PHYSICAL barriers: Rough terrain precludes a continuous wall and presents huge challenges for those trying to patrol the Mexican border A craggy border canyon in San Diego is difficult to monitor and too rough to handle much of a fence, so the federal government has come up with an idea - it will fill the 230-foot-deep chasm and get the dirt to do it by lopping off the tops of two nearby mesas. By smoothing out Smuggler's Gulch with enough dirt to fill 70,000 dump trucks, the U.S. Border Patrol expects to be able to better patrol one of San...