The route surveyed by Captain Gunnison passed around the northern end of the ravine of the Colorado River, which almost completely separates the Southwest from the United States. Farther south, within the United States, were only two available points at which railways could cross the cañon, at Fort Yuma and near the Mojave River. Towards these crossings the thirty-fifth and thirty-second parallel surveys were directed. Second only to Governor Stevens's in its extent was the exploration conducted by Lieutenant A. W. Whipple from Fort Smith on the Arkansas to Los Angeles along the thirty-fifth parallel. Like that of Governor Stevens this route was not the channel of any regular traffic, although later it was to have some share in the organized overland commerce. Here also was found a line that contained only two or three serious obstacles to be overcome. Whipple's instructions planned for him to begin his observations at the Mississippi, but he believed that the navigable Arkansas River and the railways already projected in that state made it needless to commence farther east than Fort Smith, on the edge of the Indian Country. He began his survey on July 14, 1853. His westward march was for two months up the right bank of the Canadian River, as it traversed the Choctaw and Chickasaw reserves, to the hundredth meridian, where it emerged from the panhandle of Texas, and across the panhandle into New Mexico. After crossing the upper waters of the Rio Pecos he reached the Rio Grande at Albuquerque, where his party tarried for a month or more, working over their observations, making local explorations, and sending back to Washington an account of their proceedings thus far. Towards the middle of November they started on toward the Colorado Chiquita and the Bill Williams Fork, through "a region over which no white man is supposed to have passed." The severest difficulties of the trip were found near the valley of the Colorado River, which was entered at the junction of the Bill Williams Fork and followed north for several days. A crossing here was made near the supposed mouth of the Mojave River at a place where porphyritic and trap dykes, outcropping, gave rise to the name of the Needles. The river was crossed February 27, 1854, three weeks before the party reached Los Angeles.