Why did slavery arise in the chesapeake for african americans in the 1600s?

Chesapake is an area in Virginia where the biggest plantation and production of tobacco is held. White slaves rendered their services for 5-7 years in exchange for allowing of passage to America to have their share of tobacco crops, however during 1600s, their labor became scarce paving way for the african americans. 

The biggest obstacle for the railroads heading east from California was the Sierra Nevada Mountain range. It lies on the easternmost part of the state and runs down inland California for about 400 miles.  

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