Colorado Geology Photojournals

A Tribute to Colorado's Physical Past and Present

Right: Trees and snow mark major Laramide uplifts in green and white while salmon pink marks the Colorado Plateau in this true-color satellite image of Colorado and surrounding states, courtesy NASA, ^Visible Earth

Colorado in first snow, courtesy NASA, Visible Earth,



Groundwork articles



Dinosaur Ridge and Vicinity

A Tale of Two Uplifts and a Sea Dragged In By the Uplift Next Door

Dakota Hogback west of Denver, from its summit.
Dakota Hogback, looking south from the crest

On this page


Dinosaur Ridge and Vicinity

A Tale of Two Uplifts And a Sea Dragged In By the Thrust Belt Next Door

East To West, Young To Old

The Golden Fault

Page References
Last modified 10/22/04
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Dinosaur Ridge and Vicinity

Dakota Hogback from the crest of Dinosaur Ridge

Twenty miles or so west of Denver on I-70 is ^Dinosaur Ridge (DR), a particularly prominent segment of the Dakota hogback heavily visited for its famous, easily accessible dinosaur trackways and fossils, striking rock exposures, panoramic views and outstanding ridge-crest trail, and many nicely-done interpretive markers. After umpteen visits, DR remains a family favorite, and my kids don't even like to hike. On moving here from California, it was a most welcome introduction to the natural wonders of the greater Denver area, and it's the one place we strive to take all our out-of-town guests. 

Geologically, the DR area has been heavily studied, early on because of its excellent and tantalizing natural exposures, its proximity to Denver, long a hotbed of geoscience talent, and a number of nearby oil seeps. More recently, much of DR's interior has been laid open to direct inspection by the immense road cut I-70 uses to punch through its north end. Accordingly, the DR area is as well worked out as anyplace along the east flank of the Front Range, but it still holds many secrets, the most valuable of which may be the exact subsurface geometry of the adjacent Front Range Precambrian core. Working out this geometry has become a holy grail of sorts for geophysicists from far and wide — in no small part because of the great bounty oil explorationists have placed on its head. There are many reasons to believe that below 10,000' or so, the Precambrian core of the Front Range overhangs a triangle of upturned petroleum-bearing strata (most notably the elsewhere highly productive "J" sandstone of the upper Dakota Group) to form a fault trap containing potentially huge volumes of oil and gas, but no one's ready to gamble the many millions it would take to drill the technically challenging overhang once, much less to find its production sweet spot. 

Since DR is also a very prominent range-front landmark identifiable at long distances from many directions, it will serve as the focal point for this article. Unless otherwise specified, all references to the Front Range, east flank of the Front Range, mountain front, range front, and so on refer to the Dinosaur Ridge area in this article.

Caveat Viator

A word of caution to the Front Range traveller: The east flank of the Front Range is exceedingly complex. It varies substantially in both geology and geometry along strike and resists all but the broadest of generalizations. Understandings gained at DR don't necessarily translate even 10 miles to the north or south. I'll try to highlight some of the more interesting variations encountered within the range-front segment covered here — roughly from Morrison to Rocky Flats.

Nearby Attractions []

Near Dinosaur Ridge are several other mountain-front scenic and recreation areas popular for good reason with locals and tourists alike:

All these areas offer worthwhile views and hiking and biking trails with many fascinating exhibits on local geology and paleontology. In fact, you'd have a hard time finding another spot exposing as much of Colorado's geologic history in one easy day trip. The exposures in the area provide excellent opportunities to observe first-hand evidence of two of the region's defining events of the last 300 Ma — the Ancestral Rocky Mountain and Laramide uplifts. These uplifts commenced over 200 Ma apart but share many similarities, some visible right here.

Together, Red Rocks Park and Dinosaur Ridge expose an uninterrupted sequence of Late Paleozoic to early Cretaceous sedimentary rocks tilted up against the crystalline Precambrian core of the Front Range. For the most part, these were shed to the east from the Ancestral Rocky Mountains starting around 300 Ma. Just across C-470 to the east is Green Mountain, an erosional remnant of a much larger apron of alluvial fans shed to the east off the Front Range block as it rose during the Laramide Orogeny starting around 72 Ma. Along its western base are exposures of late Cretaceous sandstones marking the beginning of the Laramide uplift.

The area also offers stunning views of and access to the abrupt eastern front of the Rocky Mountains, here embodied by the Front Range foothills. At nearly 320 km in length (from Cañon City to southern Wyoming) and up to 80 km in width, the Front Range is the largest and 2nd highest of the Laramide Rocky Mountain uplifts. Among many other attractions, the Front Range is home to Mount Evans, Pikes Peak and Longs Peak—three of Colorado's tallest Fourteeners. The last two are the only Fourteeners in the Southern Rockies located off both Colorado's main structural lineaments — the Colorado Mineral Belt and the Rio Grande Rift.


A Tale of Two Uplifts And a Sea Dragged In By the Thrust Belt Next Door

The rocks in and around DR tell a dramatic story of two mountain-building uplifts and an intervening invasion by the planet's largest known inland sea. And who best to summarize a dramatic tale but a bona fide drama expert?

