Physiographic features

The Bluff Hills and the Southern Rolling Plains (EPA Level IV Ecoregions 74A and 74C, respectively) of the Southern Mississippi Valley Loess (MLRA 134) are located in southwest Mississippi and southeast Louisiana. The areas lie within the Coastal Plain Province of the Atlantic Plain. The underlying geology consists of marine deposits of sand, silt, clay, and lignite of the Pascagoula, Hattiesburg, Catahoula and Citronelle formations. The Bluff Hills, which bound the Mississippi River floodplain, are capped by loess deposits often greater than 50 feet thick (Chapman et al., 2004). The adjacent terraces of the Southern Rolling Plain are loess mantled as well.

“Loess” is the geologic term of German origin that refers to widespread deposits of homogeneous layers of friable, porous silt mixed with minor amounts of clay or fine sand (Heinrich, 2008). The loess mantle, created by well-sorted windblown silt, was deposited during the Pleistocene age. Its source was glacial sediment from glacial meltwater that was flowing down an extensive braided stream system depositing large volumes of silt over the floodplain of the Mississippi River (Heinrich, 2008). Glacial meltwater ceased flowing when southern edges of ice sheets stopped melting in fall and winter, thereby creating dry conditions on the previously flooded Mississippi River Valley. Strong seasonal winds blew across dry floodplains and eroded large quantities of silt-sized sediment, and transported it out of the Mississippi alluvial valley and deposited it on adjacent uplands and terraces (Heinrich, 2008). Over thousands of years, the silt accumulations created loess deposits that are many feet thick (Heinrich, 2008). The Bluff Hills and Southern Rolling Plains are covered mainly with 2 separate layers (and ages) of loess deposits, the older and lower Sicily Island loess and the younger Peoria loess at the soil surface. In Louisiana, the Sicily Island Hills (composed of basal strata of undifferentiated Catahoula Formation, overlain by Citronelle Formation deposits) are mantled with deep loess (25 to 30 feet thick in places) on the ridgetops and some entire side slopes.

Where blankets of loess are thicker than 6 feet, the soils formed entirely in loess. Where loess deposits are less than 6 feet thick, soils reflect the nature of the underlying parent material (McDaniel, 2001). Thick loess areas produce intensely dissected terrain with excessively steep slopes and ridge and ravine topography (McDaniel, 2001). The Bluff Hills and the Sicily Island Hills tend to have deeper, calcareous loess and steeper, much more dissected topography than the Southern Rolling Plains to the east. The loess mantle on the Southern Rolling Plains begins to thin and become more acid in the east as it transitions to the Southeastern Plains. Stream gradients in the Bluff Hills are high with narrow drainageways and floodplains, while the stream gradients become lower with broader floodplains in the Southern Rolling Plains.

This Site is located in the very steep, highly dissected Sicily Island Hills in Louisiana and the very steep, highly dissected Bluff Hills adjacent to the Mississippi River floodplains and the very western portion of the uplands of the Southern Rolling Plains in Mississippi and Louisiana. In extreme southwest Mississippi and adjacent area in Louisiana, the site is in an area locally known as the Tunica Hills. This site occurs on long, narrow, winding ridgetops and steep and very steep side slopes, upper parts of drainageways, and sharp breaks near ridgetops. Slopes mainly range from steep to very steep (12 to 90 percent), and erosion is a severe hazard. Many of these sites are in eroded to severely eroded areas with gullies cutting through unstable sideslopes and drainageways.

Table 2. Representative physiographic features

Climatic features

The Southern Bluff Hills portions of MLRA 134 in Mississippi and Louisiana has a warm, humid climate, with fairly long summers and relatively short winters. The result is a long growing season and abundant plant growth. As you move northward in this region temperature trends lower and Precipitation is not as well distributed. This change in distribution does not imply that there is a rainy season and dry season, however there is a change in distribution. Water is a definitive part of this landscape, largely due to the combination of low elevation and fairly abundant rainfall in most years. Mean annual precipitation ranges from 50 to 70 inches over this region, and is fairly well distributed throughout the year. There have been very few years when less than 50 inches of precipitation has fallen. Snow is a rarity, however chances increase as you move Northward through the region. Growing seasons are long, typically from late February to late November. Hurricanes and tropical storms impact the climate of this region predominately in the southern areas, with some impact occurring nearly every year in some areas. However, devastating storms do not occur too often, and heavy rain is usually the biggest concern compared to wind damage. The following climatic data are averages from the four weather stations listed below. Temperature and precipitation may vary considerably from that listed for each month. Site specific weather data should be used for land management decisions. For site specific weather conditions, obtain data from a weather station close to the site. Information can be accessed from specific weather stations at http://www.wrcc.dri.edu/coopmap/ or http://www.wrcc.dri.edu/summary/climsmla.html.

Table 3 Representative climatic features

Climate stations used

• (1) NATCHEZ [USC00226177], Natchez, MS

• (2) PORT GIBSON 1 NE [USC00227132], Port Gibson, MS

• (3) WOODVILLE 4 ESE [USC00229793], Centreville, MS

• (4) BATON ROUGE RYAN AP [USW00013970], Baton Rouge, LA