for Democratic nominees outside the 1912, 1932, 1936, and 1964 landslides. The region was also the only one in antebellum slave states to back the insurgent candidacy of Robert La Follette in 1924; in fact, Comal was La Follette’s top county in the nation with 73.96% of the vote, and Gillespie and Comal were the only counties south of the Mason–Dixon line to give a plurality to his “Progressive” ticket. Geography Because of its karst topography, the area also features a number of caverns, such as Inner Space Caverns, Natural Bridge Caverns, Bracken Cave, Longhorn Cavern State Park, Cascade Caverns, Caverns of Sonora and Cave Without a Name. The deeper caverns of the area form several aquifers, which serve as a source of drinking water for its residents. Wonder Cave in San Marcos was formed by an earthquake along the Balcones Fault. From east to west, Texas Hill Country is where the Southern United States ends and the Southwestern United States begins. Several tributaries of the Colorado River in Texas — including the Llano and Pedernales Rivers, which cross the region west to east and join the Colorado as it cuts across the region to the southeast – drain a large portion of the Hill Country. The Guadalupe, San Antonio, Frio, Medina, and Nueces Rivers originate in the Hill Country. This region is a dividing line for certain species occurrence. For example, the California fan palm (Washingtonia filifera) is the only species of palm tree that is native to the continental United States west of the Hill Country's Balcones Fault. The region has hot summers, particularly in July and August, and even the nighttime temperatures remain high, as the elevation is modest despite the hilly terrain. Winter temperatures are sometimes as much as 10°F cooler than in other parts of Texas to the east. In popular culture The area experiences a fusion of Spanish and German influences in food, beer, architecture, and music that form a distinctively "Texan" culture separate from the state's Southern and Southwestern influences. For example, the accordion was popularized in Tejano music in the 19th century due to cultural exposure to German settlers. Devil's Backbone is an elevated, winding stretch of Ranch Road 12 between San Marcos and Wimberley, then Ranch Road 32 continuing through to Blanco. It has long been the subject of ghost stories. Folklore about it appeared in a 1996 episode of NBC's