Morton County is drained by the Cimarron River, which enters the state of Kansas from the west at a point about 5.5 miles north of the Kansas-Oklahoma line, and flows east-northeast across the county. The valley of Cimarron River is half a mile wide and 100 to 225 feet deep. Rounded bluffs are more predominant on its norther section, while on the southern edge there are sand dunes. Although a periodic river which seems to be dry most of the time, the Cimarron plays a key role in the sustainability of this area.
Cimarron Valley
The rocks that crop out in Morton County are all of sedimentary origin. The Alluvium sub-division constitutes the foundation of the Cimarron Valley; and it is made of sand and gravel containing some silt and clay. Because of the impermeable consistency of the Alluvium sub-division at the bottom of the river, the constant flow of water has not eroded the valley deeper into a canyon. Instead the river has eroded and carved most of the valley widening the riverbed. This effect in combination with constant winds blowing from the west has exposed the top layers of rock of the Ogallala formation.
Sandstones of the Ogallala Formation
The Ogallala formation is made of sand, gravel, clays and other porous light materials. Some of these materials were cemented together to form porous sandstones. In the High Plains, the most common rock in the Ogallala Formation is a porous sandstone made up of quartz and feldspar grains that are cemented by very fine-grained calcium carbonate. Because these rocks look like concrete, they are known locally as mortar beds.
Another kind of sandstone—a hard, dense, gray-green rock—also occurs in some parts of the Ogallala Formation. This type of sandstone is cemented with opal. Opal consists of silicon dioxide (SiO2), like quartz, plus an indefinite amount of water. It never forms as crystals and cannot be scratched by a knife, though it is slightly softer than quartz. The opals from the Ogallala may be colorless, white, or gray and are found with a white, cherty calcareous rock. Most of the Ogallala is underground, but it crops out in many places. The Ogallala is one of the chief sources of ground water in western Kansas because it holds and transports water, making possible for springs to flourish from the ground.
Jurassic Rocks at Point of the Rocks
The only area in Kansas where jurassic rocks are at the surface is near Point of Rocks, a landmark on the Dry Route of The Santa Fe Trail. Near this landmark, travelers of the SFT found a reliable source of water in nearby natural springs because the hard rock of the soil forced the water to come back to the surface. It also constitutes a prominent landmark for travelers because Point of Rocks is the 3rd highest point in the state of Kansas with the elevation at its summit being 3,540 feet.
During the Jurassic, sandstones and shales were deposited over the western one-third of Kansas. These Jurassic formations were then covered during the Cretaceous and are found only in the subsurface. Jurassic rocks are very important at this particular location because they are older than any other rock in the area and they are harder. In Kansas, rocks of Jurassic age, about 175 million years old, underlie much of the western part of the state being Point of Rocks the only place where we can find them on the surface. The map below shows where the jurassic and the Ogallala formation rocks meet and outcrop to the surface together.
To get credit for this earthcache you must visit the Point of Rocks location in the Cimarron National Grasslands and examine some rocks in the area. Send the answers to the following question by email. It is not required to have a successful log but I recommend taking pictures of this remote but fantastic landscape in SW Kansas.
At the earthcache coordinates examine the type of rocks. Describe its consistency and color.
- What materials do constitute the rock you are examining?
- Why do these materials give that rock its colors?
- Are you standing at the Ogallala Formation? How do you know?
Examine the rocks at the Point of Rocks #1 coordinates. You will need to climb down using a steep trail.
- What are the major differences between the top rocks and the ones you find at the bottom?
- Can you tell what type of rock you are standing on? How can you identify it?
This earthcache has been placed with the permission of the Cimarron Ranger District Office in Elkhart, KS.