Balance Rock is in the northern section of the Wachusett Reservation. The route below starts at the Wachusett ski area. If the ski area is crowded, you might want to use the trailhead on Bolton Road (it’s marked on the map below). From either trailhead, the trail to Balance Rock is only about one-half mile.
Balance Rock is a glacial erratic perched atop another glacial erratic. These two rocks have been balancing since the glacier receded, perhaps 15,000 years. The chances of two rocks balancing is slim, but over the course of ages, slim chances sometimes come to pass; such balancing rocks can be found in many countries.
Below is a picture of Balance Rock. Note the angular shape of the two glacial erratics that comprise Balance Rock.
[Update 2023: I now believe that these rocks were intentionally balanced by Native Americans. This is more likely, in my view, than an accidental balancing. The top rock has the upward-pointing shape that the natives were so fond of. Native Americans often balanced rocks on other rocks; I’ve seen many examples of this (pictures here). I still believe that the two boulders are glacial erratics; they were carried by a glacier, but not balanced by a glacier.]
Erratics tend to be angular rather than rounded because they were riding on the glacier, not crushed beneath it. When a rock is crushed beneath a glacier, it tends to be flattened and smoothed on the front side (the side that the glacier contacts first), and “plucked” on the back side (plucking means that the ice tears off chunks of rock, chunks that are then carried by the glacier, becoming erratic boulders). Here’s a diagram of a glacier moving over a large rock:
The term “roche moutonée” means smoothed rock (literally, rock that resembles hair that has been smoothed down with sheep fat). The smoothing occurs on the front side, or upstream side, and on the top, while the back side is left rough and broken.
If you want to see more erratics, there are several near Balance Rock, and an unusually large erratic at Wachusett Meadow, on a trail called Glacial Boulder Trail. If you want to see thousands of erratics in one area, visit the Beaver River Preserve in Rhode Island, a small preserve packed with boulders.
When I first heard about Balance Rock, I wondered why teenage pranksters hadn’t pulled down the top rock. I later learned that this is indeed a problem for some balancing rocks, and has led to some intentional demolitions, but Balance Rock has survived so far, perhaps because both the top and bottom rocks are very heavy and firmly seated.
Both the top and bottom rocks that comprise Balance Rock seem to be made of a rock called gneiss, which is known for banding or striping. Here’s a close-up picture of Balance Rock (the bottom rock):