Deep Time I: Stratigraphy & Sedimentary Record

1. Deep Time I:Stratigraphy & Sedimentary Record • Presented by Dr. Richard Alley • The Pennsylvania State University

2. ARCHES NATIONAL PARK: Stories in Stone Photos by R. Alley

3. Dr. Anandakrishnan, Arches National Park. The La Sal mountains, background, are named for the salt beneath them. Motion of the salt made joints that isolate fins that make the arches, such as the one next to Dr. Anandakrishnan’s shoulder.

4. End-on (top) and angled (bottom) views of fins, Devils Garden, Arches National Park. The tough sandstone has been broken by parallel, vertical joints, and weathering along those joints isolates “fins” of sandstone that then are eroded to make arches.

5. Desert varnish at Double Arch. The arrow points along a crack through which rainwater flows to form desert varnish. Enlargement of the crack will eventually lead to rockfall, changing the arch.

6. CAUSE student Raya Guruswami in Glen Canyon (right), and a cliff in Canyon de Chelly National Monument (top). Rockfalls from the sandstone cliffs have left arch-shaped amphitheaters. Similar falls from fins help make arches.

7. The sandstone of Delicate Arch started as a sand dune (note bedding, top-right picture).

8. Landscape Arch is the world’s longest natural stone arch. Notice numerous large blocks (a few are shown by red arrows) that have fallen from the arch, many since the park was founded, plus joints that make additional falls likely (black arrows). The close-up on the right shows the source of a recent fall. White and yellow lines connect points that are common to both pictures.

11. BRYCE CANYON: “A Hell of a Place to Lose a Cow” –Ebeneezer Bryce

12. Although Bryce Canyon is higher, cooler and wetter than many of the nearby parks, life is still tough on the unstable slopes of the canyon. Here, 2-inch-tall dwarf columbine (right and upper right) and a dewy Oregon-grape leaf struggle to grow at Bryce.

13. Guidebooks almost always refer to Bryce as a “fairyland.” The wonders of differential erosion—softer rocks, and rocks along cracks, go faster—really are amazing.

14. The Park Service now discourages “naming” rocks, because visitors are so disappointed when erosion then changes those rocks. This one was named Queen Victoria back when naming was condoned. The layered sedimentary rocks are beautiful by any name.

15. Trees in Bryce Canyon, with CAUSE students Sameer Safaya (far right) and Sam Ascah.

16. A decade or so ago, the Park Service put the logs (yellow arrow) across the stream bed to slow burial of a trail by sediment. As we’ve seen for dams, rocks filled the space upstream of the logs, and erosion happened downstream. Fast erosion supplied lots of rocks to fill the “lake” above the dam quickly.

17. Bryce’s rocks are mostly limestones, but include river gravels such as the conglomerate shown here. Many types of clasts are present; the orange one in the middle is a conglomerate itself, and the clasts in it include several types of sedimentary rocks. This picture shows a long and complex story—try telling it.

18. Sedimentary rocks, Waterton Lakes-Glacier International Peace Park. Colored lines follow a folded layer. Right-side-up at the green line becomes mostly upside-down at yellow, right-side up at orange, and slightly upside-down at red, as shown by the black arrows.

19. Small region (about 6 inches across) of fossil sand dune in a cliff at Canyon de Chelly National Monument. Wind from the right blew over the dune and cascaded down, forming the slanting layers. Then the top was eroded, and nearly horizontal layers were added above. This is right-side-up. Rust deposition after formation of the dune colored some of the layers, making the layering easier to see.

20. Here is part of the fossil sand dune from the previous slide. The picture in the upper-left corner shows the original, right-side-up sand dune. You might see the others if nature had flipped the rocks various ways. The blue arrows, which point in the “up” direction, will help you follow the flips, but as an experienced GEOSC 10 geologist, nature can’t fool you any more.

21. A cliff below the South Rim of the Grand Canyon, along the Bright Angel Trail. The redder Hermit Shale (below the yellow line) is a flood-plain deposit, and sand from the Coconino Sandstone dunes (above the yellow line) fell into a huge mud crack extending perhaps 20 feet downward (below the red arrow). A sand dune blowing onto the flood plain of the Nile could produce a similar deposit today. This is right-side up; the sand fell down into the mud crack.

22. These mud cracks are from the Flagstaff Limestone of central Utah, very similar in age and setting to the Bryce Limestone. This slab is right-side up; the cracks go down.

23. Mud deposited in a small lake near the sand dunes dried and cracked in the sun. More mud washed in and filled the cracks. After the mud hardened, the rocks were split apart, and the upper piece turned over; Dr. Alley’s index finger points to the crack-filling mud.

24. Lava flow, Sunset Crater National Monument, Arizona. Bubbles tend to rise in lava flows, as they do in soda, but to be trapped beneath the quick-cooled upper layer. Often, bubbles will be bigger and more numerous toward the top of a flow, as in this one. This flow is right-side-up.

25. Along I-70 in central Utah. Rocks below are sediments from a coastal environment, and are standing up on end (look just above the geology student, shown by the black arrow). Above is a fossil soil or paleosol (blue arrow), and above that are whitish lake sediments. Try telling the story shown here.