Among the gently rolling 'dobie hills in North Delta, Devil's Thumb stands tall -- apparently impervious to the forces of erosion that have worn down the surrounding soft clay hills.
Late this summer, Dr. Dave Noe led an excursion to Devil's Thumb, following a trail that at first climbed gently through an arroyo, then grew steeper as it ascended Petrie Mesa to the landmark that gave Delta's golf course its name.
The hike, offered by the Western Slope Conservation Center and Delta County Libraries, was the most recent in a series of geologic presentations by Dr. Noe of Paonia. A retired geologist, he spent years in field research and map making on the Western Slope, including work with the Colorado Geologic Mapping Program.
"In retirement, I am focused on educating the general public about the wonders of geology as they relate to the places where we live and in our everyday lives," he said.
While he has a Ph.D. in geological engineering from the Colorado School of Mines, he shares his extensive knowledge in an engaging manner, using terms easily understood by everyone on the hike.
He pauses frequently on the ascent, both to give hikers a breather and to point out surrounding features.
He explains the 'dobies are made of Mancos shale that was deposited during the Cretaceous period, maybe 80 to 100 million years ago. "We see the Mancos shale is largely deposition -- mud being deposited in a shallow seaway for a long amount of time."
The Mancos shale formation stretches from Grand Junction on the north, to near Ridgway to the south and east into the Paonia area. The mud was deposited across western Colorado in layers up to 4,000 feet in thickness. Shale is a mud that's turned into rock. When the mancos shale erodes, it leaves soft hills that in many places are known as badlands. "Around here, they made adobe bricks from the clay soil, so these are called the adobes or the 'dobies," he explained.
He points down to the gravel and dark rock -- basalt -- lying on top of the Mancos shale. Those deposits are much newer, he said.
Basalt is characterized by its dark color and is usually associated with Hawaii because it is volcanic rock, Dr. Noe explained. The basalt originated on Grand Mesa, where 23 separate lava flows have been documented. Over time, landslides broke up the rock and created many of the lakes on Grand Mesa. Glaciers also busted up the rock and when they melted, basalt was carried down through the valleys. With the aid of gravity, water and mud flows traveled for miles and miles down the valley, creating gigantic deposits of mud and leaving behind pieces of basalt.
There was never a volcanic cone as one might envision from a middle school science project. The geological clues point to molten magma erupting into what was once a valley. Lava filled the depression and then tailed away from the mesa. Dr. Noe speculates that landslides or glacial flow took the debris into the valley that was Petrie Mesa. "Then at some point the side of Petrie Mesa failed and this basalt got dropped down here."
Petrie Mesa once stretched across the horizon. Now it has two arms. "Petrie Mesa used to be a longer feature, but the edge of it has degraded and worn away, leaving some awesome cliffs," Dr. Noe said.
Gravity, erosion and weather have combined to create a landscape that appears stark to many, but which provides specialized habitat for rare native plants. Careful searching can also reveal fossils of clams, ammonites and shark teeth.
Dr. Noe points to another source of erosion -- wind, which has smoothed the sides of Devil's Thumb. Dr. Noe has observed the wind swirling up over a ridge and swirling around the "thumb."
Concentrated wind erosion -- yet another force that forms the landscape that we see today, he said.
"You can walk right up to what's almost fresh mancos shale," he said. "You can see all the layers in the mud -- mud stone we would call it. You can also see white layers of gypsum that formed during more recent periods. Erosion has brought the shale up close to the surface and now rain water can percolate down into the cracks. What happens is that there are some reactions within the shale that bring out sulphur and calcium. With oxygen from the water, it all gets turned into gypsum."
A more resistant type of Mancos shale known as prairie canyon member may form the resistant cap that's kept the "thumb" pointed upward.
To Dr. Noe, the surrounding topography, the signs of mancos that's been deposited and redeposited, the gravel and the basalt, all form a patchwork through time. His trained eye can spot where streams once existed and differentiate between formations that are millions of years old, versus those that are the result of more recent forces of nature. Every rock has a story -- and to Dr. Noe, it's a fascinating story of the geologic history of western Colorado.