The Short Version, As Told Over the Phone By Someone Else's Teenage Daughter

Well, it got like reeeeeally boring around here and there was like nothing to do and OMG, it seemed like forever!!! But then like there was this really big concert happening down south with all the big continents coming together and you know, like jamming, and Africa tried to park next to Arkansas but it like didn't slow down, duh, and so it smacked right into it instead and it even got Texas, New Mexico, Colorado and even Utah all rumpled up, too! So Colorado got 2 really big lumps that time, but after a while they just fell apart and made this really big mess with gravel and sand and mud all over the place. But then it like started raining a lot and all these big weeds started growing everywhere in the mud and OMG, then a bunch of big dinosaurs just came in and started stomping all around like they owned the place. And then something else smacked into California and everything over there just got like shoved over and all piled up on Utah, and Utah got like too thick or something, it made Colorado sink, it was so heavy. So then this really big sea just barged in like it owned the place or something, and there was all this water everywhere, and it left a bunch of sand and mud and then some more sand all over the place. And then that crazy Farallon plate, well it like flapped up under Colorado instead of going down like it was supposed to. Can you believe it???? And it just started shoving stuff around, so the big sea just left and Colorado got all rumpled up again, but this time, there were like lots of lumps everywhere. No, duh, it like made these big mountains and they were covered with all these volcanoes all over the place at first but then the volcanoes just fell apart and made another big mess of volcano pieces everywhere, but the mountains, they like didn't fall apart completely so they're still hanging around and OMG they're still getting bigger, but they keep leaving these really big messes, you know, like gravel and sand and mud all over the place. So whazzup with you?

Now for the longer version...

The Great Unconformity

Pangea assembled at 225 Ma and disassembled at 135 Ma; courtesy USGS, This Dynamic Earth, 600 Ma, most of Colorado and surrounding areas had been reduced to a vast featureless plain of bare 1.7-1.4 Ga metamorphic and granitic Precambrian basement rock. In the Dinosaur Ridge area, the erosive regime persisted another 300 Ma. Precious few traces of events going back to 1.4 Ga had survived the deep regional erosion, and the sediments that might hold clues haven't been identified. The resulting gap in Colorado's geologic record is known as the Great Unconformity; at Dinosaur Ridge, it spans no less than 1.1 Ga,— nearly 1/4 the age of the planet. 

It is upon this stage that our tale of two uplifts and a large sea unfolds.

The Ancestral Rocky Mountains

During Late Pennsylvanian time, around 300 Ma, plate tectonic stresses emanating from a continent-continent collision related to the assembly of the Gondwana supercontinent far to the southeast began to buckle up a dozen or so discrete fault-bound northwest-trending uplifts stretching in a band from Arkansas to Utah. The two largest ranges rose in Colorado and are now collectively known as the Ancestral Rocky Mountains (ARM). The eastern range, here called Frontrangia, stood roughly where the Front Range stands today; Uncompahgria, the western range, was centered roughly where the Uncompahgre Plateau lies today.

Following the initial ARM uplift, Colorado entered a long and relatively quiet period of continuous terrigenous, marine and transitional sediment accumulation lasting nearly 200 Ma. As Frontrangia  rose and then completely eroded away between 300 and ~220 Ma, it shed thousands of feet of largely terrigenous debris onto the surrounding Precambrian basement surface in all directions. These sediments were laid down flat — first as syntectonic range-front alluvial fans and finally as broad coastal flood plain muds similar to those being deposited now in the Gulf states — and remained flat until the Laramide orogeny some 200 Ma later tilted them to the 30-110° dips observed along the east flank of the Front Range today. 

Fountain flatirons at Red Rocks

Progressively finer Frontrangia debris shed to the east contributed entirely to the coarse syntectonic gravels and sands of the Late Pennsylvanian Fountain Formation and substantially to 

  • the Permian desert dune and fluvial sands of the early Permian Lyons formation

  • the muddy coastal plain redbeds of the late Permian through Triassic Lykins and Ralston Creek formations

  • the colorful claystones and minor fluvial sands and lacustrine limestones of the Late Jurassic Morrison formations. 

The Mid-Jurassic breakup of the supercontinent ^Pangea brought a wet temperate climate and lush vegetation to a previously arid Colorado between Ralston Creek and Morrison time, and with the moisture and vegetation came burgeoning populations of dinosaurs. Dinosaur-wise, the Morrison is one of the most fossiliferous formations on the planet.

In addition to the debris washing off its own ARM, Colorado from time to time supplemented its Triassic and Jurassic sedimentary piles with both fine and coarse sediments derived from highlands far to the west in Utah, where troubled plate interactions along the west coast of North America threw up high mountain chains on a recurring basis. In the earliest Cretaceous, renewed uplift in Utah spread a pulse of distinctive chert-bearing conglomeratic fluvial sands across the state. of the Lytle Formation across the state.

The Cretaceous Interior Seaway

The Lytle sands marked Colorado's last stand as dry land for the next ~30 Ma, for close on their heels came the largest inland sea (an extensive body of salt water floored by continental rather than oceanic crust) the planet has ever known. For much of its stay, between 100 Ma and 72 Ma, the Cretaceous Interior Seaway (CIS) had most of Colorado under 600' of saltwater. It flooded in simultaneously from the Arctic Ocean and the Gulf of Mexico to fill a broad trough-like downward flexure of western North America representing at least in part an isostatic response to the combined weight of the many thrusts sheets already stacked to the west by the early to late Cretaceous Sevier orogeny, and it receded only with the onset of Laramide uplift. By the time the Colorado was once again above water, the seaway had left behind a thick region-wide blanket of originally flat-lying marine and transitional Cretaceous sediments, including the

The Niobrara's small hogback east of Dinosaur Ridge was quarried away shortly after the settlement of Denver, and Early Jurassic sediments are for some reason absent throughout central and eastern Colorado. Otherwise, all 200 Ma-worth of the post-ARM, pre-Laramide sediments deposited east of Frontrangia are exposed in the Dinosaur Ridge area today.

