Seen here is the triangular shadow cast by Mount Hood in northern Oregon, USA. The photo was taken by Scott Hotaling who made the ascent from the south side during pre-dawn hours.

As for why a jagged mountain casts a perfectly triangular shadow, APOD explains:

“The triangle shadow phenomena is commonly seen from the tops of other large mountains and volcanoes. A key reason is that the observer is looking down the long corridor of a sunrise (or sunset) shadow that extends to the horizon. Even if the huge volcano were a perfect cube and the resulting shadow were a long rectangular box, that box would appear to taper off at its top as its shadow extended far into the distance, just as parallel train tracks do.”

 

Mount Hood is a stratovolcano located about 50 miles (80 km) east-southeast of Portland. In addition to being Oregon’s highest mountain, it is one of the loftiest mountains in the nation based on its prominence (the height of a mountain’s summit by the vertical distance between it and the lowest contour line encircling it). According to the US National Geodetic Survey (NGS) Mount Hood is 11,249 feet (3,429 m) tall. The peak is home to 12 named glaciers and snowfields. [Source]