In very early human history, complex sanitation systems were not needed because settlements were usually very small, like the village below. However, as cities became larger and population densities increased, one major advancement was the development of which helped to channel waste waters away from human populations more effectively than in the past

Rocks, as they are compressed, begin forming mountains above the earth's surface when two continental plates converge. the continental crust increases in depth as the mountains grow above. the himalayan mountains formed at a convergent plate boundary in this manner. the rocks are smashed together causing them to due to the intense heat and pressure from the colliding plates and eventually forming rock. a) melt; igneous b) layer; sedimentary c) recrystallize; metamorphic d) melt into the earth's interior; metamorphic