The Pebble Plains

Long ago, during the last glacial maximum of the late Pleistocene to be exact, the San Bernardino Mountains were home to California’s southernmost glacial field. By today's standards it's hard to believe the San Bernardino Mountains were once home to a glacier. To the north and west of the mountains lies the arid Mojave Desert. And to their south, the mountains are flanked by the very dry San Bernardino Valley. 

The San Bernardino Mountains rise from some of the driest portions of California. While many of the peaks within the range rise to near 10,000 feet in elevation, they too are relatively dry. But during the last glacial maximum of the late Pleistocene (~ 18-20 ka), when a much wetter and colder climate existed, abundant winter snow accumulated on the north face of San Gorgonio Mountain, the highest peak within the San Bernardino Mountains. And it accumulated in great quantities, ultimately leading to the development of a glacier on the north flank of San Gorgonio Mountain.  

Aerial view looking southwest over San Gorgonio Peak. The cities of San Bernardino and Riverside are in the distance.                                       Image: Google Earth

​According to a paper published in Geology by Lewis A. Owen of UC Riverside, the development of a glacier on San Gorgonio Mountain came about by way of several coincidental conditions. One such condition was a substantial increase in precipitation, which was a widespread trend throughout California during the Pleistocene. A second condition was the southward migration of the mid latitude jet stream, which not only supplied more atmospheric moisture but also pulled colder arctic air further south. The southerly jet stream caused a significant decrease in summer temperatures that made the accumulation of year-round snow possible. And a third condition was the lateral transfer of snow from the mountain's south-west faces to its north-east faces, which was a net result of San Gorgonio's north facing orientation amidst a prevailing south westerly wind pattern. San Gorgonio's north facing orientation assured snow field accumulations remained net positive during the cold-wet periods of the Pleistocene, allowing snow accumulations to eventually develop into glacial ice fields. 

The presence of glaciers on San Gorgonio Mountain points to how cold and wet the San Bernardino Mountains were during the Pleistocene. This is also evident in the topography and terrain you see today. Moraines and glacial cirques on San Gorgonio's north-east face provides evidence of previous glacial activity and outlines of now dried lakebeds within the adjacent valleys indicate how much water previously flowed through the landscape. 

Map of the Big Bear Lake region. Big Bear Lake itself was constructed as a reservoir. To its east lies Baldwin Lake, the now dried remains of a natural Pleistocene era lake. Image adapted from Google Earth.

One of the more prominent remains of the colder and wetter past is the footprint of a Pleistocene era lakebed named Baldwin Lake. Baldwin Lake lies in the eastern portion of the Big Bear Valley, a prominent and now populated valley within the San Bernardino Mountains. The Baldwin Lake was not likely a direct result of the glacier that once existed on San Gorgonio - the Big Bear Valley lies one valley north of the Sana Ana Canyon, which drains the north face of San Gorgornio - but the climate that graced them both during the late Pleistocene was very likely the attributing factor.

Today, Baldwin Lake is an ephemeral lake that looses more water than it receives. In winter, snow melt and rain help fill the lake somewhat, only to have it then evaporate under hot and dry summertime conditions. (Baldwin Lake also suffers from a dam placed at the western end of the valley, with the dam being responsible for the formation of Big Bear Lake reservoir). Baldwin Lake grew to its greatest extent at a time when a glacier existed on San Gorgonio. Since that time, a hotter and drier climate has caused the lake to recede, leaving behind a unique habitat niche.

As Baldwin Lake receded during the Halocene (10 ka - present), the fine grained silt and clay soil particles that were once submerged within the lake became exposed. These fine grained silts and clays create a soil type that contrasts from the more coarse soils elsewhere in the valley. In some areas the fine grained soil particles mixed with larger rocks and pebbles that had either washed into the former lake or been captured by a previously rising lake level. In such instances, that mixing of coarse and fine grained particles has resulted in an edaphic niche termed the pebble plains.  

Aerial view looking east over the Big Bear Valley and Santa Ana Canyon areas. The Santa Ana River flows through the Santa Ana Canyon. The north face of San Gorgornio drains into the Santa Ana River. Image adapted from Google Earth.

