This is the ancient shore of the Baltic Ice Lake.  We can only imagine the landscape 10,000 years ago, when the Kurzeme peninsula was covered with vegetation reminiscent of a tundra, and waves from the icy lake dashed against the ancient shoreline.  We know that the lake was full of icebergs of various sizes.  Today the cliff is more than 20 km long (some 9 km of it are in the Slītere National Park), and it is as much as 42 m high.  It is the most impressive cliff of its type in all of Latvia.  At the foot of the cliff are the Irve flatlands (the name comes from the Liv 'īrva' or 'īra', both of which refer to deer).  The flatlands are the former bottom of the Baltic Ice Lake.  The Blue Hills are crossed by many small streams and rivers (Pitragupe, Lorumupe, Kaļķupe, Mazupe, etc.).  Their valleys are the basis for a branched and very complicated system of waterways which look particularly impressive when the trees are bare.  In many places the cliffs and valleys display small areas of sandstone from the Narva suite.  These are Latvia's oldest sediment cliffs that can be seen above ground.  Although the Blue Hills are very impressive in scope, there are only a few places where there are important viewing areas (a viewing tower at the Dundaga-Mazirbe road, the view from the Šlītere lighthouse, the view from Mežlīdumi).  The mightiness of the ancient shoreline can truly be enjoyed from the Cirste-Mazirbe road, the Cirste-Riepaldi road, Mežlīdumi and the Vīdala-Melnsils road).  It is worth visiting the area in the springtime, when the slopes of the Blue Hills seem to be sinking under the fresh greenery.