Answer:
it's the regolith....hope this helps you
C Horizon - Also called regolith: the layer beneath the B Horizon and above the R Horizon. It consists of slightly broken-up bedrock. Plant roots do not penetrate into this layer; very little organic material is found in this layer. R Horizon - The unweathered rock (bedrock) layer that is beneath all the other layers.
B) Cultural groups may be isolated and thus become more distinct.
This is because traveling over mountains can be hard or dangerous which will result in little long distance travel and in result little interaction with other groups