The ridge that forms the skyline is called the Garden Wall, seen in this view looking north from Logan Pass in Glacier National Park, Montana. The rocks that have been exposed by glacial erosion are Proterozoic sedimentary rocks, with absolute ages between 0.9 and 1.5 Ga.

The rocks of this area are part of the Belt Supergroup, a sequence of sedimentary rocks of Proterozoic Age found through this region of the Rocky Mountains in Montana and as far west as the vicinity of Spokane and Colville in Washington state. The ancient rocks of these jagged mountains reveal sedimentary processes that operated in shallow waters of either a gigantic lake or a large, shallow bay of the ocean.

The rocks contain evidence of the presence primitive, single-celled bacteria at the time the sediments formed and indirect evidence that there were no multicellular organisms such as worms at that time. None of the fine-grained sediment layers were disturbed by any tracks, burrows or worm tubes. Such layers in Phanerozoic sediments always contain evidence of at least a little biological disturbance.

The present landscape was shaped by the erosive and depositional action of glaciers in the last 2 Ma, during the Pleistocene epoch. The Garden Wall is an ar�te, a jagged ridge carved by glacial erosion.

Stromatolites

The structures in the rock in this picture are called stromatolites. Stromatolites are created by films of photosynthesizing bacteria (a type of algae) that live on the floor of shallow bays in the ocean. These stromatolites provide evidence that such bacteria were present at the time the sedimentary layers were forming. Select the image to see a larger view. Use your browser's back button to return to this page.

Ripple Marks

The surface of this rock is a bedding plane (the original surface of a layer of sediment) that shows ripple marks. Moderate water currents flowing across or sand or silt forms the ripples. Many of the layers of Proterozoic sedimentary rock at Logan Pass have ripple marks, consistent with the deposition of silt and sand in tide flats. Selectthe image to see a larger view. Use your browser's back button to return to this page.

Mud Cracks

This bedding plane shows mud cracks, indicating that the sediments were deposited in water and then dried up from time to time. The mud cracks are also consistent with the deposition of sediments in a tide flat environment. Select the image to see a larger view. Use your browser's back button to return to this page.

Unlike tide flat sediments of the Phanerozoic eon, none of the sand, silt or mud layers in the Proterozoic rocks of Glacier National Park shows signs of the presence of worms or any other burrowing animals.

Glossary terms that appear on this page: glacier; stromatolite; sedimentary rock; mud crack