There have been several international projects which drilled cores from the icecaps in Greenland and Antarctica. One of the interesting results is that the cores show layers, and the layers can be counted fairly easily.
There are four obvious questions:
The short answer is: direct counting shows the Earth has to be more than 50,000 years old. Indirectly counted layers are more than 420,000 years old. Ice dates agree with radioactive dates and the ice layers show astronomical cycles.
For an overview of the subject, read