High on the Sierra crest in Yosemite National Park you will find the two largest glaciers in the 209.

One is the Lyell Glacier beneath the 13,114-foot Mount Lyell that is also the highest peak in Yosemite National Park.

The other is the nearby Maclure Glacier.

They serve as the headwaters of Tuolumne River on the Lyell Fork.

It is the same Tuolumne River that carries water into Hetch Hetchy Reservoir and through Modesto before joining the San Joaquin River at California’s newest state park — Dos Rios — some 18 miles south of Manteca.

The two glaciers can be reached hiking south out of Tuolumne Meadows.

You hike the Pacific Crest Trail about 11.5 miles towards Donahue Pass.

The initial 8.5 miles are flat before the climb towards Donahue Pass starts.

At 11.5 miles, you will head south toward Mt Lyell. The trek up to the glacier requires Class II-III climbing skills on the easiest route.

The glaciers have been receding during most of the last 150 years they have been studied since the days of John Muir’s initial explorations.

As such they are a reminder of climate change as well as the fact climate change has, for the most part, always been driven by the forces of nature.

Yosemite National Park, and especially the valley, is teeming with features that are the handiwork of glaciers.

There have been at least four major glaciation periods in the Sierra that helped create Yosemite.

The periods are dubbed the Sherwin, Tahoe, Tenaya, and Tioga.

The Sherwin was the first.

It is believed it did the major evacuation responsible for the bulk of the carving of Yosemite Valley and various canyons in the park.

The Sherwin glaciation period ended a million years ago and lasted 300,000 years.

Radiocarbon dating puts the start of the last glaciation period — the Tioga — at 28,000 years ago. It ended roughly 15,000 years ago.

Scientists believe most of the ice in Yosemite melted away due to natural warning roughly 10 years ago.

During the so-called “Little Ice Age” was when the last major advance of glaciers worldwide occurred with the time period being between the years 1250 AD and 1900 AD was when the present-day glaciers in Yosemite were formed.

All of this might sound a tad wonky but is only skimming the surface of Sierra glaciers.

It helps underscore that Yosemite Valley wasn’t carved into its current configuration by a one and done glaciation age.

The abundance of glacial features within the 1,189 square miles of Yosemite National Park is amazing.

During the Ice Age, most of the Sierra — including Yosemite — was covered with glaciers. To give an idea of their size, the Tuolumne Glacier that helped carve the route of the Tuolumne River at one point was 67 miles long.

The Lyell and Maclure glaciers were formed during the much more recent Little Ice Age that took place between the 16th and 19th centuries.

The Lyell glacier has been steadily retreating since the 1800s.

It has retreated around 70 percent since 1883.

A 2013 study contends it is no longer a glacier and should be reclassified as essentially a large ice patch as it has stopped moving. Yosemite National Park information sheets, however, still note that it is a glacier.

The most recent United States Geological Survey of California glaciers was in 2006.

At that time, there were 497 glaciers.

The USGS also listed 788 smaller ice bodies that don’t meet the criteria to be classified as glaciers.

The glaciers as defined by the USGS 19 years ago covered some 25 square miles of California and have since shrunk in size. The smaller ice bodies surveyed in 2006 covered roughly five square miles.

Most experts today put the number of glaciers in the Sierra at less than 100.

As for the Lyell Glacier, some experts believe it could disappear within a decade or so with the Macclure Glacier eventually following suit.