South Pass was the most important landmark on the Oregon Trail; the key to westward migration. Without South Pass, wagon travel across the continent would have been impossible--and Oregon and California would probably not have become a part of the United States. Yet, there's no narrow gorge here--this gap in the Rockies is miles wide.
Emigrant Lorenzo Sawyer:
"Most emigrants have a very erroneous idea of South Pass, and their inquiries about it are amusing enough. They suppose it to be a narrow defile in the Rocky Mountains walled by perpendicular rocks hundreds of feet high. The fact is the pass is a valley some 20 miles wide."
South Pass crossed the continental divide and hence marked the boundary between the United States and Oregon Country. Even though the emigrants were now in Oregon, there was no reason to celebrate. They were still only half-way to their destination. There were a thousand miles yet to travel.