Water erosion begins when runoff from rainfall flows in a thin layer over the land in a kind of erosion called?

Distance that lewis and clark traveled from st. louis to ft. clatsop near the pacific ocean.

According to the earth science reference tables, during which geologic time period were the continents of north america,south america, and africa closest together?

Based on the information in the graph, what can you conclude about humans in the world in general?