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Rock groups

Dear Mr. Alice:

When I walk on the beach, I notice that rocks of the same size seem to be clustered together. All the big ones are together, and all the small ones are with other small ones.

-- Wanderer, Solana Beach

While you're padding across the sand, the beach itself is moving too. In Southern California, sand and rocks are continually heading south with the prevailing longshore currents. During times of heavy surf in the winter, the veneer of sand is stripped off the cobblestone base that makes up many of our beaches. New sand is redeposited by the gentler waves of spring and summer. But on a smaller scale, individual rocks are being shoved around too. Rocks of roughly the same size will congregate because it takes the same amount of wave energy to get each one of them moving (the erosion velocity) and keep them moving. When the energy drops below a certain point (the fall velocity), rocks of the same size (actually, the same weight) will stop moving at roughly the same time. Enough of this pushing and shoving, and you'll eventually see the tide line covered with clumps of cobbles that look about the same. Beach contour and some other factors also affect where sand and rocks end up, so this is the shorthand version for you folks who are saying, "Aw, who cares, Matt? Let's get on to something juicy."

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Dear Mr. Alice:

When I walk on the beach, I notice that rocks of the same size seem to be clustered together. All the big ones are together, and all the small ones are with other small ones.

-- Wanderer, Solana Beach

While you're padding across the sand, the beach itself is moving too. In Southern California, sand and rocks are continually heading south with the prevailing longshore currents. During times of heavy surf in the winter, the veneer of sand is stripped off the cobblestone base that makes up many of our beaches. New sand is redeposited by the gentler waves of spring and summer. But on a smaller scale, individual rocks are being shoved around too. Rocks of roughly the same size will congregate because it takes the same amount of wave energy to get each one of them moving (the erosion velocity) and keep them moving. When the energy drops below a certain point (the fall velocity), rocks of the same size (actually, the same weight) will stop moving at roughly the same time. Enough of this pushing and shoving, and you'll eventually see the tide line covered with clumps of cobbles that look about the same. Beach contour and some other factors also affect where sand and rocks end up, so this is the shorthand version for you folks who are saying, "Aw, who cares, Matt? Let's get on to something juicy."

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4S Ranch Allied Gardens Alpine Baja Balboa Park Bankers Hill Barrio Logan Bay Ho Bay Park Black Mountain Ranch Blossom Valley Bonita Bonsall Borrego Springs Boulevard Campo Cardiff-by-the-Sea Carlsbad Carmel Mountain Carmel Valley Chollas View Chula Vista City College City Heights Clairemont College Area Coronado CSU San Marcos Cuyamaca College Del Cerro Del Mar Descanso Downtown San Diego Eastlake East Village El Cajon Emerald Hills Encanto Encinitas Escondido Fallbrook Fletcher Hills Golden Hill Grant Hill Grantville Grossmont College Guatay Harbor Island Hillcrest Imperial Beach Imperial Valley Jacumba Jamacha-Lomita Jamul Julian Kearny Mesa Kensington La Jolla Lakeside La Mesa Lemon Grove Leucadia Liberty Station Lincoln Acres Lincoln Park Linda Vista Little Italy Logan Heights Mesa College Midway District MiraCosta College Miramar Miramar College Mira Mesa Mission Beach Mission Hills Mission Valley Mountain View Mount Hope Mount Laguna National City Nestor Normal Heights North Park Oak Park Ocean Beach Oceanside Old Town Otay Mesa Pacific Beach Pala Palomar College Palomar Mountain Paradise Hills Pauma Valley Pine Valley Point Loma Point Loma Nazarene Potrero Poway Rainbow Ramona Rancho Bernardo Rancho Penasquitos Rancho San Diego Rancho Santa Fe Rolando San Carlos San Marcos San Onofre Santa Ysabel Santee San Ysidro Scripps Ranch SDSU Serra Mesa Shelltown Shelter Island Sherman Heights Skyline Solana Beach Sorrento Valley Southcrest South Park Southwestern College Spring Valley Stockton Talmadge Temecula Tierrasanta Tijuana UCSD University City University Heights USD Valencia Park Valley Center Vista Warner Springs
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