Argillite – Golden Bronze Slate from Montana


We are currently getting started on a large paving project on Vashon Island, WA.  It involves over 1000 sq. ft. of Argillite Flagstone mortared onto the large walkways surrounding a new home.  I’m sure I will be posting photos of our progress, but I thought I would do some research on the material and get more familiar with its origin.

Argillite exists in various parts of the world, but he nearest area where it is quarried is in the NW corner of Montana – it is also known as Pritchard Ledgestone.  The material is a deep blue-gray on its interior, but the exterior takes on a variety of colors including tan, yellow, burgundy, bright orange, deep red and brown.  Fossil traces are often found amid a swirl of sunburst colors.


This is one of the more ancient building stone varieties, having formed in the Precambrian Period over 1 billion years ago.  It is sedimentary clay that was deeply buried and then exposed to heat and pressure during the formation of the Rocky Mountains until it re-crystallized into a kind of slate.

The stone is mined from the Pritchard Formation, a large deposit of stone around the Plains-Paradise area of Montana just west of Missoula.  A couple of the quarries I found are Rock Solid Inc. (, K2 Quarries ( and Montana Rockworks (

I have worked with this material many times in its different forms – other varieties within this family of rock are known as Montana Slate (light to dark), Autumn Flame and Montana Bronze.  They all have slightly different characteristics, but overall they are very hard and not inherently workable due to the high amount of fracturing to which it is prone.  It is very attractive in color and shape, but don’t expect to be able to shape it easily unless you plan on using a heavy diamond blade to do so.

But Argillite has a certain way about it that, if you can adapt your style of workmanship, has a unique character of its own.  Used to make patios, walls, or veneers, it brings a very linear quality as well as a rough and rustic one.


by Mark Shepherd

(206) 618-0558

Bringing the finest in Stone Masonry to the Greater Seattle Area.

Natural stone walls, patios, steps and rockery of all sizes.

Mortared stone veneers, fireplace surrounds, and interiors.

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