Question

This scenic image is from Monument Valley in the northeastern corner of Arizona. The bedrock in this region consists of layers of sedimentary rocks. Although the prominent rock exposures ("monuments") in this photo are widely separated, we can infer that they represent a once-continuous layer. Discuss the principle that allows us to make this inference.

Short answer

The principle of lateral continuity allows us to infer that the separated rock formations were once a continuous layer.

Step-by-step solution

Understanding the Principle of Original Horizontality

Sedimentary rocks are deposited in horizontal layers. This is because sediments settle out of bodies of water and build up layer by layer, typically following the Earth's gravitational pull. Thus, the principle of original horizontality suggests that these rock layers begin as flat, parallel deposits.

Applying the Principle of Lateral Continuity

The principle of lateral continuity posits that sedimentary rock layers initially extend laterally in all directions. This means that when observing separated rock formations, we can deduce they were once part of a continuous sheet of sedimentary rock that was later eroded or disrupted by geological events.

Observing Modern Topography for Evidence

In regions like Monument Valley, the existing topographical features—the scattered rock monuments—are evidence of elements of the terrain that resisted erosion, while the areas between them have been worn away. This supports the inference that these features were once part of a larger, continuous layer of sedimentary rock.

Key concepts

Sedimentary Rocks

Sedimentary rocks are fascinating geological formations that tell the intricate story of Earth's history. These rocks form from the accumulation of sediment, such as mineral and organic particles, which settle and solidify over time. They often originate in bodies of water like rivers, lakes, and oceans, where the sediments layer upon each other.

• The process begins with weathering and erosion, breaking down existing rocks into small particles.
• These particles are then transported by wind, water, or ice to new locations.
• Eventually, they settle out of the transporting medium, layering over existing deposits.
• Compaction and cementation then transform these layers into solid rock, creating layers over millions of years.

Such layering is vital because it leads to the next principle essential in geology: the principle of original horizontality.

Original Horizontality

The principle of original horizontality is a foundational concept in geology. It proposes that sedimentary rock layers are originally deposited horizontally due to gravity. This is because sediments naturally settle out in flat, parallel layers. Therefore, any deviation from this horizontality seen in older rocks indicates that geological processes have altered them after their initial deposition.

Some critical points include:

• Sediments settle in water in stable, horizontal layers due to gravity.
• Non-horizontal sediment layers suggest post-depositional geological influences, such as folding, tilting, or faulting.
• This principle helps geologists read Earth's history and understand past geological events.

Recognizing this original horizontality aids in deciphering complex geographical features and past movements of Earth's crust.

Monument Valley Geology

Monument Valley, with its iconic landscape, offers a perfect showcase of the principles discussed. It is located in the northeastern corner of Arizona, displaying stunning rock formations amidst arid desert conditions. These formations are primarily composed of sedimentary rocks.

• The towering "monuments" are remnants of vast sedimentary rock layers that once blanketed the entire region.
• Over millions of years, natural processes such as erosion, wind, and water have sculpted these rocks, leaving behind isolated formations.
• The principle of lateral continuity suggests that these scattered rock features were once part of a continuous layer that covered more of the landscape.

Monument Valley stands as a natural wonder, demonstrating the potent forces of nature that reveal the Earth's dynamic history. It also highlights how methods like the principle of lateral continuity can unravel the secrets of such breathtaking geological features.