On mohs scale, ice has a hardness of 1.5. why can a glacier cause the grooves and parallel scratches (striations) found in bedrock of granite and gneiss? answer

The glacier carries along with ice, rocks that create the grooves and scratches. As a simple example, paper is relatively soft, yet sandpaper is quite capable of sanding substances with a mohs scale hardness of 9 or lower. Reason is quite simple. The sandpaper has embedded on its surface, silicon carbide which has a hardness of 9 to 9.5. And glaciers can carry boulders and rocks along their bottom surface which in turn can grind and scrape softer materials.

Glaciers are bodies of dense ice that moves constantly due to its hefty weight. Glaciers are known to cause glacial striations, characteristic scratches to the underlying rock. Glaciers tend to have rocks or boulders embedded on them, causing glacial striations. Glacial striations have important role in determining movement of the glacier and thus often studied.