[1] Made up of a mix of prairie, savanna, and woodland,[2][3] it forms part of the boundary between the more heavily forested eastern country and the almost treeless Great Plains,[2][3][4] and also marks the western habitat limit of many mammals and insects.

[4] Ecologically, the EPA includes the Cross Timbers as part of the vast Great Plains, which comprise Level I Ecoregion 9.0, stretching from central Alberta in Canada to northern Mexico.

The towns of Sand Springs, Sapulpa, Ada, and Shawnee, Oklahoma fall within this large area; Bartlesville and Okmulgee lie on the eastern edge.

A fairly narrow strip dividing the Eastern and Western Cross Timbers, the Grand Prairie differs in physiography, topography, and land use from both of these, as it is much more nearly level and better suited to agriculture.

A broader, southern extension of the Grand Prairie, found only in Texas; it is underlain by limestone rather than sandstone, and serves as a physiological and vegetational transition to the Edwards Plateau, which it borders to the south.

The region features a limestone substrate as opposed to sandstone, and has greater topographical relief and denser and different vegetation than other parts of the Cross Timbers.

Covering a fairly small area in south-central Oklahoma and underlain by a unique mosaic of several different minerals, this region includes the town of Ardmore.

[2] The Arbuckle Mountains are located in a small area nestled in between regions 29g and h; it is made of folded, rather than dissected, limestone, sandstone, and dolomite, and features the greatest topographical relief of the entire Cross Timbers, though not the highest elevations.

"[16] Rachel Plummer, while a captive of the Comanche in 1836, described it as "a range of timber-land from the waters of Arkansas, bearing a southwest direction, crossing the False Ouachita, Red River, the heads of Sabine, Angelina, Natchitoches, Trinity, Brazos, Colorado...the range of timber is of an irregular width, say 5 to 35 miles wide...abounding with small prairies, skirted with timber of various kinds — oak, of every description, ash, elm, hickory, walnut and mulberry...the purest atmosphere I ever breathed was that of these regions.

They may be considered as the "fringe" of the great prairies, being a continuous brushy strip, composed of various kinds of undergrowth; such as black-jack, post-oaks, and in some places hickory, elm, etc., intermixed with a very diminutive dwarf oak, called by the hunters, "shin-oak."

Most of the timber appears to be kept small by the continual inroads of the "burning prairies;" for, being killed almost annually, it is constantly replaced by scions of undergrowth; so that it becomes more and more dense every reproduction.

The Underwood is so matted in many places with grapevines, green-briars, etc., as to form almost impenetrable "roughs," which serve as hiding-places for wild beasts, as well as wild Indians; and would, in savage warfare, prove almost as formidable as the hammocks of Florida.Robert Neighbors and Rip Ford reached the "Cross Timbers, two parallel strips of timber region that ran down the middle of Texas", in 1849 while blazing an emigrant trail from Austin to El Paso.