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For a variety of reasons, ice on lakes is not as gravity-bound as open water. Ice can form various sorts of ridges and bulging surfaces. You can take sights off an ice "horizon" but be aware that it's probably less reliable than a water horizon. 

Looking for something to do with your sextant on a frozen lake? How about setting up a sextant test range...

Make sure the ice is really solid and safe. Place a small platform or table in the middle of a clear area. Then get in your snowmobile and stake out a circle with a radius of about two miles. Plant six to ten stakes at regular intervals around the circle (but not precise locations). Place your sextant on its side on the table and measure stake-to-stake angles. Measure all neighboring pairs around the circle. They should total up to 360°00.0' exactly. If not, your sextant has an issue, and you can investigate further. 

Historical note: in the 18th century the mathematician/navigator Jose de Mendoza y Rios (Spaniard who lived in London and frequently wrote in French), an expert in lunars and author of multiple articles and books in navigation, suggested this method for testing sextants. In that era he suggested that this could be done using the numerous church steeples visible in the distance around any city or large town like London or Paris or Madrid.

Frank Reed