Geological processes created the complex landscape of southeastern Idaho and helped determine the routes covered wagons took along the Oregon Trail. In the eighteen forties, fifties, and eighteen sixties, trail emigrants often wrote in their journals about the two volcanic cinder cones visible north and south of this area and other geographic features along the way. Six hundred thousand years before the pioneers, a forty-mile-long lava flow from the valley to the west blocked the Bear River's original northward course and forced it south into its present route. Though that basaltic bedrock is now covered with soil and vegetation, evidence of volcanic activity remains visible in the surrounding area, including basaltic cinder cones and several volcanic domes. The cinder cone north on Highway 30, later named Alexander Crater, has been partially destroyed by quarry activity, while the cone south of the highway, the emigrant landmark called Old Crater, looks much as it did when travelers passed in the eighteen forties and fifties. These ancient lava flows provided a level landscape and, together with generally straight valleys, created a relatively good road for pioneers and wagons heading toward Oregon. Diaries and journals from explorers, emigrants, and pioneers on their four- to five-month journey across the prairies and mountains preserve their impressions of rocky chasms, deep cracks, and old craters along the road.