The Laramide Orogeny

Around 72 Ma, a deep-seated regional deformation known as the Laramide orogeny began to push up the Rockies and to a lesser extent the Colorado Plateau in a patchwork of discrete north- and northwest-trending block-like uplifts stretching from southern Wyoming to central New Mexico. Resistant 1.7-1.4 Ga metamorphic and granitic Precambrian basement rocks cored the uplifts.

Four Corners, May, 2002, courtesty NASA, Visible Earth,
Four Corners, NASA

For the most part, the Laramide regional deformation reactivated basement-penetrating normal faults left over from a series of Late Proterozoic continental rifting events affecting the entire western two-thirds of the US, including the Colorado Province. Thousands of feet of Mesozoic and Paleozoic pre-Laramide sediments resting on basement prior to the Laramide were bent upward along steep reverse and and shallower thrust faults flanking the rising Laramide blocks even as erosion stripped the same strata from block summits.

Today, eroded Laramide uplifts still dominate the regional topography: Forests and snow nicely color the Laramide uplifts green and white in the satellite photo of Four Corners at right. They underpin Colorado's most prominent highlands, including the Medicine Bow Mountains; the Front, Park, Gore and Tenmile, Mosquito and Sawatch Ranges; the San Juan Mountains; and the Uncompahgre and White River Plateaus, and the Uinta Uplift

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East To West, Young To Old

From from Amuedo and Ivey, 1978, _Coal and Clay Mine Hazard Study and Estimated Unmined Coal Resources, Jefferson County, Colorado_
Dakota Hogback and Golden Fault cross-section
I-70 road cut through the Dakota Hogback

A drive west along I-70 from Denver to the eastern foothills of the Front Range is a trip back into deep time, as this roughly west-trending section just north of I-70 through the town of Golden shows. West and older are both to the left in the diagram. The bump left of center is the Late Jurassic through Cretaceous Dakota Hogback. The hogback is less pronounced at Golden than it is at the geologically famous "I-70 road cut" (right) just east of the Morrison Exit, No. 259. 

Range front in morning ligjt
North and South Table Mountains

At Golden, the Paleocene basalts atop North and South Table Mountains cap prominent mesas of sediments trapped in the Denver basin, a large and deeply sagging range-front basement depression brimming with ~13,000' of late Paleozoic to late Cenozoic sediments shed eastward from the current and Ancestral Rockies. The vertical structural relief between the bottom of the Denver basin (below the diagram and off to its right) and the top of nearby Mount Evans (off to the left) is at least 22,000'.

Denver from Genesee Mountain; 1.7 Ga metavolcanic gneiss in foreground
Denver from atop 8200'  Genesee Mountain

Off to the right and a little south of this section, Denver nestles within the Colorado Piedmont, a 1,000' deep range-front trough excavated by the North and South Platte, Arkansas and Canadian Rivers and their tributaries in Late Tertiary time. In its entirety, the Piedmont stretches from southern Wyoming to northern New Mexico. At Denver, Early Paleocene Denver Formation and Late Paleocene Dawson Formation gravels, sands and muds floor the Piedmont trough as the uppermost strata of the Denver basin.

Green Mountain

Green Mountain

West of Lakewood, the topography along I-70 gets more interesting as the deeper strata of the Denver basin bend upward against the Precambrian core of the Front Range uplift. Starting at Green Mountain (right), hillside exposures begin to reveal their structure.

Green Mountain is the erosional remnant of a once extensive flat-lying syntectonic apron of alluvial fans deposited all along the east side of the rising Front Range during the Laramide mountain-building event. These gravels rest unconformably on east-dipping Late Cretaceous strata, including older syntectonic gravels, tilted upward to varying degrees during the uplift of the east side of the Front Range block during the Laramide Orogeny.

Cropping out along the lower western slopes of Green Mountain are the Latest Cretaceous through Early Paleocene Arapahoe Formation and Denver Formation. These originally flat-lying syntectonic conglomerates and sands reflect different levels of erosion in the rising Front Range. The Arapahoe is mostly debris from the Mesozoic and Paleozoic sedimentary cover, while the Denver is largely composed of basaltic clasts (rock fragments) coeval with the 65-63 Ma Early Paleocene basalt flows preserved as a caprock on North and South Table Mountains just a few miles north of I-70 at Golden. Erupted early on during the Laramide, Front Range volcanics probably found their way to the surface along leaky Laramide faults cutting the full thickness of the crust, if not the entire lithosphere. The Arapahoe and Denver Formations now dip to the east along with older Denver basin strata. 

Capping Green Mountain is the Late Paleocene Green Mountain conglomerate, a flat-lying collection of gravels composed almost exclusively of pink granitic Precambrian clasts shed from the Front Range to the west after its volcanic cover had been breached by erosion. Its lack of tilt indicates that Laramide uplift of the Front Range had largely subsided by the Late Paleocene. Visit geologist Dick Gibson's ^Green Mountain page site for more information.

Laramie and Fox Hills Formations

White Fox Hills bluffs at he base of Green Mountain

Older near-vertical beds of light pink beach sands of the lower Laramie Formation (70-75 Ma) crop out along the west margin of Green Mountain Park, where they underlie both the Denver/Arapahoe Formation and the Green Mountain conglomerate. Nearly vertical light yellow beach and bar sands of the Fox Hills sandstone crop out at the northwest base of Green Mountain along the south side of I-70, where they underlie a horizontal unconformity covered with white alluvium. These barrier island sands were deposited by the retreating Cretaceous Interior Seaway as the earliest Laramide deformations broadly lifted Wyoming and Colorado out of the water.

