Complex sentence: The range reaches its widest point north of Goose Creek at nearly three miles, south of the creek the Catoctin vanishes into the piedmont countryside near the northern terminus of the Bull Run Mountains at Aldie.
Split into simpler sentences: The range reaches its widest point north of Goose Creek at nearly three miles. South of the creek the Catoctin vanishes into the Piedmont countryside near the northern terminus of the Bull Run Mountains at Aldie.