Rimrocks of Billings
The city of Billings is bordered on the North and East by majestic layers of sandstone that form the Rimrocks. The rims rise as much as 300 feet above the city.
This is a unique area. You are standing on the Eagle Sandstone formation of the upper Cretaceous period and are looking across a valley that was formed by the erosive power of the waters that are the Yellowstone River. The river has worn its way down to the Telegraph Creek formation that was formed during the middle Cretaceous period under a fairly deep sea. Dinosaur remains have been found in the Telegraph Creek formation[1]. The Eagle Sandstone formation was deposited during a period of receding water depth. The elevation drop from where you are standing to the river bed is about 330 feet.
Now turn and look at the rock face behind you. For those of us who have no formal training in the field of geology are amazed to find alluvial, or river bed, gravel on the top of the rims. These alluvial gravel beds are also found across the river in the Four Dances Area. This layer is known as Terrace Level 4[2] alluvial gravel of the Pleistocene epoch (2,588,000 to 11,700 years ago) of the first part of the Quaternary Period. This layer was formed in relatively shallow water. This layer of gravel rests directly on top of the Eagle Sandstone and the lower 1 to 5 feet is held in a calcite conglomerate. The cobble and pebble size clasts consist mainly of granite, granite gneiss, schist and quartzite. This conglomerate formation is found only in a few places in the Billings area, and in other areas may be as much as 20 feet in depth.
Question 1: What does this conglomerate resemble?
Now for more information and the answers to the following questions go the N45o 47.893, W108o 29.252, about 0.15 mile on the trail.
Question 2: How long ago was the Eagle Sandstone deposited?
Question 3: In this area which direction does the layers of sandstone tilt?
Please email your answers to us - do not post them! - and log your find. Thanks for learning more about our earth.
[1]Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press.
[2]Geologic Map of the Billings 30' x 60' Quadrangle, Montana by David A. Lopez 2000
See also: US Geological Survey
Congratulations to lakeman&tagalog for FTF!