Crossbedding--reading the rock

What is crossbedding?

The sandstone was deposited on a nearly flat plane, but the beds below were laid down on a sloping surface, rather like brush strokes going one way and then another way on a canvas.

Cross-beds are the groups of inclined layers, and the sloping layers are known as cross strata.

Cross bedding forms on a sloping surface such as ripple marks and dunes, and allows us to interpret that the depositional environment was water or wind. Examples of these are ripples, dunes, sand waves, hummocks, bars, and deltas. Cross-bedding is widespread in three common sedimentary environments: rivers, tide-dominated coastal and marine settings.



NAME  Hungarian Falls


KMZ  download HungFallsGTour.kmz

SUBJECTS Sandstone cross-bedding

Author:  Clive Wardell

What was the original environment?

In the Jacobsville Sandstone at Hungarian Falls we interpret a river. Rivers travel across the surface in quite variable styles. The river carries its heaviest and coarsest sediments in floods. Finer sediment will be carried and then laid down as the energy of the water drops below its capacity to carry the weight of heavier grains. These grains are deposited on the river bed and more are added on top over time.

During the deposition of the Jacobsville Sandstone the rivers were filling a rift valley from highlands and the deposition came in channels flowing towards the center of the rift valley (low). With rain the energy of the water running steeply downhill could be large and could carry large amounts of material! Look at the cartoon on the next page to get the whole history and placement and also try the link below to a little animation.

Filling a rift: Animation of alluvial fans

What is a sedimentary Rock?

Animated bedforms and Crossbeds: USGS

Here is another good example of cross-bedding from an area that is regarded as very dry, but which must have had a great deal of running water in the past!

How do cross-bedded layers relate to currents?
The pattern of beds also allow you to figure out the direction that the water was moving and therefore which part of a shoreline was sea and which was land or the direction the river was moving. A good example of this occurs below!

The picture below shows that sediments are forming with cross –bedding and ripple marks today as we read these words!  By knowing what environments result in modern beds we can interpret old beds.

By looking at the positions and directions of deposition in the Jacobsville, we can interpret the directions of ancient rivers. So in what directions did the waters (rivers and streams) move the sediments about 1 billion years ago?

This work was supported by  National Science Foundation award #0831948.

Geoscience Big Idea #3            Big Ideas
Earth is a complex system of interacting rock, water, air, and life.

1. The Fault     2. Dam    3. Crossbedding   4. Potholes    5. Plunge Pool   6. Bigger River?    History