The Window, Weminuche Wilderness Area, San Juan Mountains, Colorado [AAW00]

The Window is a rectangular notch in a ridge line visible from various locations along the Continental Divide National Scenic Trail (CDT) and various approach trails in the southeastern portion of the Weminuche Wilderness Area. I spent a week backpacking in the area and it often popped into view as we rounded a corner or crested a ridge.

The Window as seen from the southwest at the top of Rincon la Osa.

The first impression when a geologist is confronted with a vertical planar outcrop like the Window is to assume some sort of intrusive origin - molten rock injected into vertical fractures forming a dike for instance. Or perhaps a structural origin - bedrock folded up into a vertical position through some sort of compressive mountain building event. 

View from the southeast.

First impressions can be wrong. A short steep climb provides insights into the geology of the situation. 

The image below is a closeup of the northern wall of the window. There are two contacts between different rock types that shed some light on the geology of this feature - or at least allow me to speculate wildly with a valid starting point. The conclusions, however wrong and incomplete, are my own.

Lithologic contacts along the northern ridge of the window.

The gray rubble at the bottom of the rock wall is one rock type and the vertical wall with large slabs spalling from its side is another rock type. The contact between them is a bit irregular but very obvious just based on color and the change in slope in the above image. The lower gray rock is a poorly welded ash flow tuff - meaning that it is an ash flow that erupted from a volcano and covered the ground surface in an undetermined thickness of hot volcanic ash and total devastation. Anything alive, except the most hardy extremophiles in the area would have been immediately snuffed out.

The volcanic ash deposited in such a way tends to be very hot, as you can imagine. The heat of the ash deposit can partially melt it creating a "welding" effect that forms solid rock known as a welded ash-flow tuff. The efficiency of this welding effect can vary. The gray rock in the image above is poorly welded. It's crumbly. It's an unstable foundation for the overlying rock. As the tuff weathers away, the overlying rock breaks off too - as seen in the large slabs that appear to be ready to flake off any moment now.

A closer view of the overlying rock reveals that it is made up of chunks of various sizes and kinds of volcanic rock in a matrix of fine grained ash and mud.

Close up rock unit overlying the poorly welded ash flow tuff..

Give it a name: deposits composed of volcanic fragments generated through mudflows associated with volcanic activity are called lahars. Based on texture and composition, this rock fits the bill. So we have a volcanic mudflow deposit resting on top of a volcanic ash deposit.

But, there's one more contact between rocks that is also interesting. This contact is in the middle of the lahar(s).

Close up view of contact between two lahar deposits.

There is an obvious horizontal line in the above image that separates a pinkish gray rock on the bottom from the bluish gray rock on the top. That line is a contact between two lahar deposits. So, this mudflow catastrophe happened twice. Not too surprising. Volcanoes are known for repeated earthquakes and eruptions that could have generated mudflows from the top and sides of the volcano. There is yet another interesting aspect of the contact between the two deposits. See the two large tan colored boulders that are intersected by the contact line between the two lahar deposits? Here's a closer close up view.

Closer close up view of the contact between the two lahar deposits.

These boulders cross the contact between the lower pinkish gray and upper bluish gray lahar deposits. They attest to a least one more interesting event.

As you can see, the contact between the two lahar deposits is a nice clean line in the outcrop. There is no significant mixing of the two mud flows. The surface of the lower pinkish gray lahar had to be somewhat firm prior to the deposition of the upper bluish gray lahar - at least firm enough to support the weight of the upper mudflow as it was pouring out over the surface of the lower mudflow. 

While other interpretations are possible, the following is a likely sequence of events and my current working hypothesis based on the observations made during my brief visit.

1. Explosive volcanic eruption of the poorly welded ash flow tuff. Total devastation.
2. Earthquake and/or volcanic eruption causing a mudflow covering the ash flow tuff with the lower pinkish gray lahar deposit. This probably occurred in a valley or other low lying area. Semi-total devastation.
3. Deposition of the boulders onto the surface of the lower pinkish lahar deposit through an explosive event. Imagine boulders flying through the air landing with a resounding splat in the soft-ish mud of the lower pinkish lahar. Or, less prosaically, just a simple rock fall from a steep cliff face. Only highly-localized devastation.
4. Yet another earthquake and/or volcanic eruption causing a mudflow covering the lower pinkish gray lahar deposit with the upper bluish gray lahar deposit. Semi-total devastation.

It was a cold and windy visit. I did not stay long for that reason and also those large overhanging blocks of lahar that seemed ready to spall off at any moment added to the inhospitality of the Window that afternoon. All these thoughts about the deposition of these rock units came flooding into my head for the few moments I stood there, shivering and taking in the expansive view and reveling in the freedom of hiking in wild country for days and days and days. The kind of moments I live for.