Witwatersrand
The "Witwatersrand" is a geological formation deep inland in South Africa. It is the "white water ridge": The defining line that lets water flow to the Indian or Atlantic oceans.
In Joburg, there is a road running right on top of that ridge. Driving that road while it rains means being present for a great divide: Rain falling on the one side runs to the Atlantic Ocean. Water on the other side, to the Indian Ocean.
A few meters makes hundreds of kilometres difference.
So it is with the edges in life.