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Amateur Naturalist: Surface Geology Of Cerros Del Abrigo

on November 13, 2016 - 8:48am
Amateur Naturalist: The Surface Geology of the Cerros Del Abrigo
By Robert Dryja
The Valles Caldera reflects both the impact of human activity and the resilience of nature to recover. The most direct human impact is seen in the logging roads cut in parallel for hundreds of miles upon the mountains. The indirect impact is seen in the intensity and magnitude of the Las Conchas forest fire. The Las Conchas forest fire may not have been so great if smaller, repeated fires over the earlier years had been permitted to occur.
Smaller fires would have consumed the fuels that otherwise accumulated for a major fire. This essay will consider the impact of logging and the Las Conchas forest fire on the surface geology of the Valles Caldera. The next essay will consider how the forests have responded.
The Cerros Del Abrigo is one of the large lava domes that can be seen to the north side of the Valle Grande. The Cerros Del Abrigo is an example of the interaction of logging and fire upon a mountain. A hiking trail uses a logging road to circle the Cerros Del Abrigo. The varying impact of the logging and forest fire on the surface geology is visible along the trail.
Logging roads were cut in parallel and spiral down the Cerros Del Abrigo. Trees growing on the slopes between the logging roads then were clear cut. A steel cable was connected between machines on upper and lower parallel roads. The trees were broken down as the cable was pulled across the ground between the machines. (A garden weed eater with its spinning plastic wire provides a small scale analogy.)
The clear cutting was spread over about ten years starting in the 1960’s. It ended approximately 45 years ago in the early 1970’s. Stands of conifer and aspen trees began to grow again on the stripped slopes. However the Las Conchas forest fire in 2011 burned much of this new growth. Trees that then were up to forty years old were killed. Barren ground was created again.
Rain water and snow melt flow down the clear cut and burnt sides of the mountain rather than being slowed and absorbed by forest growth. Some sides of the Cerrros Del Abrigo are very steep. The result has been the continual washing away of top soil, leaving a rocky surface. The more level slopes may have grasses growing on them to cover the rocks. Steep or shallow, however, all these slopes are covered with the burnt trunks of trees that had started to grow after the logging ended.
The logging roads follow a grade as they spiral down the mountain. They provide a path which concentrates rain and melting snow. Gullies approaching four feet deep are eroded in sections of the logging road that now serves as the hiking trail.
The cutting of gullies is first seen at the start of the hiking trail at the base of the Cerrros Del Abrigo. The trail follows a logging road that goes straight down the slope. One of the wheel tracks now is a gully.
The logging roads had their surfaces compressed as a result of the heavy machinery passing repeatedly over them. Any top soil suitable for plant growth was destroyed.
Erosion since the 1970’s continues to remove any developing new top soil in areas devoid of living trees. The exception occurs where a section of forest is not burnt. The adjacent forest absorbs and slows the movement of water. Detritus of leaves, needles and bark falling onto the road year after year provides the start of soil. The road bed in these areas now has grasses and herbaceous plants growing on it. Gullies have not developed.
Several rock slides occur along the trail on the south side of the mountain. These slides look like straight streams composed of smaller to larger rocks and boulders. They extend to the base of Cerros Del Abrigo and then spread out. The smallest stream is about 1,000 feet long and 50 feet wide where it crosses the trail. The largest stream is about 2,800 feet long and 200 feet wide where it crosses the road, cutting deep gullies in it. It spreads out to 400 feet at the base of the mountain. The rock streams occur where a slope is particularly steep and also burnt. Water channels develop with these conditions and wash rock downward. There is relatively little plant growth among the burnt tree trunks on the rocky slopes.
Felsenmeer is a Germanic word that translates as “sea of rock”. A felsenmeer is created by the freezing and thawing of water. Water enters cracks in the top layer of a field of rock. It then breaks the rock apart when it freezes. A field of jagged boulders eventually develops. A felsenmeer also is called a rock glacier. Whereas a glacier is mass of ice, a felsenmeer is a mass of broken rock created by the freezing/thawing of ice within it. The boulders in felsenmeer do not slide downwards, unlike the rock streams that develop during water erosion.
There are two felsenmeers on Cerros del Abrigo. The hiking trail passes by the base of the larger one. An almost vertical wall of boulders suddenly rises up about thirty feet. It extends up the slope for about 1,000 feet and is up to 400 feet wide. The second felsenmeer is smaller. It is about 600 feet long and up to 100 feet wide. The hiking trail crosses the upper ends of both felsenmeers. Unlike the rock streams, the felsenmeers appear to be very old. Their boulders are a dark grey in color and often covered with lichen. They may have formed during the last ice age, 10,000 to 20,000 years ago.
A field of rock cannot burn. The felsenmeers therefore may have contributed to blocking or reducing the intensity of the fire in the forest adjacent to them. Both of the felsenmeers have unburnt forest growing to one side of them. This contrast makes the felsenmeers even more dramatic when compared to the other burnt areas.
The surface geology of the Cerros Del Abrigo therefore has been affected in several ways by the loss of forest cover. The steeper slopes often remain rocky and eroded half a century after the logging ceased. Several streams of rock flow down these slopes. All sides of the Cerros Del Abrigo are scared with lines from the cuts made by the logging roads.
In contrast the felsenmeers may have protected the forest adjacent to them by reducing or blocking the passage of forest fires. A successfully growing forest may be the result of growing close to the rockiest of areas.
A logging road cut into the steeply angled side of a mountain in the Valles Caldera. Courtesy/Historical Society of New Mexico, Jemez Mountains Railroads
Clear cutting combined with a forest fire has resulted in rocky barren slopes half a century after the logging ended. Courtesy photo
A gully following along the side of a stony logging road that now is a hiking trail. Courtesy photo
A stream of rocks and boulders flows across the hiking trail. Courtesy photo
The base of the larger felsenmeer appears suddenly in an unburnt section of forest. Courtesy photo
The hiking trail crosses the top of the smaller felsenmeer. Note some large conifers are growing near the felsenmeer as if the forest fire was blocked or not as intense next to a field of rock. Courtesy photo