South Michigan EarthCaches 


Blue Ridge Gravel Pit EarthCache Part 2 – Cross Bedding 

Connection to the Earth Science Curriculum

Essential Lessons:

The Blue Ridge Gravel Pit provides an opportunity for visitors to discover two interesting stories that involve orange-brown boulders or fragments of sandstone found that tell of times when the climatic conditions were vastly different in Michigan than they are today.

Earth Science Literacy Principles

  • Big Idea 2.1. Earth's rocks and other materials provide a record of its history.
  • Big Idea 3. Earth is a complex system of interacting rock, water, air, and life.

Common misconceptions

  • Glaciers are ice and snow deposits on the land were no more than 50 feet thick.
  • Glaciers most recently covered Michigan millions of years ago.
Key Earth Science/Geological Vocabulary Words:

Cross-Bedding: Layers of sediment within a rock unit that do not run parallel to horizon. Cross-bedded sediments often appear to be tilted.

Esker: A long winding ridge of gravel, sand, etc., originally deposited by a meltwater stream running under a glacier.

Glacial Till: Unconsolidated, heterogeneous mixture of clay, sand, pebbles, cobbles, and boulders deposited onto the landscape by retreating glaciers.

Laurentide Ice Sheet: A massive sheet of ice sheet that covered most of Canada and a large portion of the northern United States between 95,000 and 20,000 years ago.

Outwash Plain: Sediment deposited by melting water onto the landscape in front of a glacier.

Pleistocene Epoch: A small unit of time within the Cenozoic Era that encompasses the most recent glacial advances on our planet between 11,700 and 2.58 million years ago.

Wisconsinan Advance: The most recent period of continental glaciation in the northern hemisphere that occurred between 10,000 to 70,000 years ago. The height of the Wisconsinan Advance occurred nearly 20,000 years ago when glaciers made their furthest advance toward the equator.

Developed by: Mark Reed

The Blue Ridge Gravel Pit is owned by the Jackson Country Road Commission and provides an opportunity for approved visitors (Permission must be obtained, see access information below) to discover two interesting stories that can be told that involve orange-brown sandstone boulders or fragments found within it. These boulders tell a story of when climatic conditions were vastly different than what they are today in Michigan.

Materials needed for visit:

Permission from the Jackson County Road Commission, the information provided, GPS, magnifying lens, rock hammer, safety goggles, and a sketchpad or camera.

Description:

The Blue Ridge Gravel Pit owned by the Jackson County Road Commission, is located east of US 127 about 6.5 miles south southeast of Jackson, Michigan. It is an interesting site in many respects due to the fact that the mining of sand and gravel has exposed the interior of a glacial esker. The esker measures over 5 miles in length and its sediments are likely from a variety of locations from within both Michigan and Canada. The Blue Ridge Gravel Pit provides a window into the past that tells of a time over 10,000 years ago when a flowing "river" within the ice was able to move both melt-water and sediments onto the landscape in front of a retreating glacier.

An esker is a "stream-tunnel" within a mass of ice that deposits a tubular structure onto the landscape as a glacier retreats.

Coordinate: N 42° 09.639′ W84° 21.307′

As the melt-water flowed through an ice tunnel known as the Esker, coarse to fine sediment would be deposited as the velocity of the water varied. As glaciers retreated or that seasonal temperatures would change during the year, the relative volumes and velocity of the melt-water would also determine the amount of sediment that could be carried through the ice tunnel prior to being deposited onto the outwash plain.

Figure 1: A cross-bedded dune – note the tilted layers on the leeward side.


In most cases, sediments are typically deposited in a horizontal manner. This concept is known as the Principal of Original Horizontality. This suggests that if a tilted sequence of sedimentary rocks is found, it is likely that some geologic process has most likely altered the orientation of the rock from its original position. However, it should be noted that cross-bedded sediments appear to be tilted due to both erosion and depositional histories.

Figure 2: Cross-bedding is a product of erosion and re-deposition


Cross-bedding occurs when particles are eroded by wind or water and are deposited on a slope. The easiest way to think of cross-bedding is to think about dune migration. The Wind erodes a surface and piles sand particles to create a dune. If the particles have enough energy, they will continue past the crest, tumble down the steep side, and be deposited (Figure 1). If the wind continues to blow, some of the upper sand will be eroded and re-deposited causing the dune to migrate. If the dune is cut along its longitudinal axis, it will show a series of tilted beds that illustrate both the horizontal erosion and tilted deposition due to the migration of the slope.

Figure 3: The fast-flowing water of this stream was able to erode sediments that had been previously deposited. The front surface of the delta is creating a modern cross-bed as new material is being re-deposited as it rolls down the slope.


Geologists can use cross-bedded sediments to determine both the direction and the velocity of an agent of erosion such as air or water. Cross-bedding also occurs when the flow water is fast enough to erode and transport sediment until it drops over a slope where the water velocity is slower prompting the sediment to be re-deposited.

Melt-water from glaciers is rich with sediments and likely has a large enough velocity that it is able push sediments around to form a deltaic structure. As sediments push past the front edge of the delta into a glacial lake, they likely would lose velocity and fall as a deposit (Figure 2). As more and more sediments dropped out, some deposited material would be eroded and subsequently re-deposited elsewhere. An example of a delta structure and cross-bedding in action is shown in Figure 3. Here, particles have been eroded by the stream flow and were re-deposited as the velocity of the particles slowed when they tumbled down slope in deeper water.

Logging Activity /
Question 1: (Caution – Safety Advisory)

Sketch or photograph this structure and describe the types and sizes of the sediments within it. Using your compass, what was the direction of the flow?