The massive Hotlum Glacier clings to the upper flanks of the northeast side of Mount Shasta.
Mount Shasta is home to the 4 largest glaciers in California. On the northern slopes one can observe the Whitney and Bolam Glaciers while on the east side are the Hotlum and Wintun Glaciers. Of these two eastern glaciers, the Hotlum is the most notable because it is California’s largest glacier. The vast field of ice extends 1.25 miles down the side of the mountain, descending about 3,000 feet. Measuring the combined width of its multiple lobes, the Hotlum Glacier is nearly 1 mile wide at its base. It really is a singularly impressive sheet of ice. Yet, despite being notable for all of these reasons, there is another reason, more difficult to discern and more mysterious than the others. This is the fact that the Hotlum Glacier is the only glacier in California to have an extant medial moraine.
This may sound somewhat anticlimactic, so please allow a quick digression to explain what a moraine is. As glaciers form they begin to move downhill. The massive blocks of ice carve, scour and cut away at whatever feature lies along their path. This slow, gradual grinding away at the landscape is what has formed such famed landmarks at Yosemite Valley and the Kiger Gorge on Steens Mountain. However, all the debris that is worn away by the passage of the glacier is moved along with the glacier, pushed aside by the downhill motion. Debris is also pushed by the head of the glacier as well. These are called lateral and terminal moraines, respectively.
Medial moraines begin as lateral moraines, but when two glaciers meet and form a single glacier, or if two lobes of one glacier recombine, the lateral debris becomes locked in the center of ice. They appear as long, dark lines amidst the bright white and blue of the glacial ice. These features are easily recognizable in glaciers from such places as Alaska, the Alps, and the Himalayas. Here are two examples from Glacier Bay National Park in Alaska:
Medial moraines do not typically forms on small or moderately sized glaciers. They are the domain of the great glaciers, the long rivers of ice that flow from mountains at extreme latitudes or altitudes. In terms of glacial activity, California is far to the south of where most of the conditions exist for extensive glacial activity. The Sierra Nevada has a few glaciers, but most are just vestigial remnants of the vast ice fields that once covered that mountain range. Furthermore, any evidence of medial moraines, assuming there were any, has long since been washed away. Only on Mount Shasta, and only on the Hotlum Glacier is there evidence of such a feature in California.
The red marks the current extent of the Hotlum Glacier. The yellow lines mark medial moraines.
Today, just as it must have done in the past, the Hotlum Glacier splits in two, descending down the flanks of Mount Shasta in two large segments. These are separated by a large rock outcropping that divides the flowing ice. Something similar, though no doubt larger, must also have taken place in the past, for just below this large outcropping is the beginning of the medial moraine. It would appear that this protruding rock divided the glacier into two large flows. Below the rock protrusion the flows converged again, this time carrying the debris carved away from Mount Shasta by the glacier. This debris was then carried downhill as the ice slowly crept its way down the mountain.
As all glaciers at this latitude have done over the last several millenia, the Hotlum Glacier has receded. This has left the medial moraine isolated from the ice that created it. Nonetheless, the geology is still evident and it is easy to envision how much larger the glacier was in the not-too-distant past. If we accept the terminal moraine, which lies at the end of the medial moraine, to represent the full extent of the glacier’s size, then the massive ice sheet would have extended almost 6 miles down Mount Shasta, descending 7,500 feet over its journey. This would have been a truly staggering chunk of ice. Its measurements would have significantly exceeded Mount Rainier’s Emmons Glacier, the largest glacier in the coterminous United States.
Click to enlarge these two illustrations showing the Hotlum Glacier’s medial moraines:
It must have been a magnificent sight to behold. Observing the massive, slow-moving river of ice rising just below Mount Shasta’s summit and creeping all the way down to its lower flanks. Indicative of its size, there was not just one medial moraine on the Hotlum Glacier. The primary one was the largest but it was flanked by two smaller, parallel moraines. They follow the same path of flow and feature the same, distinctive sweeping curve in the first quarter of their length. Indeed, it is this curve that makes the nature of the moraines apparent even to the untrained eye. It is easy to visualize only the top of these moraines visible of the large field of ice that surrounded it.
The upper section of Mount Shasta’s medial moraine.
With the lack of ice surrounding the moraines now, it is not as easy to recognize them at every angle. While their upper reaches extend above the treeline, the preponderance of their bulk is now below this point and is forest cover has taken root midst the rocky terrain. The moraine are also long and low, which does not cause them to stand out from most angles. Nonetheless, once an observer understands what to look for, the medial moraines become much more apparent.
The medial moraines aren’t the only ones that remain to testify to the former scale of the Hotlum Glacier. Both of the main halves of the glacier have below them large fields of smaller, successive terminal moraines. As the glacier receded it would periodically advance again, covering a short distance and pushing a small pile of debris forward before it would melt and shrink again. This is what has caused the distinctive wave patterns around the treeline areas near the medial moraines. The smaller of the two fields lies below the larger of the Hotlum Glacier’s two current halves. However, there are several fields of terminal moraines in the area between the medial moraine and the Hotlum-Bolam Ridge, which seperates the Hotlum Glacier from the Bolam Glacier. The fields of terminal moraines are a dramatic retelling of the Hotlum Glacier’s story, of massive growth and then a protracted age of advance and recession, finally leading to today’s diminished-but-still-massive glacier.
There are not many vantage points that yield good views of California’s lone medial moraine and it has largely gone ignored, overlooked or unappreciated. Despite this, it is one of the most fascinating geologic features not just on Mount Shasta, but possibly in all of California. Only places like Yosemite Valley offer a correspondingly easy-to-understand visual of the power, nature and scale of the glaciers that once covered the majority of the mountains of the Golden State.
A great way to remedy this would be the construction of a “Moraine Vista Trail”. There is the ridge south of the Whaleback and immediately east of the moraine offers spectacular views not just of the medial moraine but also of the entire east side of Mount Shasta, which is the least viewed and least appreciated side of the mountain. There is good, paved road access and the land is in the midst of a logged area, which means it is unlikely to be used for little else. It would be great to build a pathway that could be used to educate people on California’s glacial past, present and possible future. It would also give hikers a chance to enjoy a new area and views of the mountain that are currently available the few who venture onto that side of the mountain. Such a trail would be a marvelous addition to the Mount Shasta area’s trail network!
A proposed route for the Moraine Vista Trail: