Many motile bacteria (for instance, Escherichia coli) inoculated at some point in a semisolid nutrient medium can form population waves: bands or rings. The formation of these motile structures is due to chemotaxis. The population waves when they are drawn together can form two types of non-motile structures. Firstly, the population waves can collide. Secondly, in certain conditions, the waves can slow down and stop without coming into contact directly with each other. In this way demarcation zones are formed. The mechanism of the occurrence of the demarcation zones has been unknown. In this paper we show that formation of these zones is due to lack of nutrients (which at the same time act as attractants) within the narrow gap between individual bacterial populations.