Wednesday, May 13, 2015

How a Valley Became a Mesa, and Why a River Runs Through It

The town of Golden, Colorado is right next to the foothills of the Rockies, but the most striking feature there is a mesa called Table Mountain. It's one of those utterly western-looking landmarks you would never find in the east. You almost expect to see John Wayne riding past it. Table Mountain actually has two parts--North and South Table Mountain--and the famous Coors brewing company sits in the canyon between them. Here's a picture of North Table Mountain from the foothills above Golden. You can see part of South Table Mountain on the right.

Photo by Ross Mays, from Lookout Mountain
I love hiking on top of Table Mountain, because it's like an grassy oasis in the middle of the Denver sprawl. It's surprisingly expansive once you get up there, and the view of the Front Range is spectacular.There's also another view of Table Mountain that's even more impressive, at least to me, but it's a view you have to use your imagination to see. To see what I mean, take a look at the picture below.

Photo by Ross Mays, ancient landscape drawing based on Drewes, 2008 (see that paper for a more accurate diagram)
This is a geologist's view of Table Mountain (or actually an amateur's version a geologist's view). One of the great things about geology is that it lets you see multiple landscapes all at once. You see the modern landscape, of course, but in your mind's eye you also see older landscapes superimposed on it. It's like looking at the world through goggles that let you see the contours of past worlds, and tell you how they gave rise to today's landscapes.

So what's going on in these past worlds? What's up with those volcanoes? The picture shows North and South Table Mountain from the south, from the slopes of another hill called Green Mountain. The sketches show ancient landscape features from different times in the past. They didn't necessarily exist at the same time.

To see how these ancient landscape gave rise to the modern one, it helps to start by considering the ridges of upturned rock you can find north and south of Golden, at places like Red Rocks and Dinosaur Ridge. In the picture below, you can see some of those layers where I-70 cuts though the Dakota Hogback just south of Golden (notice the man on top, walking forward in time from the Jurassic to the Cretaceous Period.)

The upturned rocks along the Front Range are sedimentary layers dating from 300 million years ago (at places like Red Rocks) to about 70 million years ago. Like all sedimentary rocks, they were more or less flat when they were laid down. They're still flat to the east of the Rockies, and buried under younger rocks. Along the Front Range, though, they turn sharply upward and emerge from deep underground. That's no coincidence, of course--the Rockies are what pushed them upright, when they first started to rise about 70 million years ago. What we don't see today, except through geology goggles, is that the same rock layers extended across the rising Rockies. These layers were pushed high in the air, where they began to erode away, exposing the harder, older granite and metamorphic rock that makes up most of the Front Range today (except for a few remnants of sedimentary rock, stranded high in the mountains).

All these geologic upheavals caused magma to rise from deep in the earth, creating the Cretaceous volcanoes sketched on the far left in the second image. These volcanoes soon began eroding, too. Together with eroded the sedimentary rocks lifted by the Rockies, they formed a layer of coarse gravel at the base of the new mountains. This gravelly rock now forms the bottom layers of Table Mountain. The volcanoes are gone, but you can pick up a piece of volcanic gravel--rounded by ancient rivers--and know they were once there. Some of the last dinosaurs once walked on this gravel as it rose, including a T rex that left a tooth behind on the future flanks of the mountain.

When that T rex was alive, though, Table Mountain didn't exist. In fact, Table Mountain began as a valley, in the Paleocene Epoch, just after the extinction of the dinosaurs.* In those days there was another volcano near the north end of a shallow valley where Table Mountain is now. It erupted four times over a period of about a million years, sending lava flows down the valley. Over time this lava solidified into a kind of volcanic rock called shoshonite. The volcano mostly eroded away, leaving a remnant known as Ralston Dike, which you can still see north of Table Mountain. The Rockies kept eroding onto the plains, and more layers of sediment covered the lava flows. Finally, the mountains were buried so thoroughly in their own debris that they were little more than hills projecting above the ancient plains.

So how did the Rockies come back, and how did a valley become Table Mountain? Like this: Just a few million years ago (the exact time seems to be controversial) complex tectonic forces lifted the entire region about a mile into the air. That's why Denver is a mile high today, even though it's on the plains. This regional uplift caused rivers coming out of the mountains to begin cutting downward, carving the deep canyons along the modern Front Range and stripping away the top layers of debris on the plains near the mountains.** This great event--still going on today--is known as the Exhumation of the Rockies.

As the great plains eroded, the sediments above and around the Table Rock lava flows eroded along with them. But when the hard volcanic rocks were finally exposed, they resisted erosion and protected the soft layers of rock below them. That's how mesas form--a hard layer of caprock on top keeps the rocks underneath from eroding. The valley had become a mesa, capped by lava flows from an ancient volcano.

But how did the mesa become two mesas, with a river (Clear Creek) running between them? Why didn't the creek go around the mountain, instead of straight through it? The answer is that Clear Creek once flowed across the old high plains, above the lava flows of Table Mountain. As it started cutting down into the plains, it encountered the hard volcanic rock. By this time, though, its course was already set--it was trapped within its own channel. So it kept on cutting downward, though the hard lava flows. Along with other streams in the area, it kept eroding the old high plains surface. Eventually, Table Mountain emerged from under the debris, with a river running though it, and an amazing story to tell.


Table Mountain Shoshonite Porphyry Lava Flows and Their Vents,Golden, Colorado / Harald Drewes, 2008

Geology Underfoot Along Colorado's Front Range / Lon Abbott and Terri Cook

*The first epoch of the Cenozoic Period. The Mesozoic ended with the extinction of the dinosaurs. The North American boundary between these two time periods (known as the K-T Boundary) was first identified on South Table Mountain.

** There are still remnants of these ancient high plains. Interstate 80 in southern Wyoming follows one of them, called The Gangplank. This old surface of the ancient plains rises almost to the top of the mountains, which is why the interstate was built there. This is also where the first transcontinental railroad crossed the Rockies.