​Like many of the West’s ecological niches, what has come to serve as a limitation has also afforded opportunity. Here, the abundant clay content of the pebble plains causes the soil to have a high shrink-swell capacity that heaves during winter frosts. The heaving during winter frosts tends to push the larger quartzite pebbles up towards the top layers of the soil profile, where they accumulate as stone fragments. Those stone fragments give the pebble plains its stony or "pebblelike" structure. As temperatures warm the thin, stony soil heats up fast, becoming very hot and dry for the remainder of the year. This soil profile, combined with persistent wind and high solar insolation, makes for a very tough growing environment that in turn creates a unique edaphic feature. On one hand the pebble plains are unsuitable for the typical contemporary vegetation of the area, but, on the other hand, they are suitable for plant types not typically found within the area.

During the Pleistocene we know the climate throughout the whole of southern California was colder and wetter than it is today. Thus, many plant species that were adapted to cold sub-alpine conditions were able to find comparable conditions at lower elevations. They were also able to migrate southward to lower latitudes where colder temperatures offset the stronger sun aspect. Overall, this meant many species from the sub-alpine habitats of the Sierra Nevada mountains were able to migrate south to the San Bernardino Mountains. And so, during the Pleistocene, floral components of alpine and sub-alpine habitats were able to have a much wider distribution. Many of these plant species found their way to the San Bernardino Mountains.

​As the climate warmed in the years following the Pleistocene, some of those sub-alpine plant species were forced to move upslope to the higher peaks and ridges where colder and wetter conditions remained. Other plant species could not survive the climatic shift and over time became extirpated from the San Bernardino Mountains. This resulted in a range contraction of sub-alpine plant species within the San Bernardino Mountains. However, the tough and harsh growing conditions of the pebble plains were able to retain some those sub-alpine. Owing to their evolutionary abilities to survive tough and harsh conditions, such as those within a sub-alpine environment, a collection of sub-alpine plant species persist on the pebble plains to this day. The ability to grow amidst such harsh conditions, like high solar insolation, desiccating winds, and freezing wintertime temperatures, gives them the competitive advantage they need to outcompete the more contemporary vegetation of the area.

Upslope from the pebble plains are stands of Pinyon Pines and Junipers, which are telltale plants of cold and dry environs. Pinyons and Junipers thrive where most plants cannot. But, down on the pebble plains, the stands of Pinyon Pines and Junipers concede to the harsh conditions. There, they give way to low growing perennial cushion plants or "belly plants". These cushion plants are often found in harsh sub-alpine habitats where very cold, windy, and dry conditions prevail. 

These low growing cushion plants have evolved with adaptive traits that help them sustain more extreme microclimatic conditions. The cushion plants grow low to the ground where they can escape desiccation from wind; they stem from a woody caudex below ground that harbors reserves of carbohydrates for energy and rapid growth following periods of winter dormancy or low photosynthesis; and they have glaucous (i.e. gray or bluish) leaf surfaces that reflect ultraviolet light and keeps leaf tissues cool under hot conditions. And since the edaphic conditions of the pebble plains are extreme enough to restrict the recruitment of other trees and shrubs, this opens the door for cushion plants to reproduce at a more effective rate. Over time and absent of any disturbances or impacts to their habitat, the cushion plants retain their majority presence. It’s why we can enjoy these outliers from a more distant mountain range.

Much of the cushion plants' ability to survive in a habitat far removed from their more typical sub-alpine environments is contingent upon the pebble plains having a microclimate that somewhat resembles the colder and wetter climate of their past. But, as the climate has warmed in the years following the last glacial period of the Pleistocene, the arid expanse of the Mojave Desert has moved upslope into the San Bernardino Mountains. This is problematic even for the very tough and durable cushion plants. It remains to be seen how this will play out for the pebble plains and their cushion plants. Time will tell their fate. Water years such as 2017-2018, where precipitation measured roughly 50% of normal, and the likely phases of extreme wet-dry cycles to come, may soon push these Pleistocene era relics to meet yet one more harsh condition. It may be a challenge like none they have encountered before. But, perhaps these are even tougher than we know? Perhaps.  

Additional Reading

Lewis A. Owen, Robert C. Finkel, Richard A. Minnich, Anne E. Perez; Extreme southwestern margin of late Quaternary glaciation in North America: Timing and controls. Geology 2003;; 31 (8): 729–732. doi:

Additional Resources by Michael L. Charters

Using Format