"1st Sign of the coming Mountains" a Bayard Travel Page by Toughluck

Here, you can see that Chimney rock is on a bench that comes out from the bluffs to the south. The cap rock on the Chimney is the same rock that caps the bluffs. Over time, water has undercut the caprock, tearing away the softer rock underneath, untill only the chimney as a few other isolated 'domes' to the west (Capitol Rock being one) remain of a vast plains that once stood at the top of the chimney and the adjacent bluff.

Note that the dotted line for the trails, goes south of the chimney over the rougher terrain, rather than north along the river. The North Platte and the Platte are wide meandering rivers, surrounded by low wet soils. While this provided water and grass for the oxen, it holds soft spots that can break axles, spirits and oxen.

For more sites along the Overland Trails, see:
Kansas City, Missouri
Kearney, Nebraska;
Chimney Rock, Bayard, Nebraska;
Gering, Nebraska;
Fort Laramie, Wyoming;
Guernsey, Wyoming (Trail Ruts and Register Cliff State Historic Sites).