Question: The 2170 mile Oregon Trail connected various towns along a 150 mile long valley, forms the cultural and political heart of Oregon, and is home to three of Oregon's largest cities, including which city that is the state capital?
Sentence (from "Register Cliff"): It is notable as a historic landmark for 'registering' hundreds of emigrants on the Oregon Trail (thus also the other northern Emigrant Trails that split off farther west such as the California Trail and Mormon Trail) who came to follow custom and inscribed their names on its rocks during the western migrations of the 19th century.
Relevant evidence: No