Just over the Wyoming border, the tiny town of Harrison is an attractive wide-spot-in-the-road, worth a stop to sample the huge (up to two-pound!) burgers and silky milk shakes at Sioux Sundries (308/668-2577), at the corner of Main and 2nd Streets. On the banks of the Niobrara River 23 miles south of Harrison, the Agate Fossil Beds National Monument (daily; $5 per car; 308/668-2211) contains the 20-million-year-old remains of bison-sized pigs, twin-horned proto-rhinos, and other vanished creatures.
