Temperature gauge

Hi Everyone,
Been meaning to get on this forum for ages, and a slight issue has prompted me to join. Ive a very old disco (Mreg), and was driving up north to the lakes, when the temperature gauge fell to below cold after doing a long run of over 100 miles. this seamed very strange to me...I would have thought it'd have gone up rather than down?? The water level was a bit low, so i toppped it up, but could this be a symptom of a bigger problem? any info would be well recieved,
Cheers,
Jon

Rather too low than too high.Could be a few things.Did you have the heater on ? Did this start blowing cold.
May be your thermostat is stuck open- does the temp behave normal on shorter runs.
Were you on the m way at a steady speed.If it is a diesel the cooling can be too good and almost overcool the system.