Steeply west-diipping overturned Laramie sandstone ridge at Golden
Steeply west-diipping overturned  Laramie sandstone ridge at Golden

Why the Laramie and Fox Hills strata dip more vertically than those in and west of the Dakota Hogback is unclear, at least to me.  The standard explanation is "drag-folding" due to movement on a west-dipping reverse Golden Fault during Laramide time, and that may well be so. But the nature and timing of activity on the Golden Fault is not so clear, as discussed in the following section

West of Green Mountain is the Laramide Golden Fault, the Dakota Hogback, Red Rocks Park and finally the Front Range foothills. We'll visit them all in the gallery below.

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The Elusive Golden Fault

From from Amuedo and Ivey, 1978, _Coal and Clay Mine Hazard Study and Estimated Unmined Coal Resources, Jefferson County, Colorado_
Dakota Hogback and Golden Fault cross-section

The north-trending Golden Fault (GF) runs along the east flank of the Front Range for ~24 km (15 miles). It's known primarily from well logs and seismic profiles, all of which consistently show a subsurface vertical offset of some 3.4 km (11,000') in the Precambrian-Fountain contact along the mountain front. This offset is the GF. It has few verifiable surface traces, and details of its subsurface geometry remain largely speculative, but most authorities show it as it appears in the cross-section above — a steep, west-dipping reverse fault thrown up on the west. This configuration explains how the GF brought late Paleozoic through early Cretaceous sediments deposited on Precambrian basement up and over early Cretaceous through late Tertiary sediments at the top of the Denver Basin

C-470 roughly marks the projected surface trace of the Golden fault, here just south of Green Mountain in the background

I-70 divides the GF roughly in half. The southern half runs beneath soil cover just east of the C-470 freeway (right), between Green Mountain and the Dakota Hogback. North of I-70, the GF runs west of North and South Table Mountains at Golden, where it swings west toward the mountain front to cut out the Dakota Hogback and adjacent strata. In the geologic map of Golden and North Table Mountain below, note how the westward bend in the GF truncates the Dakota, Benton and Niobara strata at top center. The bend may well reflect a change in the geometry of the Precambrian core of the Front Range in the subsurface at Golden, but no one knows for sure why the GF veers west here.

A Golden Mystery

From from Amuedo and Ivey, 1978, _Coal and Clay Mine Hazard Study and Estimated Unmined Coal Resources, Jefferson County, Colorado_
Geologic map of the Golden Fault (Clear Creek at bottom)

To this day, no one seems to know quite what to make of the Golden Fault (GF). At only 8% the length of the 320 km-long Front Range, the GF has neither the length nor the offset (well under half the total estimated Front Range uplift) to claim the starring role in the raising of the east side of the Front Range. It's often been cast as a local splinter off a deeper and much longer blind thrust or reverse fault, but seismic profiling stands against that, at least at Rocky Flats (see below).

Areas of controversy still surrounding the GF include its type, dip and subsurface geometry, but a Laramide age is fairly well established. One way to bracket the timing and direction of the last movement of a fault is to identify datable rocks that it did and did not deform or cut. Differential folding and tilting of rock units across the Golden Fault indicate that it moved up to but not during or after eruption of the still flat-lying Table Mountain basalts, which are radiometrically dated with considerable reliability at 62-63 Ma. This strong evidence puts the GF active during Laramide time but doesn't exclude pre-Laramide movement.

Most authors consider the GF a steep west-dipping reverse fault, up to the west, but some still argue for a dip to the east, and others have even proposed that it's actually a pre-Laramide normal (extensional) fault. Sure enough, if you restore the strata it cuts to their original horizontal attitude, its geometry and motion fit an east-dipping normal fault, but this last view is not widely held.

Things are much more straightforward on the west flank of the Front Range, where the well-exposed east-dipping Williams Fork Thrust is clearly responsible for most if not all of  Laramide uplift and western displacement of the block's western edge. But there are no exposed west-dipping reverse or thrust faults likely to have been responsible for the uplift of the eastern edge of the Front Range anywhere along its ~320 km length. Many lines of evidence point to the existence of such east-side faults, but they have yet to show themselves directly, and the GF may or may not be one of them.

Seismic imaging or profiling is one such line of evidence. Two profiles shot across the mountain front just north of the town of Golden at Golden Gate Canyon and Rocky Flats (the latter particularly well controlled) show the Golden Fault as merely the inboard (west) member of a pair of roughly parallel west-dipping range-front faults. The outboard (east) member, dubbed the Basin Marginal Fault (BMF, not shown on the map at right) also shows up in mountain-front profiles shot at several locations up and down the east side of the Front Range. 

Surface projections of the GF and BMF would lie ~3-4 miles apart near Golden, but neither is directly exposed. Between the GF and BMF is a block of highly deformed and rotated sediments variably exposing vertical to overturned Niobara, Fox Hills and Laramie strata at the surface along the mountain front between Denver and Boulder. This geometry explains, for example, the abrupt increase in dip observed between the ~60° Dakota beds of the Dakota Hogback at Dinosaur Ridge and the ~90° Fox Hills and Laramie strata at the base of Green Mountain just across C-470, which roughly marks the inferred trace of the Golden Fault. At Golden, the GF has cut out the Dakota Hogback, and the ~70° Fox Hills, Laramie and Pierre beds there are actually overturned. 

