The Marks of the Gods are hovering golden idols. There are forty-two in the game, so collecting ten is very easy. You can check your progress by clicking the back button to open the Player Statistics screen.
Here's a nice, comprehensive and well organized guide I used to find the marks. It's a PS3 guide, but it works for the 360.http://www.ps3trophies.org/forum/outland/113185-marks-gods-g...
You'll get the Enthusiast achievement while going for:
The Collector achievement in Outland worth 81 pointsCollect all Marks of the Gods (Single Player only)