries, burying older channels under later ones as the ocean levels rose and the ice age melted out of existence, thus forming the area known as the Malden Plain. The present surface contour at the town of Monette is 235 feet. At the site under study the elevation readings are from 237 to 240 feet above sea level.
Figure 2. Aerial View of the St. Francis River “Sunken Lands” and the Lawhorn Site
(In circle)
Figure 3. Aerial View of Drainage Ditch and Levee at the Lawhorn Site.
(To the left of the levee is the St. Francis River in its present “Sunken Lands” channel. The site limits are shown by a dotted line)
Figure 4. Contour Map of the Lawhorn Site Showing Levee, Drainage Ditch, Excavated Areas and Grid Control System
This valley story is duplicated west of the area known locally as Crowley’s Ridge where the Mississippi River flowed during the waning of the ice age. Crowley’s Ridge, it should be noted here, is an old land surface that was not eroded by the late glacial run-off waters. It is this relatively unaltered ridge of land that originally separated the Mississippi and Ohio Rivers during the formative age of the present alluvial valley. This ridge, only a few miles west of Monette, Arkansas, offered a totally different environment and was, presumably, one which the Lawhorn people made, at least, seasonal use of. The St. Francis River channel, lying close to the eastern edge of Crowley’s Ridge is the end result of these early braided Ohio River channels while today the Black River has replaced the Mississippi in the western valley.