oldest places- and watername in Scandinavia

It is well know that places- and waternames can give us information
about the previous peoples who lived in a country. It is called
substratum. For example there exist such substrata in USA and Austra-
lia (Indian and aboriginal names). But also the immigrants gave
new names to rivers and mountains, for example Rocky Mountains and
Blue Mountains (Australia).

It is well known that in Scandinavia there is no substratum of
pre-Germanic tribes. But I should like to know when the first
Scandinavian names were written and which are the oldest names? Maybe
there was a substratum of pre-germanic names till 1000 BC. So it is a
reasonable thing that such old substratum might have disappeared in
AD 800-1000. Who can give me more information?