This seaway finally retreated southeast, and coastal dunes once again crept into the area from the northwest, leaving the Sycamore Pass Member of the Schnebly Hill Formation. All the while, as more sand entered the basin.
Eventually, the shoreline was hundreds of miles away and inland dunes rather than coastal dunes became dominant. These inland dunes are preserved as the Coconino Sandstone and are the white layers of sandstone that cap many of the red rocks in this area.
After an intense summer rain, Oak Creek swells with run-off, carrying much sand, gravel, and boulders downstream. Events such as these are powerful agents of erosion, removing rocky debris that comes down from the cliffs above. Photo by Rob Harrison.
Finally, between million years ago, a different seaway came in, this time from the northwest. At first, salty mudflats were present which deposited the Toroweap Formation. As this sea moved eastward and flooded the area in seawater once again, it left a limestone layer called the Kaibab Formation. This is the rock layer that caps much of northern Arizona, including the Mogollon Rim and the Grand Canyon. However, more recent events have shaped and sculpted these rocks with a finishing flourish.
These more recent events begin only about 25 to 30 million years ago when the Mogollon Rim was formed by erosion. The Supai Group of red sandstone, deposited when the area was a floodplain about million years ago, sits atop the Redwall Formation, to a depth of about feet. On top of that is a layer called the Hermit Formation, about million years old, made of sandstone, mudstone and conglomerate.
On top of the Hermit Formation is a layer that million years ago was coastal sand dunes, and is now red sandstone up to feet thick in places. Two more layers of sandstone were then capped by a layer of limestone laid down when the sea returned about million years ago and is known as the Kaibab Formation. The so-called Laramide orogony--a round of mountain building that created the Rocky Mountains between 80 million and 35 million years ago--lifted the Sedona area and caused cracks that provided channels for water flowing from the new mountains.
Water erosion widened the cracks into broad valleys, leaving only islands of the original layers above the Hermit Formation, in the form of the red buttes, spires and towers that now surround Sedona as silent sentinels from a remote past. Have you ever wondered why the red rocks in Sedona are red in color? While many people appreciate the red rock formations, not many know how they got there.
The story of the red rock is almost as beautiful as the formations themselves. Find a Sedona vacation rental today and book your trip to Red Rock Country! Before the red-colored rocks turned into stone, the Sedona landscape was all mushy mud and sand.
It is actually divided into two sub-layers, imaginitively named the "Upper Coconino" and the "Lower Coconino". The Schnebly Hill Formation is a dark red sanstone, to feet thick, that is the major component of the "Red Rocks" of Sedona. Unlike the Coconino, the Schnebly Hill sandstone is layed down in flat-bedded horizontal layers, interspersed with multiple thin white layers of limestone conglomerate. Both the flat strata and the limestone are indicators that the layer formed under water.
However, microscopic examination of the quartz grains that make up the sanstone show that they are frosted and worn and of consistent size, clear indicators of wind-blown origin.
The conclusion is that the region was subject to periodic incursions of the sea, which reworked wind-blown sand dunes into horizontal layers. Further to the northwest this layer is crossbedded similar to the Coconino, indicating that that area was not submerged. The deep red color of the sandstone is due to hemetite iron-oxide which stains the normally white quartz sanstone.
The Schnebly Hill Formation is comprised of the Sycamore Pass top and Bell Rock bottom members, which are separated by a ten- to twelve-foot thick layer of grey-colored limestone called the Fort Apache Member, formed during a major incursion of the Pedregosa Sea. There are no fossils in the Schnebly Hill sanstones, and virtually none in the thin limestone layers.
Though often referred to as shale, the Hermit formation actually consists of various soft, easily-eroded sedimentary rocks such as siltstone, mudstone and sandstone as well as shale. Formed million years ago, it is generally a dark rust color due to iron-oxide staining, and is about feet thick in the Sedona area. Near the top of the layer are desication mudcracks, some tens of feet deep, that are filled with Coconino Sanstone.
This indicates that the sands of the Coconino covered the Hermit while the latter was still substantially saturated with water. Susequent drying caused the cracks to open, allowing the Coconino sands to filter down from above. The city of Sedona is built near the top of the Hermit Formation.
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