"To assign a more specific date to the onset of the 'anthropocene" seems somewhat arbitrary, but we propose the latter part of the 18th century, although we are aware that alternative proposals can be made (some may even want to include the entire holocene). However, we choose this date because, during the past two centuries, the global effects of human activities have become clearly noticeable. This is the period when data retrieved from glacial ice cores show the beginning of a growth in the atmospheric concentrations of several 'greenhouse gases", in particular CO2 and CH4. Such a starting date also coincides with James Watt's invention of the steam engine in 1784."
http://en.wikipedia.org/wiki/Paul_J._Crutzen

Production of coal in Britain:
"1700 : 2.7 million tonnes

1750 : 4.7 million tonnes

1800 : 10 million tonnes

1850 : 50 million tonnes

1900 : 250 million tonnes"
http://www.historylearningsite.co.uk/coal.htm

World Coal Production 2005:
6,079 billion tons
1995:
5,096 billion tons
http://www.eia.doe.gov/emeu/aer/txt/ptb1114.html
Just the amount of increase of production from 1995 to 2005 probably easily exceeded all coal mined from 1900 to time of stone age.

"In some areas of Britain, evidence has been discovered that indicates stone-age inhabitants collected and used coal.
Flint axes have been found embedded in layers of coal in excavations at Monmouthshire and Stanley in Derbyshire."

And as far as garbage dumps they go back to stone age also. And perhaps the later part of 20th garbage classification will be called the baby diaper era.
But point is one looking for global characteristics- e.g something showing up in sedimentary layers, rather than archaeological digs.