Geomorphology near the granite-metamorphic contact near Rincon Ridge, Sandia Mountains northeast of Albuquerque

Scenically and geologically, the northwestern Sandia Mountains are a special place. Scenically, this area is a great hiking destination because of the shield-like cliffs formed on the upper part of the mountain-front and, to the west, the sharp and steep Rincon Ridge (Photo 1, shot from an airplane by Jason Ricketts of UTEP). There are a lot of interesting geologic things here: pediments (ranging from Quaternary pediments at the heads of fans to high-level erosional surfaces midway up the mountain front), beheaded/pirated canyons, and allochthonous blocks of Mesozoic sedimentary rocks. These and other features are discussed in Stop 1 (Day 1) of the guidebook for the 2026 New Mexico Geological Society Field Conference (available for download after ~mid-October-2026 from the New Mexico Geological Society webpage). Another reference for the local geology is Geologic Map 78 (Sean Connell of the NM Bureau of Geology) and Dirk Van Hart’s excellent book: New Mexico’s Magnificent Sandia Mountain: The Complete Geologic Story.

One particularly neat geologic feature in the northwestern Sandia Mountains is the contact between the Sandia Granite and the older Juan Tabo metamorphic rocks, drawn in Photo 1. The metamorphic rocks consist of mainly of schist, gneiss, quartzite, and local migmatite. They are part of the 1.68–1.60 Mazatzal tectonic province that underwent deformation during the 1.47–1.35 Ga Picuris orogeny (Hillenbrand et al., 2025 [https://doi.org/10.1111/ter.12748]). During this orogeny, the metamorphic rocks were intruded by the 1.45 Ga Sandia Granite. The intrusion produced a contact metamorphic aureole that is detailed in Kirby et al. (1995; https://doi.org/10.1029/94TC02699).

Since I am a coleader of the New Mexico Geological Society’s Fall Field Conference this year, I wanted to better acquaint myself better with the geology at Rincon Ridge area near where Stop 1 is located. So one weekend I walked the granite-metamorphic contact north of Stop 1 to the cone-shaped peak depicted on Photo 1 (“Rincon high point”). Near the north end of the hike, my route followed the right side of the yellow line in Photo 2.

One can put their hand on the actual contact in several roadcuts along Forest Road 333 (the road between La Luz Trailhead and Tramway Blvd), as shown in last week’s postcard. Northwards, one can spatially constrain the contact within ~100 m based on float and scattered outcrop. The following presents some geologic observations and photos I took on my hike to Rincon High Point that pertain to geomorphology (note last week’s postcard described features of the bedrock).

In Photo 2, there are differences in the geomorphic expression of the mountain and tree cover on either side of the contact (yellow line). To the right (east), the mountain weathers more rounded, relief is more subdued (west of The Shield) and is more forested. The slopes underlain by granite tend to produce much coarse sand interspersed with local, scattered boulders; locally there are some hard granite outcrops making cliffs near the ridge (Photos 2 and 3). In contrast, the terrain underlain by the Juan Tabo metamorphic rocks produces steeper, rockier slopes that are notably less vegetated (Photo 4).

I would argue that the difference in slope character boils down to how they weather. The foliated fabric of the metamorphic rocks, where grains are tightly interlocked, makes a relatively strong rock. It erodes by shedding large chunks of rock from that peel off from fractures or from foliation planes. An example of the large rock fragments generated from schist is seen in the colluvium along a roadcut of Forest Road 333 (Photo 5).

The Sandia Granite, on the other hand, erodes by producing grus – a coarse sand whose grains were “plucked” from decomposing granite. This is especially true near the contact, where the granite is typically altered by fluids, producing grungy-greenish colors reflective of chlorite. In granitic rocks here, weathering of biotite into clay allows the mm-scale feldspar and quartz grains to fall out of the rock relatively easily. So easily, in fact, that on these weathered outcrops you can simply rub your hand over the granite and produce notable amounts of grussy sand. Photo 6 shows a weathering granite in Juan Tabo Canyon that is producing such grus. Note the pegmatite veins behind the dog, which are sticking out of the outcrop because they are relatively hard — the lack of biotite in these veins related hinders related hydrolysis weathering. Once liberated from the rock, the grus is easily mobilized by sheetwash and produces thick colluvial wedges or alluvial fills. Photo 7 shows such a thick accumulation of grus in the upper part of the small drainage just east of Rincon High Point.

So in addition to the scenery, with great views of the Shield to the east, one can learn a lot about slope processes and erosion by exploring the Juan Tabo drainage east of Rincon Ridge.