There are hundreds of images on this rock-carved surface at the location Leirfall (also spelled Leirfald) an hour's drive from Trondheim. Everyone seems to think that the most common image depicts footprints.

But the supposed 'footprints' never make a track or a trail. They are often in pairs, and often connected. I have studied them carefully, and think they probably depict clams or mussels, and have nothing to do with feet. 

Boats are also depicted. They are typical fjord boats, seen in rock-art many places in Norway. The river at Leirfall is now 12m above sea level, and the fjord is now 10km away. The land has risen more than 12 meters since the Bronze Age. This part of the river valley was at sea level, and the rock carvings were right near the fjord. 

There are lots of blue mussels and other bivalves in the fjord today, and I think that they were an important part of the diet, especially in the winter.

Here is Figure 13 of the publication These Rocks Were Made for Walking (Sognnes 2011) with my photograph (blue box) added.

See also these web sites for some photos and explanations:

https://no.wikipedia.org/wiki/Helleristningene_p%C3%A5_Leirfall

https://digitaltmuseum.no/011085440472/helleristningene-pa-leirfall

http://www.reuber-norwegen.de/NordTroendelag/BilderTab_NordTroendelagStjoerdalLeirfall.html