Last modified 01/03/05
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Her high mountains and deep canyons leave no doubt that Colorado's beauty is far more than skin deep. In fact, it's four-dimensional if you count time, and of course you must.
Over the last 1.8 Ga (billion years), many seascapes and landscapes have played out in the place we call Colorado—from deep ocean trenches to shallow tropical seas to beaches and offshore bars to vast desert dune fields to arid tidal mudflats, from lush redwood forests to volcanic moonscapes, from tropical volcanic islands to high glacier-carved inland mountains. The rocks now exposed at the surface in Colorado provide an incomplete but still fairly readable record of its physical journey through deep time. The otherwise inaccessible rocks glimpsed in road cuts, tunnels, mines and wells help geoscientists flesh out the story.
This website celebrates just a few of the many stories Colorado's rocks have to tell. Recounting them online is an exercise in geo-journalism, a reporting of the what, where, when, how and why of physical Colorado in a regional context while featuring points of particular interest in the Colorado landscape. (Occasionally, the who figures in as well.) If my words and photographs impart even a hundredth of the wonder and fascination that a writer like ^John McPhee or a painter like ^Maynard Dixon can stir, I'll have done a decent job.
With a subject as four-dimensional as geology, you can never have too many diagrams and photographs. I wish I had many more, but my permissions, time and artistic skills go only so far. Most of the diagrams shown here come with permission from ^Geological Society of America publications or from online public domain ^US Geological Survey and NASA resources.
Like Colorado itself, this site is a work in progress. The personal geologic vignettes and photographs gathered here come from day trips in and around the Denver area, family vacations in Colorado, geologic field trips guided by professionals, and geologically-oriented road trips around the state in the company of my good friend and fellow reader of rocks, John Barr. I plan to add more material as my travels widen. The supporting "groundwork articles" draw from many resources, some of which are acknowledged below.
Links prefixed with a percent sign "%" lead to internal or external pages opened in a new browser window. The same is true of thumbnail images with a heavy green border. If left open, the same new window will be used for all subsequent "new window" links.
Albert Einstein once said something like, "When you fully understand an idea, you can explain it in simple terms," but he never said it would be easy. I've done my best to write by that creed, but my understanding has limits, and geoscience jargon does afford a certain economy of word, thought and mental image. I've made an effort to introduce important terms along the way, but in the interest of text flow and time (mine), I've often assumed an unrealistic familiarity with geologic lingo and timescales—at least for most potential visitors.
If you find yourself in need of definitions or time frames, I heartily recommend the online geologic glossaries and timescales listed on the search page. This site follows the 1999 ^Geological Society of America timeline.
Striking the proper balance between simplicity and nuance, overview and detail, lumping and splitting is always a challenge. In a primarily observational science like geology, juggling the known with the conjectured invites even more verbiage. I've never been known for brevity, but I've done the best I can to be fair to the data without losing the mountains for the rocks.
Throughout this site, deep time units are expressed as follows:
The last two examples illustrate a potentially confusing ambiguity in the way these units are used here and in most geology texts. Logically, a stated time may represent a duration (the 30 Ma in the second example) or a specific time in the past (the 10 Ka in the last example). Feel free to add "ago" to these time units as context requires.
For now, distances appear in both English and metric units at my convenience. Eventually, they'll all be metric.
The information presented here is accurate and current to the best of my knowledge. When I first undertook this site in late 2001, with a view toward learning Colorado geology as quickly as possible by writing about it, I was an avid amateur geologist with 25+ years of geology reading and professionally- and self-guided field trips under my belt — all primarily in California. As of January, 2005, however, I'll be a graduate student in geology at the ^Colorado School of Mines.
My research for this site has been careful and reasonably complete, but all errors and misconceptions are mine. If you spot one, or any other opportunity for improvement, please let me know at email@example.com.
Thanks for stopping by.
Jeremy McCreary, MD
Skip to Acknowledgments
Most of the photojournals linked on the left below focus on a particular Colorado destination of geologic interest. They include one or more sections of introductory text followed by a photo gallery also divided into sections. The text and gallery sections are linked separately in the table below.
Many of the photojournals also include material on points of interest encountered on the way that may or may not pertain to the main destination. You'll find these side attractions included among the gallery links.
Geology plays out in four dimensions. To get a grip on at least two of them, I strongly recommend attempting to digest this site with some visual aids in hand.
I've included as many illustrations as I can get my hands on legally, but the site could obviously use many more.
Colorado's story continues to evolve—both in the rocks and in the minds of geoscientists. Her profound beauty, her spectacular exposures and her tantalizing puzzles have long attracted some of the best minds in the business, but her many overprinting processes have left a tangled web indeed. Many chapters in the story remain controversial or frankly unknown, and some just beginning to yield to investigation may take decades to tease out.
