By Ken Singer
The area around Lyons has some of the most diverse geology in the state, according to County Open Space volunteer geologist Roger Myers. He has led several hikes to point out the distinctive geologic features of the area around the town.
In September, he led a group of about fifteen on the Hall Ranch trail off Route 7 up to the top where it is joined by a trail coming from the trail head at Antelope Road, popular with mountain bikers and hikers. Pointing to the numerous bits of quartz on the ground, he showed a large vein of the mineral, which was likely blown up by dynamite from a prospector
looking for gold. Apparently, the prospector didn’t find any gold (which is often found with quartz) and Boulder County eventually purchased the 4000 plus acres of the ranch as open space. The quarry, located just off Route 7, isn’t being mined for the red sandstone, which is called Fountain, a softer sandstone that sits on a shallower layer of harder gray stone that is much more valuable as crushed rock road base.
Myers has a degree in geology and gives tours for people interested in the unique mix of rocks in this area. The exposed stone around here dates from 1.7 billion years, and involves not only the Rocky Mountains to the west, but also has the sedimentary deposits from the ancient Rockies which eroded millions of years ago. In fact, from the vantage point of Rabbit Mountain (formerly called Rattlesnake Mountain), one can see the “14ers” to the west and the plains starting just east of Lyons, stretching to the Ozarks and Allegheny Mountains for many hundreds of miles.
For millions of years, what is now Colorado was an inland sea. On Rabbit Mountain, Myers showed the group some sandstone with slight ridges which indicated wave action from the sea. The familiar red stone of Lyons quarries has its own designation called Lyons Sandstone. The layer is about 220 feet thick here, covering the 1000-foot thick Fountain Formation, which was deposited from the ancestral Rockies.
Fountain sandstone is a less dense rock than the Lyons formation, and therefore not as valuable as the stone quarried here. The iron oxide (rust) gives the rock its reddish hue. Lyons sandstone is used throughout the world for buildings and monuments.
The iconic Flatirons in Boulder are part of the Fountain formation, thrust up almost vertically from the pressure put on them by the present Rockies, driven by plate tectonics. And there are thousands of feet of additional sedimentary deposits on top of the Lyons stone.
Red Rocks, the famous music venue to the south, has newer sedimentary deposits called the Morrison Formation, and includes dinosaur fossils. The exposed black shale road cut off of Route 36 between Lyons and Boulder at Nebo Road, is newer still, and it is very easy to find fossils of small clams.
The Rabbit Mountain walk, last Thursday, was organized by Lori LeGault of the Town of Lyons Parks and Recreation Department for about a dozen seniors. Myers took the group to an overlook that gave a view of Carter Lake and Blue Mountain and he pointed out the Blue Mountain Fault, which is the result of the present Rockies pushing east.
The Boulder County Parks & Open Space has two senior hikes scheduled for 2017. The Thursday, October 26 hike features a Heil Valley Ranch walk at the Lichen Loop trailhead, and another hike on Thursday, November 30, will take place at the Pella Crossing (Hygiene Road). Although these are not geology-related, they are going to be “nature” walks. For more information, call (303) 678-6214.