If the BMF is indeed the master Laramide fault on the east side of the Front Range, the GF could represent a splinter joining it at some unimaged depth. The relatively shallow Golden Gate Canyon profile allows this possibility, but the deeper Rocky Flats profile  shows the GF dying out some 6,000-7,000' below the surface, well above the BMF. Geophysicists work hard to overcome the technical difficulties inherent in the seismic imaging of steeply dipping structures like the GF and BMF, but these relationships must still be taken with a pinch of salt. 

Paired range-front faults turn out to be fairly common elsewhere in the Laramide orogen, Wyoming's Wind River range included. The cause for the pairing remains unclear, but some think of the outboard fault as a zone of heavily sheared sedimentary strata in the outboard limb of a tightly folded syncline paralleling the master inboard fault. In that view, the GF would be the master.

Confused yet? You've got some very good company.

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Bottom of Gallery

Dakota Hogback and Red Rocks Park Gallery

Dakota Hogback at Dinosaur Ridge

In the 1st frame, the east-dipping Dakota hogback arcs to the south along the east flank of the Front Range, as seen from its crest along the highly recommended Dinosaur Ridge Trail just south of I-70. The hogback varies considerably in size, dip and exposure along strike but is particularly well-developed here with a ~60° dip to the west. Resistant Early Cretaceous Dakota sandstone here best seen at its crest caps the hogback, while softer, older Late Jurassic Morrison and Ralston Creek claystones form its lower west slope and the adjacent valley floor on the right. Uplift of the Front Range basement block (on the far right) during the Late Cretaceous Laramide Orogeny tilted these strata ~60° from the horizontal.

In the 2nd frame, an ^infrared version of the 1st, Pikes Peak (14,120') dominates the horizon at the south end of the Front Range block. It's much better seen in haze-defying infrared light. 

The 3rd and 4th frames show the Dakota hogback at Dinosaur Ridge as seen from Red Rocks Park to the west. The facing west slope is composed of porous, tree-dotted Lytle and Dakota sandstones over relatively impermeable Morrison flood plain shales. The Morrison's trademark purple, gray and green colors are hard to see in this light.

1. Dinosaur Ridge
2. Dinosaur Ridge in near IR light
3-4. West slope of Dinosaur Ridge from Red Rocks

Dakota Hogback Variations North and South of Dinosaur Ridge

[picture coming soon]
The Dakota sandstone, the Lyons sandstone and the Fountain conglomerate vary substantially and independently in cementation, section thickness and exposure all along the Front Range. Strong cementation generally makes for prominent exposures, but structural variations like the Golden Fault also play a role. This series of images shows a number of variations in the prominence of the Dakota Hogback along strike.

No hogback:  Just north of I-70 at Golden (not shown), the Dakota hogback is absent, having been cut out by the Golden Fault. The 1st frame looks west from a Laramie Formation outcrop in Golden. 

Low hogback:  Twenty miles or so south of I-70 at ^Roxborough State Park, a subdued Dakota Hogback (on the right in the 2nd frame) is much less imposing than adjacent Lyons (buff) and Fountain (red) flatirons, which are particularly well cemented here. The 2nd and 3rd frames in this series are a north-looking west-to-east sweep.

Medium hogback: The 4th frame looks north to the Dakota hogback at the mouth of Royal Gorge at Canon City.

Large hogback: The Dakota hogback is particularly well-developed at Dinosaur Ridge, as the 5th frame shows

Double hogback:  North of Boulder, a double hogback snakes along the Front Range into southern Wyoming. In the 6th frame, the Dakota hogback runs on the east (right) in this aerial view to the north as we flew east from Fort Collins. The rock capping the older hogback on the left is probably the Late Permian Lyons Formation. A low divide revealed by Colorado's 2001-2002 drought splits the normally one-piece Horsetooth Reservoir, which was cleverly dammed between the hogbacks.

1. West of Golden
2-3. Roxborough Park
4. Canon City

5. Dinosaur Ridge
6. North of Forth Collins

Fountain Formation in Red Rocks Park and Points South

Red Rocks Park:  The 1st and 2nd frames in this series look southwest to ^Red Rocks Park from Dinosaur Ridge Trail atop the Dakota Hogback just south of I-70. Bold red fluvial sandstones and conglomerates of the Late Pennsylvanian Fountain Formation rest uncomformably on the Precambrian core of the Front Range uplift here. Fountain erosional remnants form the park's flatirons and monoliths, some of which circle to form the acoustically renown natural amphitheater now known as Denver's ^Red Rocks Amphitheater. The park also offers excellent hiking among the flatirons. 

The 3rd frame looks back at the Dakota hogback to the south southeast across Fountain flatirons. The south-looking 4th frame shows a closer view of a Fountain flatiron with a patch of late November snow.

Roxborough and Garden of the Gods:  Much more impressive red Fountain flatirons and monoliths crop out to the south in ^Roxborough State Park (5th and 6th frames, both looking north) and Garden of the Gods Park (7th and 8th frames). Buff-colored desert dune sands of the Lyons Formation, barely exposed at Red Rocks, also form much more prominent hogbacks (5th frame), flatirons and fins (8th frame) in these locations. Why the difference? The Fountain and Lyons are better cemented and therefore more resistant here than they are at Red Rocks. 

