counties, including the towns of Killeen, Copperas Cove, and Lampasas as well as the Fort Hood Army base, fall within this region. Among the few major roads that cross the Limestone Cut Plains are US 281 north to south and US 84 east to west. 29f: Carbonate Cross Timbers This ecoregion exists as an enclave within the Western Cross Timbers, stretching from southern Jack County, Texas across northwestern Palo Pinto County into eastern Stephens County, as well as tiny parts of Young and Eastland counties. 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. No towns of any size lie within this area, although Possum Kingdom Lake and State Park do; the region is crossed by US 180 and Texas State Highway 16. 29g: Arbuckle Uplift 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. 29h: Northwestern Cross Timbers An extension in two branches of the Cross Timbers into southwestern Oklahoma, this area features reduced tree density and height, but also small forests dominated by sugar maple, bur oak, and live oak in deeper river canyons. The towns of Duncan, Oklahoma and Walters, Oklahoma, lie in this region. 29i: Arbuckle Mountains 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. The landscape includes many caves, sinkholes, springs, and streams. I-35 crosses this region north to south. Climatology Part of the difference in the Cross Timbers region and the surrounding regions west (drier) and east (wetter) has to do with the dry line which separates humid air from the Gulf of Mexico from the dry air of the Llano Estacado, the Texas Panhandle, and the High Plains. History The thick growth formed an almost impenetrable barrier for early American explorers and travelers. Washington Irving, in 1835, described it as "like struggling through forests of cast iron." 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,