I've assembled the "big picture" presented here as best I can from a number of sources, including
Over the years, many kind geoscience mentors have contributed to the understandings offered here, as I've acknowledged elsewhere, but the misunderstandings here are all mine.
Skip to General References
Over the last 25 years, I've had the good fortune to live in two hotbeds of geoscience talent—California's San Francisco Bay Area and Denver, Colorado. I first realized how eager most geoscientists are to share their knowledge and enthusiasm with interested laymen in 1982, when I timidly joined the Peninsula Geologic Society, a veritable "who's who" of West Coast earth science drawn primarily from the Stanford faculty and the USGS Western Division headquarters in Menlo Park, CA. At monthly meetings and on many a field trip, these early mentors gave me a solid leg up on field geology that no book could impart. Special thanks go to the late Clyde Wahrhaftig for allowing me to assist in his Marin Headlands field work, for putting up with my ceaseless questions, and for helping me learn to make some sense of the aptly-named "trash-can geology" of coastal California. At 80, Clyde could still out-hike me, and I was 30 something and in very good shape at the time.
My good luck with the Peninsula Geologic Society emboldened me to try tapping into the local talent once again on moving to Denver in 2001, and I've not been disappointed. Conversations and field trips with many geoscientists met through the ^Geological Society of America, the ^Colorado Scientific Society, the ^Colorado School of Mines, the ^Rocky Mountain Association of Geologists and this web site have been invaluable. I'm particularly grateful to Warren Hamilton, Bob Raynolds and Vince Matthews, as noted below.
When the Geological Society of America (GSA) annual meeting came to Denver in 2002, a year after our move to Colorado, I couldn't resist. A 2-day pre-meeting field trip focused on Colorado's Precambrian history turned out to be the perfect setup for the talks ahead, which in 2002 included no less than 6 half-day technical sessions devoted to the Rockies. I couldn't have asked for a better leg up on the current state of Colorado geology, but even more gratifying was the opportunity to meet and talk with many of the leading workers in the Southern Rockies.
When the meeting came back to Denver in ^2004, I signed up for two field trips, both to the northeast flank of the Front Range — an area at the heart of a long-standing and deep-seated controversy regarding the structure and tectonic style of the entire Laramide orogeny. There were fewer Rocky Mountain talks than there had been in 2002, but
I'm still patting myself on the back for signing up for a seminal pre-meeting field trip to the Front and Sawatch Ranges, ^Formation, Reactivation, and Evolution of Proterozoic Shear Zones in the Colorado Rocky Mountains: From Continental Assembly to Intracontinental Orogeny, ably led by Colin Shaw, Karl Karlstrom and Michael Williams. As the GSA meeting to follow unfolded, I came to realize that the field trip roster amounted to a current "who's who" of Colorado geology — and I'd managed to pick the brains of just about everyone there! All proved to be gracious and enthusiastic teachers, but I'm especially grateful to
To a student of the Rockies in general and the Front Range in particular, the ^2004 GSA field trip offerings presented some tough choices. There were at least four tasty Front Range trips, but one before and one after the meeting were the best I could do. I finally settled on two that brought somewhat conflicting interpretations to the northeast flank of the Front Range between Fort Collins and the Wyoming line.
A wealth of observable structural and tectonic detail makes the northeast flank a natural focal point in ongoing debates surrounding the kinematics and dynamics of the Front Range and indeed of the Laramide orogen as a whole. (For the uninitiated, kinematics studies the way things move — direction, velocity, timing — without regard to driving forces; why they move the way they do is the purview of dynamics.) Along its southern and central segments (south of Boulder), the high and steep east flank of the Front Range drops precipitously into the deepest portion of the Denver Basin along largely inferred high-angle reverse faults (among them the Golden Fault) with at least 25,000' of accumulated throw. In contrast, the range front basement north of Fort Collins is broken into a jostled array of large and small blocks that generally stair-step down to the east into a much shallower northern lobe of the Denver Basin. Between Fort Collins and Boulder is a transition zone characterized by fewer blocks and a basin floor of intermediate depth.
Vince Matthews' superbly illustrated pre-meeting trip entitled "Overview of Laramide Structures along the Northeastern Flank of the Front Range" focused on kinematics — specifically, on evidence for differential uplift of the northeast flank's basement blocks along steep (presumably reverse) northeast- and northwest-trending faults. We visited many striking examples of Paleozoic and Mesozoic sedimentary strata draped in ductile fashion over variably uplifted basement blocks separated by brittle faulting. The trip ended at a spectacular exposure of the southern edge of the large, east-dipping Blue Mountain block along the north side of Dry Hollow Creek, west of Berthoud. A markedly attenuated Fountain Formation monocline stretched over the edge of the basement block added weight to Matthews' view of the sedimentary strata as so much "melted cheese" covering the shattered basement of the northeast margin of the Front Range block.
