baugette

I am new to the art of bread  baking and I have a question for the experts, or novice, on this site.  I live at 3500 feet in a arid climate so humidity is not a problem but altitude can be.  I made a baugette recipie the other earlier this week and the finished product turned out heavy.  I used KA bread flour, sea salt and Fleichsmans active dry yeast in the dough.  I followed the instructions in the recipie but it still turned out like a lead balloon. 

Any ideas as what I did wrong in the process.