Reading "My Antonia" for the first time, I was brought up short by a description of baking bread. The technique reminded me a bit of sourdough, but I am no bread baker, so I was wondering if anyone could enlighten me on what is actually going on from a bread baking standpoint:

"She mixed her dough ... in an old tin peck-measure .... When she took the paste out to bake it, she left smears of dough sticking to the sides of the measure, put the measure on the shelf behind the stove, and let this residue ferment. The next time she made bread, she scraped this sour stuff down into the fresh dough to serve as yeast."