Eric Erslev's post-meeting trip entitled "Laramide Horizontal Shortening in the Rockies: Faulting and Folding in Oblique Backlimb-Tightening Structures of the Northeastern Flank of the Front Range, Colorado" presented the case for regional east northeast-trending horizontal shortening as the main driving force for northeast flank structure, Front Range uplift and for Laramide deformation in general.
Although colleagues often portray Matthews and Erslev as respective spokesmen for the long-opposed "verticalist" and "horizontalist" camps of Laramide dynamics, I came away from the 2nd trip thinking that the Laramide may well be big enough for both of them. Erslev's orogen-scale horizontal dynamics reconcile easily with Matthew's local high-angle kinematics if one steps back and views the northeast flank as a large high-angle backthrust belt related to limb-tightening as the eastern margin of the Front Range block thrust eastward over Denver Basin sediments during the Laramide. Could their perceived disagreement turn out to be nothing more than an artifact of focus and scale? Six wise men long famous for their early elephant observations might find grounds for optimism here.
Luckily, the GSA's Field Guide 5: Field Trips in the Southern Rocky Mountains, USA, edited by Eric P. Nelson and Eric A. Erslev, contains detailed field guides for the two trips I was most reluctant to pass up: Karl Kellogg's "The Colorado Front Range: Anatomy of a Laramide Uplift" led by Karl Kellogg, Bruce Bryant and John C. Reed, and "Continental Accretion, Colorado Style: Proterozoic island arcs and backarcs of the central Front Range" led by Lisa Rae and Thomas R. Fischer. Charley Haddox and I have been working through them as weekend time and winter weather permit.
Jeanette Hammann at ^Geological Society of America headquarters in Boulder deserves a tip of the hat for helping me arrange permissions to use graphics from two superb GSA publications, Geology and GSA Today. Since my text would have had a hard time standing without these invaluable visual aids, I'm also grateful to the GSA for its generous permission policy.
Among the many authors of USGS public domain publications whose knowledge and graphics went into this effort, explicit thanks go to W. Jacqueline Kious and Robert Tilling, authors of ^This Dynamic Earth: The Story of Plate Tectonics for all the excellent diagrams, maps and photos. This site would have been graphically barren without them.
My deepest thanks go to ^Warren Hamilton, Distinguished Senior Scientist in geophysics at the ^Colorado School of Mines, for his generous and patient mentoring on geodyanmics following an initial e-mail exchange regarding his thought-provoking 2002 GSA meeting talk later published as ^An Alternative Earth. His capacity for questions is exceeded only by the quality of his answers. Warren's advanced my understanding of global and local tectonics by orders of magnitude. I have no doubt that his overarching view of the planet and its workings and his dogged focus on the observations (as opposed to the stories told about them) will guide the way forward in geodynamics.
My true friend and geology buddy John Barr wins the stamina award hands down for listening to me puzzle out overlook after outcrop on our many, many outings together—and especially for poking holes where required. His interests range wide, his love of nature runs deep, and his ability to resist glazing over is exceeded only by that of my dogs, who don't even seem to know how. I enjoy John's company and conversation immensely.
According to an old Chinese proverb, when you're ready to learn, your teacher will appear. To my great good fortune, two excellent teachers appeared — ^Bob Raynolds, research associate and consulting geologist attached to the ^Denver Museum of Nature and Science and its exciting ^Denver Basin Project, and ^Vince Matthews, current director of the ^Colorado Geological Survey. Both sought me out (within the same week!) on finding this site, and both have been extraordinarily generous with their time and knowledge ever since — despite the fact that no good deed goes unpunished when it comes to putting up with my questions. Their perspectives on the history of Colorado geology remind me that with the excitement of any new finding or synthesis must come a healthy dose of caution and a look back to the quality of the original data.
Finally, I'm hopelessly indebted to my wife Kathy, son Matt and daughter Casey for putting up with the endless side excursions to points of geologic interest ("It'll just take 30 minutes, honey, I swear."), the incessant photos ("Wait, I can't leave without snagging that gorgeous contact over there. It'll just take 10 minutes, honey, I swear.") and the predictable last-minute book consultations ("I'll catch up with you down the trail as soon as I look up this formation. It'll just take 5 minutes, honey, I swear.") that riddle every family trip. They are more patient than I deserve, and Kathy always graciously accepts more than her fair share of the driving while I feverishly thumb through my briefcase full of geology references en route to the next attraction.
Listed alphabetically by title below are excellent online and hardcopy resources for anyone interested in the geology behind Colorado's incomparable scenery. This site relies heavily on many of them. The references likely to be most useful are marked with double stars "**". See also the more topical references at the end of each article on this site—particularly those cited in the groundwork articles Colorado Geology Overview and Earth At Work.
Unless explicitly attributed to another contributor, all content on this site © Jeremy McCreary. Comments and corrections to firstname.lastname@example.org. Nothing is bought or sold here. Spam will be forwarded to the proper authorities.