Even in the early days of humanity, the cross was a cult object, as is known from archaeological discoveries. The use of the cross dates back to the Stone Age; in European cult caves, crosses often represent the oldest rock carvings. One generally assumes a religious world formula in the representation of four interconnected, opposing cardinal points.
The cross, which is customary today, can be proved as a symbol from the 4th century onwards, and has arisen with the worship of the cross of Christ.
I have found in Net a few very nice crosses:
Title pic source: topix.com / Cross in Effingham – Illinois
Pic source: eastboca.net / Cross in Madrid Valley – Spain
Pic source: : geo-reisecommunity.de / Cross in San Salvador – Mallorca
Pic source: adelmann.blogspot.com / Cross in Rio de Janeiro – Brasil
Pic source: euralmanac.tumblr.com / Cross in Dobra Voda – Czech Republic
Pic source: flickr.com / Granite Cross in Africa
Pic source: mint.sbg.ac.at / Cross in Hohen Göll – Austria
Pic source: willgoto.com / Cross in Cederberg in Western Cape
Pic source: de.wikipedia.org / Brunnkogel Cross
Pic source: mariakumar.org / Cross by Sunset