The Green River shale that formed here was the result of fine sediment deposition in isolated lakes in the region along with organic remains.  Eventually the organic material under pressure and higher temperatures was converted into the precursor to petroleum - kerogen.  The Tollgate Rock is full of dark layers rich in kerogen particularly the black Mahogany Ridge zone.  The amount of equivalent petroleum in the Green River shale is estimated to be about 1.8 trillion barrels of oil.  Geologists estimate that about 800 million barrels are recoverable which is 3 times the amount of reserves locked away below Saudi Arabia.  The top of Tollgate rock contains the richest kerogen (light brown colored rocks but samples of the shale with kerogen can be collected at the base also.