If the Nasca Lines are so old (I saw on the Science Channel today that they are dated to 20 BC or so?) then why haven't they eroded, or been otherwise naturally buried/washed/blown away?

www.esa.int

"The oldest lines date from around 400 BC and went on being created for perhaps a thousand years.
They were made simply enough, by moving dark surface stones to expose pale sand beneath...

...The Nasca Lines have been preserved down the centuries by extreme local dryness and a lack of erosion mechanisms, but are now coming increasingly under threat: it is estimated the last 30 years saw greater erosion and degradation of the site than the previous thousand years before them."