Fountain facts:  The coarse alluvial fans preserved in the Fountain Formation record the initial uplift of the Ancestral Rocky Mountains in Late Pennsylvanian time, around 300 Ma, just as Green Mountain strata to the east record the initial uplift of the modern Rockies in Late Cretaceous Laramide time, around 72 Ma. Such deposits are said to be syntectonic. Range-front streams dumped the first Fountain gravels (300 Ma) onto a flat erosional surface of 1.4-1.7 Ga Precambrian basement; the resulting 1.1-1.4 Ga gap in the rock record is known regionally as the Great Unconformity. Around 230 Ma later, Laramide mountain-building tilted the Fountain, underlying Precambrian basement and overlying strata to their present 30-40° dips. In contrast, the youngest Green Mountain gravels were deposited flat and remain so.

Basal Fountain strata:  In the 9th frame, wind caves pock a basal unit of the Fountain Formation at Red Rocks Park. The contact between the Fountain and underlying basement rocks is difficult to pinpoint around here but can be razor-sharp in places. A plaque marks the contact in the amphitheater parking lot.

The 10th and final frame in this series shows classic syntectonic alluvial fan sediments at the base of the Fountain, again at Red Rocks Park. Their coarse angular clasts of quartz and feldspar tell of a high-energy stream bed draining nearby exposures of crystalline Precambrian basement. Since feldspar weathers quickly to clay during stream transport, feldspar clasts this large can't be far from home. Originally laid down flat along the Ancestral Rocky Mountain range front, these beds acquired their pronounced 30-40° dip to the east during the Laramide Orogeny.

1-2. Red Rocks from Dinosaur Ridge
3-4. Red Rocks flatirons up close
5-6. Fountain flatirons at Roxborough, with Dakota Hogback in the background
7-8. Lyons flatirons at Garden of the Gods
9-10. Basal Fountain strata at Red Rocks
These two frames show all the strata eroded from the east side of the Ancestral Rockies, from oldest to youngest. The 1st frame looks southwest from Dinosaur Ridge to the Front Range foothills. Below the red Fountain flatirons of Red Rocks Park is a short ridge of white Lyons Formation, a younger collection of Permian stream, beach and eolian dune sands eroded from the by then low-lying Ancestral Rockies. Lower still on the slope is the soft red Late Permian to Early Triassic Lykins Formation, an even younger redbed of micaceous mudstones with minor limestones recording brief marine transgressions from the east. By the end of Lykins time, around 220 Ma or so, the Ancestral Rockies were gone.

The 2nd frame is an opposing view looking northeast from the Fountain flatirons of Red Rocks Park to Dinosaur Ridge. Beyond the small ridge of white Lyons sandstone and the lower slopes of Lykins mudstones, we pick up the sequence again with soft, easily-eroded valley-forming Late Jurassic Ralston Creek mudstones, here seen under the road in the distance. Morrison Formation floodplain deposits form the lower west face the Dakota hogback here at Dinosaur Ridge; the upper slope and crest are in Dakota sandstone.

1. Fountain, Lyons and Lykins Formations
2. Fountain, Lyons, Lykins, Ralston Creek, Morrison, Lytle and Dakota strata

Dinosaur Ridge—Morrison Formation

The diagnostic gray, green and maroon terrigenous floodplain clays making up the bulk of the statewide, dinosaur-rich Late Jurassic Morrison Formation are poorly exposed on the west slope of Dinosaur Ridge (1st frame), but they're beautifully laid out in cross-section in the nearby I-70 road cut (on the right in the 2nd frame). Minor stream and lakebed sandstones and thin discontinuous resistant lacustrine limestones (not shown here) imply a flat and poorly drained Morrison landscape in a wet Late Jurassic climate. The wealth of dinosaur fossils and traces found in the Morrison throughout the state implies an abundance of supporting vegetation.

The 3rd frame shows more typical Morrison colors exposed in all their glory in badlands near the dinosaur quarry on the west side of Dinosaur National Monument.

Dinosaur footprint sags:  The 4th and 5th frames show fractured depressions in sandy upper Morrison Formation strata thought to be sauropod (long-necked leaf-eating dinosaur) footprints in cross-section. The marker is about 6 inches long.

1. West slope of Dinosaur Ridge from Red Rocks
2. Colorful Morrison strata in the I-70 road cut
3. Morrison strata in full color, west Dinosaur NP
4-5. Sauropod footprint in Morrison at Dinosaur Ridge

Dinosaur Ridge — Dakota Sandstone

Arrival of the Cretaceous Sea:  This sharp contact separating the colorful coastal floodplain claystones of the Morrison Formation below from the transgressive (advancing) Dakota beach sands above marks the arrival of the Cretaceous Interior Seaway. For the next 60 Ma, the area would accumulate over ??,000' of Cretaceous marine sediments. The Late Cretaceous Foxhills Sandstone at the base of nearby Green Mountain to the east marks the Cretaceous seaway's retreat at the beginning of the Laramide orogeny. Brother-in-law John Malone, providing scale, is younger than the contact, but not by as much as he'd like.
1. Morrison-Lytle contact
Dakota sandstone: Alameda Parkway (CO26) nips through the crest of the Dakota Hogback in this west-to-east sweep looking to the NW through a ridge cut also visible in the hogback photos above. The resistant, well-cemented, generally light-colored Dakota sandstone contains distinctive black to gray organic-rich "coaly" layers, but true coals are rare in the Dakota in Colorado. (Strictly speaking, this is South Platte sandstone, but I'll stick with the common Dakota appellation here.)

By Dakota time, around 100 Ma, North America had reached its present latitude. The climate was temperate. Large coastal rivers brought mature sands to the Cretaceous shoreline from high mountains far to the west. Dense coastal forests and swamps dotted the sandy landscape.

