Bighorn Sheep and Channel Fill

Bighorn Sheep and a “channel fill” are seen in this arresting photo by Britt Runyon (below), taken in the Taos Box portion of the Rio Grande Gorge. “What’s a channel fill?”, you say?

OK, I’ll explain. But first, I need to discuss the geologic history of the Rio Grande Gorge. In this part of gorge, the Rio Grande is incised into a succession of lava flows, one sitting atop the other, like a layer cake – and I’ve counted as many as 7 different layers in that cake. Before these lava flows began, let’s suppose that the Rio Grande was running through a valley, as rivers do. The first lava flow into that valley dammed up the river, causing a lake to form behind the dam. When the lake rose to the height of the lava dam, its waters began to spill out and meander over the surface of the lava flow. Depending on the amount of time that elapsed between the over-topping of the lava dam and the arrival of the second flow, the Rio Grande would (with less time) have  cut a channel into the flow, and (with more time) cut a gorge. This same process would have repeated with the arrival of each new lava flow. The Rio finally had the opportunity to cut down more fully through that stack of layers once the lava flows stopped coming.

So, what about that channel fill? In the upper part of the photo, four Bighorn sheep are seen standing on a tan layer of rock, that looks nothing like the basalt rocks seen above and to the side (when hardened, this kind of lava is called basalt). That tan rock is, in fact, hardly rock at all. It’s dirt! And how did it get there? It’s sediment that collected in a stream channel that ran across the surface of a lava flow. The part of the channel seen in the photo narrows to the right side, being a partial cross-section of the entire channel. And then what happened? Down came another lava flow, which capped that channel fill, preserving it for all time. Additionally, the molten lava that covered over the channel fill baked that material, giving it a reddish cast.

Channel fills like the one seen here are seen elsewhere, and at different levels, in the walls of the gorge, being situated upon the top of one flow or another. They really stick out, and once you know what you’re looking for, you begin to recognize them.