People had to take a lot of flour on the trail. They made bread and I read that they did this every day. They stocked up in Independence Missouri if that was their starting point for the trail.
Wheat flour was the type of flour used then. The grain had to be milled. This is a process of many steps. It’s an art and a science.
There was a flour called shorts. It was rough and wasn’t milled very much.
I will be whining about the weather today. It will be hot and humid. Tomorrow, after a cold front comes through, it will about ten degrees cooler.

First the map.

By, National park Service

The rest of the pictures are from Wikipedia Commons.

Next are two pictures of wheat fields.

By, Anne Burgess

By, David Monniaux

This is a diagram of a wheat kernel.

Adapted from a svg file  by, Jon C

This is bran.

By, Rasbak

Next is a picture of whole wheat flour.

By, Margaret Hoostrate

These are three kinds of flour. From the left, the first two are wheat and the third, on the right, is rye.

By, Mudd1

Aside from bread, Hardtack was taken along on long journeys either on land or by sea. It was bough already made. It consisted of flour, water and maybe salt. It was described as awful. Nothing like a cracker that we eat today.
This last picture of Hardtack is by, Infrogmation.