Rock On, Mother Earth!

To experience the “forest” of Arizona’s Petrified Forest National Park, visitors must imagine a lush tropical former world from more than 200 million years ago. In this first of two episodes, paleontologist Andrew Heckert prepares us, revealing four former worlds along the route to the park from Gallup, New Mexico. Ranger Hallie Larsen welcomes us at the park visitor center by explaining how logs turned to stone. Professor Heckert then guides us through hoodoos and badlands along the park’s Blue Mesa Loop trail. ----more---- Podcast chapters (M= minutes, S= seconds) are summarized here: 00M 00S OPENING: I compare descending through rock layers to visiting former worlds and explain that the road from Gallup to the park is like an elevator ride into the past. 02M 26S GALLUP, LOUISIANA?: Dr. Heckert and I discuss coal-bearing layers of the Mesa Verde Group in a road cut at the edge of Gallup. He compares its former world, populated by crocodilians, scaly garfish, and horned dinosaurs, to the swamps of today’s Louisiana. 07M 36S SEAWAY & RATTLESNAKES: Outcrops seen from I-40 record the Interior Seaway that divided North America during much of the Cretaceous Period. The Mancos Shale near milepost 13 reveals offshore environments that varied over time with relative sea level. Visible near milepost 5, the Dakota Sandstone blanketed the West with river deposits as the seaway began to flood a deeply eroded landscape. 14M 38S TATOOINE AT YELLOWHORSE: At Yellowhorse Trading Post and the nearby Arizona Welcome Center, we step out of the car to gaze up at the remnants of migrating dune fields. Andy compares the Entrada Sandstone deposits to the Tunisian landscape that stood in for the desert planet Tatooine in Star Wars movies. 19M 21S LOGS TURN TO STONE: Ranger Hallie Larsen welcomes us to the park’s Painted Desert Visitor Center by explaining how giant logs in great rivers formed jams, were buried, and turned to stone. Some wood, with incomplete petrification, is less colorful but preserves cell structure. 26M 36S BLUE MESA BADLANDS: Professor Heckert looks through the images I took hiking the one-mile Blue Mesa Loop Trail, discussing hoodoos and shrink-swell clays that are a nightmare when wet. We see a petrified log that he says most likely has been “let down.” 32M 40S A CLIFF’S STORY: Andy interprets sediment layers (see podcast image) that alternate between rivers dropping sand and gravel with floodplains hosting plant roots. 39M 18S NEXT EPISODE, THANKS: I preview Episode 4 and thank participants.

Rio Grande del Norte National Monument features the deepest canyon in New Mexico at its Wild Rivers Recreation Area. The Rio Grande River is continuously carving this gorge into the Taos Plateau, a dry plain dotted with extinct volcanoes. The cutting of the canyon began when a vast Ice Age lake in Colorado spilled over about 440,000 years ago. The Taos Plateau itself forms the floor of the Rio Grande rift valley, flanked by mountain ranges both east and west. Like the East African rift valley, famous as the home of early humans, the rift is a place where the Earth's crust is pulling apart. Join Rock on Mother Earth for a hike into the gorge with two rangers, and an interview with the lead author of a recent study of its geology. ----more----Podcast chapters (M= minutes, S= seconds) are summarized here: 0M 0S OPENING: The Rio Grande is continuously carving New Mexico’s deepest canyon into the Taos Plateau, a dry plain dotted with extinct volcanoes. Join me for a walk with two rangers (Tim Long and James Larson) and an interview with the lead researcher of a 2019 study on Wild Rivers geology (Travis Clow). 0M 50S RIO GRANDE RIFT: Ranger Long uses a wood block model to illustrate Earth’s crust pulling apart along the Rio Grande Rift. The Sangre de Cristo and Tusas mountains rise up on the flanks of the Taos Plateau, while magma rising from Earth’s mantle spills out as lava that mingles with sediments eroded from the mountains. 6M 56S PAY STATION VIEWPOINT: A 2017 U.S. Geological Survey guidebook in hand, I visit a gorge overlook to examine a “columnar-jointed” lava flow and a vista that includes numerous volcanoes, plus 25-million-year-old volcanic rocks uplifted in a fault block. 12M 34S LA JUNTA POINT: Ranger Larson describes the view of the gorge and surroundings. Ranger Long shows how water availability in the canyon creates an “inverted ecosystem.” He recounts the beginnings of the canyon, believed to have resulted about 400,000 years ago from the spillover of a vast Ice Age lake in Colorado. 17M 54S TERRACES: Dr. Clow talks about phases of canyon-deepening (incision) and floodplain formation (aggradation) that have alternated since the spillover event. He describes his research to obtain the age of remnants of former floodplains, called terraces. The results suggest that melting after glacial episodes coincided with incision. 27M 35S INTO THE GORGE: I give an overview of the round trip to the bottom of the gorge that Ranger Larson led me on. Then I describe beginning the hike with views to the north. Next, Dr. Clow explains the origin of piles of boulders (talus) eroded from the rim’s cliffs. 30M 33S TOREVA BLOCKS: An area shaded by Ponderosa Pine nestles just upslope of a tilted strip of lava rock, twice the length of a football field. Dr. Clow describes how this “Toreva block” slid down from the cliffs above without shattering into smaller pieces. 33M 48S TO THE RIO GRANDE: The rangers and I meet hikers Sue and Dudley Chelton, a couple just past their 50th wedding anniversary, who share their delight at having visited the river’s edge. Ranger Long talks about the role of water in an arid landscape. 38M 30S ROCK ART: Ranger Larson leads me, along with Floridians Kristi Lowery and Jason Buchheim, to some extraordinary petroglyphs. Dr. Clow tells the geology of the boulders that First Peoples chose to decorate. 45M 57S NEXT EPISODE, THANKS: I close by promising Episode 3 and thanking participants. Sources: https://www.blm.gov/visit/rgdnnm ; https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/3/820/4701037/820.pdf; https://pubs.usgs.gov/sir/2017/5022/r/sir20175022r.pdf