The ice caps are only about 30 million years old, and the land and oceans are similarly transformed over long time scales. The only techno signature I'm aware of that would be clearly detectable 50 million years after our civilization is gone is reinforced concrete, which should produce a worldwide layer of a limestone-like mineral with iron inclusions that would be both unique in the geologic record and difficult to explain by any natural process. If a civilization did not use reinforced concrete nor some other such material that would leave a similar signature, it would be quite difficult to detect at that timescale.