Green Mountain as seen from South Table Mountain.
The Green Mountain Conglomerate
Waters coming out of the canyon...
Green Mountain, with its notched and undulating slopes, south of Golden, has the appearance of an old lava flow. However, except for its north and northeast slopes, which were the southern boundary of the Ralston Dike magma flow, Green Mountain is composed primarily of conglomerate, which was laid down in four layers between 64 and 55 million years ago. The source of the cobbles and pebbles which make up the various layers of the Green Mountain Conglomerate was a mountain range to the west which extended from Mount Morrison, on the south, to Boulder, on the north. 
The fourth layer of conglomerate, approaching the top of Green Mountain, is course, with larger stones and boulders, indicating that the source mountains may have been closer. Petrified wood, mixed with the conglomerate and sandstone, can also be found on the upper slopes. Some of the quartzite found on Green Mountain is similar to that near Coal Creek Canyon, 18 miles northwest, suggesting that Green Mountain was part of an alluvial fan, a fan-shaped accumulation of sediment which formed where streams emerged from canyons. The terrain, between Boulder and Green Mountain, extending as far as Castle Rock, 30 miles to the southeast, would have been a broad, flat floodplain, home to tropical or subtropical forests, and subject to fires and mudflows. The streams which deposited sand and rock from the mountains, also picked up logs from the forests, now seen as petrified wood.
(1) Harald Drewes and John Townrow, "Trailwalker's Guide to the Dinosaur Ridge, Red Rocks and Green Mountain Area, 2nd Edition"
Friends of Dinosaur Ridge, (Morrison, CO 2005), pp. 19, 31 & 35;
(2) Drewes and Townrow, "Trailwalker's Guide," p. 31;
Kirk R. Johnson and Robert G. Raynolds, "Ancient Denvers: Scenes from the Past 300 Million Years of the Colorado Front Range," Denver Museum of Nature & Science, (Denver 2003), pp. 20 & 22.