Tuesday, September 11, 2012

More Colorado Geology - Mesa Verde

Most visitors to Mesa Verde National Park go there to see the spectacular cliff dwellings that are preserved in the sandstone alcoves. But these sandstones and the entire landscape tell an interesting geologic story as well. It may be a stretch to say that if there were no cliff dwellings on Mesa Verde that visitors would still go there. But the view from the top of the mesa is world-class and a little knowledge of the rocks brings another learning dimension to the locality. In July of this year I made a geologic excursion to the park with colleague Jack Share.

View to the west of the north side of Mesa Verde, which looms high above the Montezuma Valley in southwest Colorado. The great Aztec ruler Moctezuma (whose name was later corrupted to Montezuma) never came this far north but early American settlers attributed the nearby ruins to his dynasty in Meso-America (Mexico). When the Anasazi (or Ancestral Puebloans if you like) lived here in the 13th century AD, there were over 40,000 inhabitants of the Montezuma Valley, more than twice the current population of Cortez, Colorado.

The north flank of the mesa is carved into south-dipping sediments that are of Cretaceous age. As rocks fall off of the edge of this escarpment, it retreats slowly through time to the south (towards the left in this view). That means that these layers once extended up slope farther to the north (to the right) and onto the southern flank of the San Juan dome, a Laramide-age uplift present in southwest Colorado. The rate of retreat on this escarpment can likely be figured out using rates from nearby areas on the Colorado Plateau and may be on the order of about one meter every 2,000 years.

Beyond Mesa Verde to the west is the Sleeping Ute Mountain, a laccolith about 28 million years old. See my previous post on laccoliths in this area here.

Here is a view off of the edge of Mesa Verde to the north. There is a pre-existing topography on top of the mesa that is now being destroyed by the southward retreat of the escarpment. Old drainages that flow down the slope of the mesa are thus being beheaded by scarp retreat. Look beyond the top of the cloud shadow and notice a canyon cut into the top of the mesa. This is Morefield Canyon.

This part of the escarpment is called The Knife Edge and was a prominent feature early in the history of the park as the main road to the ruins was built on the slope beneath it. The road had to be relocated because of the manner in which the Mancos Shale sloughs off in the process of cliff retreat (boulders from The Knife Edge were a constant hazard on the road as well). Note the headwall of Morefield Canyon just visible in the gap in The Knife Edge. The shadowed cliff is carved in the Point Lookout Sandstone, the lowermost unit in the Mesa Verde Group. The Point Lookout Sandstone is a regressive sand, meaning that as the Mancos Seaway was retreating to the east, it left behind these beach sands about 85 million years ago.

Farther along the road is a road cut into the next formation in the Mesa Verde Group, the Menefee Formation. Here Jack explores some of the talus that has come off the cut. The Menefee is a continental stream and swamp deposit that was laid down in a setting much like the present-day Great Dismal Swamp in Virginia or the Okefenokee Swamp in Georgia. Rivers delivered sand to the landscape but these in turn were buried by thick mats of vegetation in swamp environments.

The angled beds of sandstone here are likely point bar deposits from a stream meandering on a coastal plain, some 82 million years ago. As the Cretaceous Seaway retreated to the east, it exposed a low-lying plain that was coursed by many rivers. A map of Menefee time is provided two images down. The black layers are coal seams within the Menefee Formation.

A close-up of a coal seam with plant fragments within it. Palms and conifers are common leaves found in the Menefee, as well as petrified tree trunks. Fossils such as these help determine the environment of deposition. 

Paleogeography of the seven southwestern states about 82 million years ago during deposition of the Menefee Formation. Note that the Mesa Verde area in southwest Colorado is the site of a coastal plain adjacent to the Cretaceous Seaway. Map courtesy of Dr. Ron Blakey, Colorado Plateau Geosystems. You can order a CD of these maps from CPG here or purchase a copy of our book, "Ancient Landscapes of the Colorado Plateau" here.

This map looks quite similar to the previous one - except when we zero in on the Mesa Verde region and notice that the seaway has transgressed over the area during a two million year interval. This map displays the geography of the area at about 80 million years ago and introduces the final deposit of the Mesa Verde Group, the Cliff House Formation.

A view of Spruce Tree House in Mesa Verde National Park. The inhabitants used local blocks of the Cliff House Formation to construct these dwellings. The Cliff House represents deposition as the Cretaceous Seaway crept back over the Mesa Verde area about 80 million years ago.

Square Tower House always delights visitors with its four story tower and symmetric preservation. Note the desert tapestries that streak the inside walls of the alcove. These form wherever water preferentially drips from above, allowing for the growth of desert varnish.

The largest cliff dwelling in the US is Cliff Palace with more than 250 rooms. It was an ideal location to build a small village as it receives much winter sun and summer shade. It is built within one of the larger and most well-developed alcoves in MVNP. The alcoves form within the Cliff House Formation, which consists of two massive sandstone bodies separated by a softer sandy shale. This alcove formed at the contact of the upper sandstone and the sandy shale horizon. It was here that groundwater pooled and dissolved the cement in the upper sandstone long before the canyons were cut. Upon exposure the weakened sandstone disintegrates progressively to form the alcove.

A close-up view of the heart of Cliff Palace ruin with a park ranger and lone visitor for scale on the far right. This is reportedly the very first ruin that was glimpsed by Richard Weatherill and Charlie Mason in December, 1888 and is what led to the "discovery" of this entire ruin complex.

Jack and I took the walking tour into Balcony House, my favorite excursion at Mesa Verde. The walk includes an ascent of a double-wide, thirty-two step ladder into the ruin.

Manos (hand-held grinding stones) and metates (grindiing surfaces) lay upon the "frozen" ripple marks of the Cretaceous shoreline. Certainly these people must have wondered about the lithified ripples that are so clearly exposed on the floor of Balcony House........

......especially with such detail of preservation! These are exposed in another ruin site located on Long Mesa. They look as if a foot print today could be made in soft sand. The ripple marks are symmetrical meaning that they likely formed in the broad swath of tide zone, rather than on a river floodplain. The water in a tidal zone is bi-directional and the ripples then become symmetrical in cross-section. In a river, the water is uni-directional and the ripples are formed asymmetrically.

Looking up Soda Canyon to the northeast from Balcony House ruin. The character of the Cliff House Formation changes in this direction with many more shale beds becoming common within the two massive sandstones. This records the deepening of the seawater in that direction during this time. Changes in rock type along the same horizon are known as facies changes.

Mesa Verde is an archaeologists paradise but another fascinating aspect of a visit here lies in its geologic story. What geologic thinking provides us is a way to shift time scales instantaneously - such as when we see a 1,000 year old metate laying on top of 80 million year old ripple marks. Time travel is something I enjoy very much and sharing with others is also appealing to me.

4 comments:

Teri said...

The walk along Knife Edge Trail is my favorite place in the park. I have been on that trail several times this year.

Dr. Jack Share said...

A phenomenal post! It makes me wish I was there (again). Jack

Garry Hayes said...

Nice post. My ongoing series is about to arrive in Mesa Verde too.

Joe Dirt said...

Fantastic writeup, I need to revisit MVNP with my geology eyes. Last time I was in tourist mode. I did manage to notice the ripple marks last time but not that they were tidal, great details. Thanks