GEOG 1710: Soils and vegetation in North-Central Texas.
Introduction & Background.
The North-Central Texas region provides an excellent example of
correlation between geology, soil, topography and vegetation. Rocks
underlying the region formed in the Cretaceous Period, about 100
million years
ago.
Fig. 1. Cretaceous paleogeography of the U.S.
Texas was under a shallow
subtropical
ocean during most of this period. Sand was deposited in coastal areas
(beaches, intertidal flats, river deltas), mud and clay accumulated
offshore from river mouths and limestone
formed further offshore. Layers of sandstone, shale (mud and clay),
marl
(calcareous shale)
and limestone formed from these sediments and were later tilted and
eroded, such
that their outcrops have a north-south strike and a gentle dip to the
east.
Fig. 2. Geology of north-central Texas.
Four major outcrops run through the
DFW region:
1. Paluxy sandstone in the west (Montague, Wise and Parker counties).
2. A mixture of limestones, clays (shales) and marls running through Cooke County,
western Denton County and western Tarrant County.
3. Woodbine sandstone running through eastern Denton County and eastern Tarrant
County.
4. Eagle Ford Shale, Austin Chalk (a type of limestone) and Taylor Marl running through Dallas
County.
Soils in the region closely reflect
the combined influence of geology, rainfall and vegetation.
Fig. 3. Soils of the North-Central Texas region. "4"
and "5" show locations of Figures 4 and 5 below.
Oak woodlands have developed on the sandy soils of the Paluxy and Woodbine sandstones. Alfisols (from Aluminum Al and Iron Fe) are found in these areas. These soils usually have a thin sandy to loamy surface layer and a clayey to loamy subsoil. The thin sandy soils are readily eroded by surface run-off. Leaching of the surface layer has produced iron and aluminum oxides, giving these soils a "rusty" color. The drier climate in the west supports mainly dwarfed post oaks and short grasses (on the Paluxy sandstone). Larger trees and a mixture of short and tall grasses are supported by the wetter climate in the east (on the Woodbine sandstone).
Mollisols (from the Latin mollis - soft) have developed on the limestones, clays and marls lying between the Paluxy and Woodbine sandstones (Fig. 3). These are calcareous clayey soils that support a mixture of tall and short grasslands, under the fairly dry conditions. These soils have a dark, organic-rich surface layer due to large inputs of organic material from the dense grass cover.
Mainly deep calcareous clayey soils have developed on the shales, limestones and marls in the east of the region (Fig. 3). These are Vertisols (from invert - to turn over), characterized by expansive clay that swells in the wet season and shrinks and cracks in the dry season. These movements cause vertical mixing of the soil (literally "turning the soil over"). The slightly wetter conditions in the east support mainly tall grasslands.
Topography also reflects the combined influence of geology, rainfall and vegetation. The two sandstone regions are generally unevenly eroded and hilly, whereas the limestone, shale and marl areas tend to be more uniformly eroded, making them flatter to gently sloping (although these areas can still have considerable relief where they are dissected by streams and rivers).
Fig. 4. Topography developed on Paluxy sandstone in the
Montague-Wise County border region. Notice the numerous
small hills and highly uneven surface.
Fig. 5. Topography developed on shales and limestones
to
the northwest of Denton. Although dissected by streams,
the surface is relatively even (compared to Fig. 4).
These varying combinations of
geology, soil, vegetation and topography have been used to define contrasting
"natural regions" in North-Central Texas.
Fig. 6. Natural regions of North-Central Texas:
1. Western Cross Timbers. 2. Grand Prairie.
3. Eastern Cross Timbers. 4. Blackland Prairie.
Virtual Field Trip:
Site 1. Western Cross Timbers.
Site 2. Grand Prairie.
Site 3. Eastern Cross Timbers.
Site 4. Blackland Prairie.
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