Fifty million years ago, when mammals were beginning their reign on the planet, metamorphic core complexes arose in tandem from deep within the crust in the project area. As they crept skyward the overlying strata was extended, above major detachment faults, forming horst and graben structures.
The horst and graben structures are bounded by normal faults. These normal faults tend to be high angle and trend in a north south orientation. The faults are associated with precious metal mineralization, which is often preserved in the grabens. The Rebublic and Toroda Creek grabens have been lucrative places to find gold.