1. Dakota sandstone at the crest of Dinosaur Ridge
Dakota beach ripples: Symmetrical beach-style shallow water ripple marks decorate the upper surface of this Dakota bedding plane on the east side of the hogback. (Asymmetric ripples imply a stream or wind-blown origin.) Algal mats covering the beach preserved the ripples.  

The slow westward advance of the Cretaceous Interior Seaway left behind thick Dakota sands from central Kansas through Colorado and well across Utah to boot. Accordingly, the Dakota is one of the most pervasive and most easily recognized of Colorado's sedimentary strata. Along I-70, the Dakota pops up nearly a dozen times between Denver and Grand Junction alone. 

The top frame shows the ripples slipping under younger Dakota strata. The center frame looks straight up the steep bedding plane bearing the ripples. The bottom frame includes a glove for scale.

1-3. Symmetrical beach ripples in Dakota sandstone
Sticks in stones: Fallen branches and roots left the linear casts recorded in these Dakota bedding surfaces. The temperate Dakota shoreline included extensive mangrove swamps, which crawled across Colorado along the advancing Cretaceous Interior Seaway shoreline like so much green fluff before the waters.
1-2. Mangrove branch and root casts in Dakota sandstone
Dakota trace fossils:  The myriad worm and shrimp burrows perforating this Dakota beach horizon, along with the mangrove litter and the dinosaur tracks nearby, tell of a temperate coastal environment teaming with life. 

As the Cretaceous seaway pushed west, coastal forests became wetlands became beaches became open water. The encroaching Dakota sands were well suited for burial and preservation of Colorado's Early Cretaceous coastal ecosystem. 

1-2. Mangrove branch and root casts in Dakota sandstone
Dakota dinosaur crossing:  The famous criss-crossing dinosaur trackways of Dinosaur Ridge are exposed on the east flank of the Dakota Hogback. The tracks have been shaded with charcoal for better visibility. The markers are about 6 inches long. The host Late Cretaceous Dakota sandstones have been dated at 98 My.

The larger four-toed tracks (frames 2-3) belong to a duckbilled herbivore usually identified as an iguanodontid (bird-hipped) dinosaur, but recent work suggests that the last of the iguanodontids may have died out shortly before 98 My. Some paleontologists now believe the tracks to belong to an early hadrosaurid such as Eolambia instead, but the debate is far from settled. Some of the size variations are thought to record the presence of both adult and juvenile duckbills.

The slender three-toed ornithomimid (bird-like) tracks seen in frame 4 were probably made by a much smaller and faster bipedal therapod dinosaur rather than a bird. Heron-like shorebirds existed in the Early and Middle Cretaceous, but none this large are known, and other track details also favor a dinosaur as the trackmaker here.

Roadside interpretive markers (frame 6) give life to the tracks. For more information on the tracks and the dinosaurs responsible for them, visit the well-illustrated ^Friends of Dinosaur Ridge web site.

For a real "you were there" treat, however, I strongly recommend the outstanding ^Ancient Denvers exhibit at the ^Denver Museum of Nature and Science. As of this writing, the online version is still under construction, but the exhibit's truly spectacular original large-format paleogeographic paintings are well worth a visit if you're in the area. The museum also sells a beautiful Ancient Denvers guide book. The museum's extensive mineral, ecology and paleontology exhibits provide invaluable learning opportunities for any student of Colorado's past and present.

1. Main Dinosaur Ridge trackway
2-3. Iguanodon tracks
4. Therapod track
5. Crossing trackways
6. Typical interpretive marker
Dakota-Benton contact:  The dark, organic-rich Benton Shale overlies the Dakota along Alameda Parkway at the east base of Dinosaur Ridge. 

Oil and gas cooked up from organics in the portion of the Benton still buried deep in the Denver Basin rise across this boundary to take up residence in porous Dakota sandstone traps drag-folded by motion along the Laramide Golden Fault, which around here runs along the east side of the C-470 freeway (2nd frame). The Golden Fault is a steep reverse fault reactivated 

1. Dakota-Benton contact
2. Inferred Golden Fault trace

Dakota unconformity: Broad lowland rivers draining high mountains of the Sevier orogeny far to the west in Utah crossed the sandy Dakota coastal plain in many places. The coarsely cross-bedded Dakota sandstone on the left in the 1st frame is a fossil river bar nicely exposed in the north side of the I-70 road cut in the upper Dakota. The interpretive marker here explains the sudden change in bedding plane orientation as the signature of an abrupt and sustained reversal in river flow, but others believe that the unconformity reflects a basement disontinuity far below.
1. Dakota unconformity

North and South Table Mountains at Golden

Town of Golden, CO between North and South Talble Mountains Historic Golden, Colorado nestles against and between North and South Table Mountains. (The latter is on the right in east-looking frame 1.) A series of 4 well-dated 63-64 Ma Laramide basalt flows (2nd frame) caps both mesas. The mesa slopes below the cap are the poorly consolidated Paleocene strata of the Denver Formation (3rd frame) and Arapahoe conglomerate (4th frame), both of which consist largely of earlier Laramide andesitic volcanic debris. A thickness of Denver Formation sediment separates the earliest flow from the others — hence the double cliffs. All the flows consist of shoshonite porphyry — an unusually potassium-rich fine-grained basalt peppered with large feldspar crystals — and appear to have erupted over a span of 1 Ma or so. 

Clear Creek, a large Front Range stream headwatering near the Continental Divide, splits the originally intact flows into into north and south mesas after spilling out of a spectacular range-front canyon. From Golden, CO6 takes Clear Creek canyon up and the west to Idaho Springs — a drive not be missed. The upper canyon follows the Idaho Springs-Ralston shear zone, an important section of the east end of the Colorado Mineral Belt. The  meandering course of the lower canyon and the creek's improbable run directly through the Table Mountain flows tell of a gently sloping range front surface built upon mid-Tertiary sediments long since removed during exhumation of the Colorado Piedmont.  

