Rocks for Brains
Methow Series, Specimen 5

Summary

Location Methow Domain
Type Conglomerate
Unit Virginia Ridge Formation
Terrane Methow Terrane
Period Cretaceous (Albian - Cenomanian)…110 Ma
Found Confluence of Early Winters Creek and the Methow River


The Virginia Ridge Formation is a sedimentary sequence of mudstone, siltstone, chert sandstone and chert pebble conglomerate beds. These rocks date to middle cretaceous period when the Methow Terrane was part of a marine sedimentary basin. During the late cretaceous period, faulting and folding tilted the sedimentary beds, examples of which can be seen from Hwy 20 on the valley walls between Mazama and Winthrop. This specimen is composed of moderately rounded to angular gray and black chert clasts and is poorly sorted: .25 to 2.0 inches in diameter. Cement between chert lithics is predominantly quartz with a minor amount of calcite.

Petrography

Chert:Gray to black clasts, moderately rounded to angular, .25 to 2.0 inches in diameter, poorly sorted.
Volcanics:Clasts in minor amounts.
Quartz: Minor mineral in cement as secondary mineral due to weathering. Too small to measure. Larger euhedral grains about 1.5mm in diameter.
Cement- Predominately quartz with minor calcite.

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Origin and Depositional History


Early Cretaceous marine sedimentation from a western / southwestern source began to form the Virginia Ridge Formation. This was predominantly black mud (forming mudstones and siltstones) but periodically there were periods of greater energy to allow for the formation of chert sandstones and chert conglomerates. Chert Clasts of the latter two types lessen in size to the east / northeast of the formation.

Late Cretaceous and Paleocene movement along the Pasayten fault followed by Eocene intrusive igneous activity exposed rocks of the Virginia Ridge formation to erosion from both continental and alpine glaciation.

22,000 – 18,000 years BP Alpine glaciers advance
17,000 – 13,500 years BP Continental Cordilleran Ice Sheet advance/retreat
11,000 - 9500 years BP Alpine glaciers advance (higher valleys)
7000 years to present Fluvial erosion in the Methow River Watershed


The erosive forces of glacial advance and retreat and subsequent fluvial erosion further exposed the rocks of the Methow Terrane resulting in the topography (and river rocks!) we see today.