The mountainous sections of the Great Plains were formed long before the remaining areas were outlined by erosion. Uplift of the Black Hills and the Central Texas Uplift began as the continental interior was raised and the last Cretaceous sea was displaced, 65 to 70 million years ago. They stood well above the surrounding plains long before any sediments from the distant Rocky Mountains began to accumulate at their bases. In southern Colorado and northern New Mexico, molten rock invaded the sedimentary layers between 22 and 26 million years ago. The Spanish Peaks were formed at this time from hot magma that domed up the surface layers but did not break through; the magma has since cooled and solidified and has been exposed by erosion. Elsewhere the magma reached the surface, forming volcanoes, fissures, and basalt flows. A great thickness of basalt flows accumulated at Raton Mesa and Mesa de Maya between 8 and 2 million years ago. Volcanism has continued intermittently, and the huge cinder cone of Capulin Mountain was created by explosive eruption only 10,000 to 4,000 years ago. Most of these volcanic masses were formed before major downcutting by the streams began. Other igneous intrusions and volcanic areas in the northern Great Plains similarly were formed before the streams were incised. To examine the origin of the present landscape and of the landforms typical of the various sections of the Great Plains, it is convenient to begin with the Black Hills, the Central Texas Uplift, and the Raton section simply because they were formed first—they existed before the other sections were outlined. BLACK HILLS The Black Hills is a huge, elliptically domed area in northwestern South Dakota and northeastern Wyoming, about 125 miles long and 65 miles wide (fig. 10). Rapid City, S. Dak., is on the Missouri Plateau at the east edge of the Black Hills. Uplift caused erosion to remove the overlying cover of marine sedimentary rocks and expose the granite and metamorphic rocks that form the core of the dome. The peaks of the central part of the Black Hills presently are 3,000 to 4,000 feet above the surrounding plains. Harney Peak, with an altitude of 7,242 feet, is the highest point in South Dakota. These central spires and peaks all are carved from granite and other igneous and metamorphic rocks that form the core of the uplift. The heads of four of our great Presidents are sculpted from this granite at Mount Rushmore National Memorial. Joints in the rocks have controlled weathering processes that influenced the final shaping of many of these landforms. Closely spaced joints have produced the spires of the Needles area, and widely spaced joints have produced the rounded forms of granite that are seen near Sylvan Lake (fig. 11).