Judging from the large volumes of volcaniclastic debris captured in the Paleocene Denver Formation, volcanics must have  erupted widely during the opening of the Laramide orogeny, but the generally fine grain of the debris puts most of the eruptive centers some distance off to the west — certainly not at the range front. Nevertheless, the flows at Golden are Colorado's only preserved Laramide volcanic edifices (they may lie in a protective graben), and their vents are only a few miles to the north. Moreover, the Table Mountain flows are the only evidence of basaltic volcanism in the Laramide. Laramide volcanism in the Front Range was no doubt intense, but it was no match for the erosion that followed.

The ^Colorado School of Mines (CSM) campus takes up the foreground in the 1st frame in this series. Behind it, the massive Coors brewery sprawls along Clear Creek between the mesas. The 2nd, 3rd and 4th frames are exhibits along the CSM geologic museum's worthwhile geology trail. Prominent overturned ridges of near-white fossiliferous Laramie sandstone run through the west end of the campus (frame 5).

Frames 6 and 7 look north toward Golden from the parking lot on the north side of the I-70 road cut through the Dakota Hogback — 6 in the morning and 7 in late afternoon. The morning light on the crisp October day in frame 6 nicely shows off the range front. The famous Fountain flatirons just south of Boulder tilt up steeply against Front Range Precambrian basement in the distance at left center. Golden lies in the middle distance, with North and South Table Mountains on its right. The 1-2° southeast dip of flat erosional surface atop the table mountains shows that there has been little local range-front uplift since the capping basalts flowed at 63-64 Ma.

West of Golden, an intriguing westward bend in the Golden Fault cuts out the Dakota Hogback. In the late afternoon photo (frame 7), it's easy to see the Dakota Hogback at the north end of Dinosaur Ridge petering out in the right foreground. A small hogback reappears north of Golden, but it's not well seen in these photos. 

The 8th frame is a plan view of South Table Mountain and its basalt cap from 8,200' Genesee Mountain in the foothills to the west. This perspective clearly shows the cap to be a remnant of a larger flow. The most likely vent site is an intrusive body and dike complex in the foothills to the north northeast near Ralston Creek. The age and composition of the intrusives there match those of the capping basalts. Thinning of the basalt cap to the southeast is also consistent with this source.

1. Golden, CO between North and South Table Mountains
Basalt boulder from the flow capping the table mountains
2.Table Mountain basalt
Denver Formation
3. Denver Formation
Arapahoe conglomerate
4. Arapahoe conglomerate
Laramie sandstone
5. Palm frond in Laramie sandstone
Range front in morning ligjt
6. Mountain front at Golden, late afternoon
Range front in late afternoon light
7. Mountain front at Golden, late afternoon
8. South Table Mountain from Genesee Mountain

Green Mountain

East of Dinosaur Ridge across C-470 and the Golden Fault is Green Mountain, topped by a flat-lying pile of syntectonic gravels shed in alluvial fans from the Laramide Front Range uplift to the west. Upended Fox Hills sandstone is quarried in the light-colored patches at the base of Green Mountain.

The lower frame captures two generations of syntectonic gravels: The Late Cretaceous Green Mountain Conglomerate on the horizon behind the Dakota Hogback records the initial Laramide uplift of the current Rockies, while the Late Pennsylvanian Fountain Formation in the foreground at Red Rock Park records the initial uplift of the Ancestral Rockies

I-70 Road Cut and the Front Range Foothills

The geologically famous I-70 road cut through the Dakota Hogback west of Denver beautifully exposes (from left to right) ever older, east-dipping South Platte, Lytle, Morrison and Ralston Creek strata in this shot of the south side. The same Mesozoic strata arch discontinuously across the Rockies to the opposing west-dipping Grand Hogback at Newcastle, 155 miles to the west. 

The buff and black Dakota strata shown on the far left in the 2nd frame are easily recognized throughout Colorado. Gray, green and maroon  claystones of the equally recognizable Late Jurassic Morrison Formation stand to the right of the Lytle-Morrison contact seen in the 3rd frame.

Informative plaques along the footpath along the other (north) side of the road cut recount some of the stories the rocks have to tell. Access both sides of the road cut via Alameda Parkway (CO26) or the Morrison Exit, I-70 #269.

This view to the southwest from Dinosaur Ridge shows the Fountain Formation flatirons of Red Rocks Park resting on the Front Range foothills, here composed of hard Precambrian gneiss and pegmatite of the 1.7 Ga Idaho Springs Formation. 

Mount Vernon Canyon:  West of the Dakota Hogback, I-70 climbs up onto the crystalline basement of the Front Range foothills via Mt. Vernon Canyon, seen in the 1st frame looking west from Dinosaur Ridge. Road cuts in the canyon expose 1.7 Ga pink granites and pegmatites and dark gray to white banded gneisses of the Front Range core. The sedimentary cover is long-gone here but reappears on the west flank of the Front Range just east of Dillon, where Dakota strata dip west.

The 2nd frame looks back to the east toward Denver from atop Genesee Mountain (8,284') near the top of Mt. Vernon Canyon. In the foreground is an outcrop of 1.7 Ga metavolcanic gneiss of the Idaho Springs Formation.
Denver from Genesee Mountain; 1.7 Ga metavolcanic gneiss in foreground
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