MARYLAND GEOLOGICAL SURVEY 171 



is SO powerful as to determine tlie difference between what is called 

 continental and marine climates. 



The valleys in a mountain regior. have greater extremes of temperature 

 than the mountain tops, being usually warmer during the day and in 

 summer, and colder at night and during winter, because the cold air 

 flows down the slopes and accumulates in the depressions. The effect of 

 the nature of the soil and soil covering is also important. The mean 

 temperature of the soil is always higher than that of the air above it. 

 There are great differences, however, in the amount of heat which differ- 

 ent soils return to the air. In rocks the temperature is higher at all 

 depths and at all times of the year than in the overlying air, consequently 

 rocky soils give up more heat to the air than other kinds. In sandy land 

 the upper layers only are warmer than air, while moist lands or l)ogs 

 are colder because much of their heat is lost in causing evaporation. 

 A covering of vegetation lowers the temperature of the soil, and changes 

 in temperature over grass and forests are less than over bare soils. In- 

 cidentally forests conserve the rainfall, returning it slowly to the streams 

 and diminishing the evil effects of drought. 



The position of a place with reference to the prevailing path of 

 storms determines the frequency of rainy days, the cloudiness, the winds, 

 and all the variable phenomena called weather, which are non-periodic 

 in occurrence. 



THE PHYSIOGRAPHIC FEATURES OF CALVERT COUNTY. 



In order to correctly interpret the climate of Calvert County it is 

 essential to have some knowledge of its physiographic features, but as 

 complete details will be found in other portions of this volume, it will 

 only be necessary to give here a brief recapitulation of the main facts. 



Geologists divide the region from the Appalachian chain to the Atlan- 

 tic coast into three well-known physiographic provinces: the Appalachian 

 Eegion, Piedmont Plateau, and Coastal Plain. In Maryland the Coastal 

 Plain includes all that portion of the State lying east of a line extending 

 from southwest to northeast through Washington, Baltimore, and Wil- 

 mington, Del., or about one-half the area of the State. The Coastal 



